Chapter 23 Airway Volumes, Flows, and Pressures P.729

Definitions

• Compl iance : Ratio of a change in volume to a change in pressure . It is a

measure of dis tensibi l i ty and is usually expressed in mill i l i ters per centimeter

of water (mL/cm H2O). Compliance commonly refers to the lungs and chest

wal l . Breath ing system components, especia lly breathing tubes and the

reservoir bag, also have compl iance.

• Expiratory Flow Rate : Rate at wh ich gas is exhaled by the patient expressed

as volume per uni t of time.

• Expiratory Flow Time : Time between the beginning and end of expiratory f low

(Fig . 23.1).

• Expiratory Pause Time : Time from the end of exp iratory f low to the s tart of

inspiratory f low (Fig . 23.1).

• Expiratory Phase Time : Time between the start of exp iratory f low and the

start of inspiratory f low. It is the sum of the exp iratory f low and expiratory

pause times (Fig . 23.1).

• Inspiratory Flow Time : Period between the beginning and end of inspiratory

f low (Fig . 23.1).

• Inspiratory Pause Time : That portion of the inspira tory phase time during

which the lungs are held inf lated a t a fixed pressure or volume (i .e., the t ime

of zero f low (Fig. 23 .1). I t is also called the inspira tory hold , inf la tion hold ,

and insp iratory plateau .

• Inspiratory Phase Time : Time between the start of inspiratory f low and the

beginning of expiratory f low (Fig. 23 .1). I t is the sum of the inspiratory f low

and inspiratory pause t imes. The insp iratory pause t ime:insp iratory phase

t ime (T IP :T I) may be expressed as a percentage.

• Inspiratory:Exp iratory Phase Time Ratio (I :E rat io): Ratio of the inspiratory

phase time to the exp iratory phase t ime. For example, an I :E ratio of 1:2

means tha t the inspiratory phase t ime is one thi rd of the venti la to ry cycle

t ime.

• Inspiratory Flow Rate : Rate at which gas f lows in to the pat ient expressed as

volume per uni t of t ime.

• Minute Volume : Sum of al l tidal volumes within 1 minute.

• Peak Pressure : Max imum pressure during the inspiratory phase t ime (Fig.

23.1).

• Plateau Pressure : Rest ing airway pressure during the inspiratory pause.

There is usua lly a lowering of airway pressure f rom peak pressure when

there is an inspiratory pause (F ig. 23.1). This lower pressure is cal led the

plateau pressure .

• Pos it ive End-expiratory Pressure (PEEP): Pos it ive pressure in the ai rway a t

the end of exhalat ion.

• Resis tance : Ratio of the change in driv ing pressure to the change in f low

rate . It is commonly expressed as centimeters of water per l i ter per second

(cm H2O/L/second).

• Tidal Volume : Volume of gas entering or leaving the patien t during the

inspiratory or expira tory phase time, respect ively.

• Venti la to ry (Respira tory) Rate or Frequency: Number of resp iratory cycles

per uni t time, usually per minute.

• Work o f Breathing : Energy expended by the patient and/or venti lator to move

gas in and out of the lungs (1). I t is expressed as the ratio of work to volume

moved, commonly as joules per l i ter. It inc ludes both the work needed to

overcome the elas tic and f low-resistive forces of the both respiratory system

and apparatus.

View Figure

Figure 23.1 Flow, volume, and pressure curves from a ventilator that produces a rectangular inspiratory flow wave. A: This represents controlled ventilation with no inspiratory pause. The end-inspiratory pressure will equal the peak pressure. B: With an inspiratory pause, there is a decrease from peak pressure to a lower plateau pressure. C: This illustrates the effect of continuing fresh gas flow during inspiration. The inspired volume increases, and the peak pressure falls, then rises.

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General Considerations

The Ventilatory (Respiratory) Cycle Airway pressure with controlled venti lat ion is shown in Figure 23.1. There is a r ise

in p ressure with no preced ing negat ive pressure. A fast r ise to peak pressure

sugges ts too high a f low. The peak pressure wi l l increase if tidal volume,

inspiratory f low rate , or res istance increases or compl iance dec reases . A decrease

in peak pressure may resu lt f rom a leak, spontaneous inspiratory effort by the

patient, a dec rease in res istance, or an increase in compliance.

Figure 23.1B shows the respira tory cycle wi th an inspiratory pause. If the pause is

long enough, a p la teau pressure wil l occur a t the end of inspiration . The plateau

pressure is usually preceded by a higher peak pressure .

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Plateau pressure depends on t idal volume and the total s tat ic compl iance but is

independent of resistance (2).

Compliance and Resistance

In the past, compliance and resistance measurements during anesthesia were

diff icul t and involved bulky apparatus. They can now be measured accura tely on a

real-t ime basis with relat ively compac t equ ipment.

Compliance Compl iance measurement may be dynamic or s ta tic . Dynamic compl iance is

ca lcula ted by d iv iding the difference in volume by the dif ference in p ressure at two

points during the vent i latory cycle. Th is is not a true measure of total compliance,

because the ai rway pressure inc ludes the pressure needed to overcome resistance

(3).

Static compliance is ca lculated by using the end-inspiratory occ lusion pressure

(4,5) (Fig. 23.22). Condi t ions of zero gas f low are achieved by employing an

inspiratory ho ld or occluding the expiratory port long enough to allow ai rway

pressure to reach a cons tant value . This pressure, commonly te rmed plateau

pressure , represents the elastic recoil of the tota l respira tory sys tem at end-

inf lat ion volume.

Static Compliance = Tidal volume/Plateau pressure - Posit ive end-exp iratory

pressure (PEEP)

In adul ts , normal total s tatic compliance is 35 to 100 mL/cm H2O. In children,

normal s tat ic compl iance is greater than 15 mL/cm H2O (6).

Total compliance ref lec ts the elast ic propert ies of the lungs, thorax, abdomen, and

the breathing system. Using muscle relaxants wi l l increase chest wall compl iance

but wil l no t affec t lung compliance, so in paralyzed patients, changes in compliance

ref lec t mainly al tera tions in lung compl iance.

Resistance When gas f lows th rough a tube, energy is lost. This is ref lec ted by a decrease in

pressure . The pressure drop can be expressed as the product of resistance and

f low rate . For a given tidal volume, a high resis tance may be overcome by using a

lower f low for a longer time or a higher driv ing pressure . During control led

venti lation , if there is an inc rease in ai rway resistance, the pressure needed to

deliver a given tidal volume wil l increase. This can usually be supplied by the

venti lato r or the person squeezing the reservoir bag so tha t inspira tory f low is not

af fected. Because exhalat ion is passive, expiratory f low depends on the elas tic and

resist ive forces of the lungs and the resistance in the expiratory l imb of the

breathing sys tem and airway dev ice.

Total res istance, wh ich may d if fe r during inspiration and exp irat ion , is determined

predominantly by the res is tance of the pat ient 's ai rway, the tracheal tube, and the

breathing sys tem. Decreased airway ca liber from bronchoconstric tion, secret ions,

tumor, edema, a foreign body, or airway c losure is associated wi th inc reased

resistance. Tracheal tube resistance depends primari ly on its in ternal diameter.

Partial tube obstruction by secretions, kink ing , or other p roblems wil l cause

increased res is tance. Breathing system resistance is affected by the length and

inte rna l diameter of i ts components and is inc reased by sharp bends and

constric t ions.

Total airway resistance can be est imated by using the dif fe rence between peak and

plateau pressures, which is normally 2 to 5 cm H2O. If there is an increase in

resistance, a higher peak pressure wi l l be necessary to produce the same f low.

Plateau pressure, however, depends only on compliance and wi l l not be affected by

resistance. Therefore, if the inspiratory f low and tidal volume remain constant but

resistance increases, there wil l be a greater d if fe rence between the peak and

plateau pressures.

Measured Gas Composition The compos it ion of the gas being measured wil l af fect the accuracy of f low-

measuring devices (7,8). Dif ferences in densi ty and v iscosity of the gases can

induce an error in f low measurement. The composi tion of carrier gas has a greater

impact on v iscosity than volati le anesthetic agents, whereas densi ty is more

inf luenced by volati le agent concentrat ions (9).

For accuracy, the f low-measuring dev ice should be associated wi th a gas monitor

that can make corrections to gas f low caused by changes in gas composi tion . If a

gas such as xenon that is not measured is p resent, f low measurements may be

inaccurate (8).

Respiratory Volume and Flow Measurement A respirometer (sp irometer, vent i lat ion or respira tory meter or mon itor,

venti lometer, volume measuring dev ice, f low monitor, respiratory flowmeter) is a

device tha t measures the volume of gas passing during a period of t ime through a

locat ion in a f low pathway (10).

Moni toring respiratory volumes and f lows can aid in detect ing breath ing system

obstruct ions, disconnect ions, apnea, leaks , venti la tor fai lure , and high or low

volumes in spontaneously breath ing patien ts as wel l as in those whose venti la tion

is contro lled. Some can detec t reversed f low, an indicat ion of an incompetent

unidi rect ional valve or a leak. A discrepancy between expired and inspired tidal

volume should suggest a leak .

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A decrease in t idal volume assoc iated wi th the tracheal tube migrating into a

bronchus may be detec ted (11). Al though there are other ways of detecting these

problems, such as observ ing chest wal l movements, monitoring breath sounds ,

capnometry, and airway pressure moni toring , the use of a volume monitor provides

additional pro tec tion. One study found that for detec ting and c lassify ing breathing

system faul ts , a volume moni tor was better than ai rway pressure or carbon d ioxide

moni toring (12). In 1999, the American Society of Anesthesiologists (ASA) strongly

encouraged qual itat ive moni toring of the volume of expired gas. The anesthesia

workstat ion standard requires a device to moni tor the patient's exhaled tidal or

minute volume or both (13).

Respiratory volume moni toring may fa il to detect some problems. With ai rway

occlusion, there may be enough f low during expiration resul t ing f rom compression

of gas within the breathing system during inspiration to prevent the respirometer

alarm f rom being ac tivated. If the sensor is attached to the patient 's tracheal tube

or supraglottic ai rway device, a disconnect ion between the sensor and the

breathing sys tem wi ll not be detected if the patient is spontaneously breathing . If

the sensor is in the exha la tion s ide of the breathing system, the disconnection wi l l

be detec ted. It is possible to have fai r ly normal f low with esophageal intubat ion.

A high-volume alarm may be useful to de tect unant ic ipa ted increases in t idal

volume (14). This may be due to improper venti la tor settings or inc reased gas f low

into the breathing sys tem during inspirat ion, resu lt ing from a hole in the vent ilator

bellows, an inc reased insp iratory:expiratory (I :E) rat io , or f rom the f lowmeters (if

there is no f resh gas flow compensation or decoupling). During pressure control

venti lation , a decrease in compliance wil l resul t in an increased t idal volume.

Older respirometers were stric tly mechanical dev ices. Newer respirometers convert

f low into an e lectronic signa l that is processed and displayed. Elec tronic processing

enhances alarm capabi l i ty. Alarm limi ts should be set as close as possible to the

displayed tidal or minute volume without producing an unacceptable inc idence o f

false-posi t ive a larms (15).

Equipment Ventilator Bellows Scale

Venti la to rs that are used in anesthesia are discussed in Chapter 12. I f the

venti lato r has a bel lows, there is usual ly a scale on the bel lows housing . This scale

can prov ide a rough estimate of t ida l volume de livered into the breathing system

but is no t an accura te es timate of the volume delivered to the patien t because of

was ted vent ilation secondary to gas compression and dis tension of components o f

the breathing system. I f the fresh gas f low adds to the t idal volume during

inspiration (see Chapter 12), th is added volume wi l l not be represented on this

scale.

View Figure

Figure 23.2 Wright respirometers. A: This small instrument can be handheld or inserted into the breathing system. It has two dials: a large peripheral one and a smaller one on the upper part of the main dial. The small dial indicates volumes up to 1 L and the large dial up to 100 L. Note the reset button on the side. B: This version has three dials. The top small dial reads up to 1 L, the large dial indicates volumes up to 100 L, and the bottom small dial reads up to 10,000 L. Note the on-off control and the directional flow arrow. (Courtesy of Ferraris Medical, Inc.)

Wright Respirometers

Description Typical Wrigh t respirometers are shown in F igure 23.2. They are supp lied wi th

adaptors to fac il i tate connection to a mask, ai rway device, or breathing sys tem.

There is an ON-OFF control in the form of a s liding stud and a spring-loaded reset

button to se t the hands o f the scales to zero .

An infant vers ion that can measure volumes down to 15 mL is avai lable (Fig. 23 .3).

I ts dead space is 15 mL. An e lec tronic vers ion is also available (15).

The internal construction is shown in Figure 23.4 . Gas entering through the outer

casing is di rected through a series of tangential s lots enclosed in a cy lindrical

housing and strikes a vane, causing i t to rotate. The vane is connected by a

mechanical gear system to the hands on the dial so that a reading corresponding to

the volume of gas pass ing through the dev ice is registered.

Evaluation Most studies have found tha t the Wright respirometer over-reads at high flows and

under-reads at low f lows (16,17,18). Pulsati le f lows can cause addit ional over-

reading. It wi l l give s lightly higher readings wi th mixtures of ni trous oxide and

oxygen than for ai r and wi l l s l ightly over-read in the presence of xenon (8).

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View Figure

Figure 23.3 Infant version of Wright respirometer. The outer scale goes up to 500 mL, and the inner scale goes up to 5 L. (Courtesy of Ferraris Medical, Inc.)

Advantages of the Wright respirometer inc lude its small s ize and light weight. Its

low dead space makes it sui table for use between the patien t and the breath ing

system.

The main disadvantage is that i t has no alarms. I t is somewhat di ff icul t to read and

does not give respiratory rate. A clock is necessary to de termine minute volume. It

does not read bidi rectional f low. Maintenance can be expensive. Many instruments

that are in use are inaccurate because of poor mechanical cond ition. I ts portabil i ty

can resu lt in inaccuracy due to pocket di rt and a high incidence of damage f rom

being dropped. Guards that are designed to reduce damage from phys ical abuse

are avai lable. I t needs to be c leaned and dis infected between patien ts .

Spiromed

Description

The Spiromed is an electron ic respirometer that is designed for use wi th North

American Drager breathing systems (19,20). As gas f lows through the monitor, i t

forces a pair of rotors to counter-rotate (Fig. 23 .5). Attached to the axle of one of

the rotors is a four-pronged armature with a smal l magnet at the tip of each prong.

As exhaled gas f lows through the sensor, the rotor and armature spin in un ison.

Located at approximately the 12 and 7 o'c lock posi tions are two transistors that

turn ON in the presence of a magnetic f ield. As the armature that carries the

magnets rotates, the trans is tors are turned ON and OFF. These paired pulses are

transmitted through the sensor cable to the inte rface panel and then to the

processor.

View Figure

Figure 23.4 Internal construction of Wright respirometer. Gas entering the casing is directed through a series of tangential slots and strikes the vane in the center, causing it to rotate.

View Figure

Figure 23.5 Spiromed. (See text for details.)

The number of paired pulses is related to the volume of gas tha t passes through

the sensor over t ime. The to tal number of pulse pairs counted during each

exhalation determines the tidal volume. The speed a t which the exhaled gas f lows

through the sensor determines the dura tion of each pulse pa ir. Rapid gas f low

causes the ro tors and armature to spin quick ly, and shorter pulses are produced as

the trans istors rap id ly cycle. With slower gas f low, longer pulses are produced. The

processor analyzes the pulse lengths and displays the information as the exhaled

waveform.

Sixty seconds of cont inuous data is required for the in it ia l display of the respira tory

rate , and the displayed read ing is recalculated af ter each exhalation. For an

exhalation to be counted as a “valid b reath,” the processor mus t count at least 80

mL. All exhaled gas volumes, regardless of s ize, are counted and inc luded in

ca lcula ting the minute volume. The sensor in the breath ing system is shown in

Figure 23.6.

The Spiromed sensor recognizes the di rection of gas flow by monitoring the phase

rela tionship between the pulses in each pu lse pair. When the gas f lows “forward,”

the pulse from one transisto r leads the pulse from the o ther transducer because of

the armature's rotat ional direct ion. I f gas f lows in the wrong direc tion, the order

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of the pu lses is reversed and the processor recogn izes this as reverse f low. If two

consecutive pulse pairs are in reverse order, a reverse f low alarm is generated.

View Figure

Figure 23.6 Spiromed in place in breathing system.

Evaluation This instrument is programmed to measure t idal volumes equal to or g reater than

150 mL. If the t ida l volume is less than 150 mL, the instrument wi l l automatica lly

add two or more consecut ive tidal volumes and reduce the recorded frequency

accordingly. The minute-volume display remains correct.

The accuracy o f tidal volume measurement is reported as ±0 .04 L, minute volume

as ±10% of reading or 0.1 L, and resp iratory rate as ±8% of reading or 1

breath/minute.

D-Lite Gas Sampler and Flow Sensor

Description The f low sensor (Fig. 23.7) fo r this device is a modif ied F leisch pneumotach with a

two-s ided Pitot tube (21,22,23,24). The Fleisch pneumotach measures f low by

measuring the pressure difference across a f low resisto r (capil lary tube) in a tube.

The Pi tot tube uses two sensing tubes to make a d if fe rential p ressure

measurement. One tube faces the di rec tion of f low (to tal pressure), and the other

faces the opposi te di rect ion to measure the static pressure . The dif ference in

pressure between the total pressure and sta tic pressure is the dynamic pressure,

which is proportiona l to the square of gas f low.

The sensor body (Fig. 23 .8) consists of a straight tube wi th a combined 15-mm

female/22-mm male connector on the pat ient end and a 15-mm male connector on

the machine end. Two smal l hollow pressure tubes perforate the side of the tube

and extend in to the lumen. Each makes a 90-degree turn ins ide the lumen so that

the end of one tube faces the breathing system and the o ther end faces the pat ient.

A gas sampling port is also present. A double-lumen tube conduc ts the f low s igna l

as a pressure dif ference to the pressure sensor ins ide the moni to r.

The sensor is placed between the breathing system and the pat ient. A f il ter or heat

and moisture exchanger may be placed on e ither s ide of the sensor. If placed

between the patient and the sensor, mucus and humid ity wil l be prevented from

entering the gas sampling tube. If the sensor is placed between the patient and the

heat and moisture exchanger, a higher compl iance wi l l be observed than if i t is

placed between the heat and moisture exchanger and the Y-piece (25).

View Figure

Figure 23.7 D-Lite flow sensor and gas sampler. The patient end has a 15-mm internal and 22-mm outside diameter connector to fit a mask or tracheal tube connector. The other end has a 15-mm outside diameter connector. Because the pressure tubes point in opposite directions, gas flows can be measured during both inspiration and exhalation. Note that one pressure tube is larger than the other to avoid misconnection of the tubings. There is a gas sampling port on the opposite site of the sensor.

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View Figure

Figure 23.8 Sensor for D-Lite flow sensor and gas sampler A: The gas sample port is at the top. Note that the pressure line attachments utilize male and female connectors. B: Attachment for pressure tubings.

During inspirat ion, gas moves f rom the breath ing system toward the patient. The

pressure in the hollow tube fac ing the breathing system and the pressure in the

tube that faces away from the di rect ion of gas f low are measured. Since the

pressure tubings face in opposi te di rections, s imilar measurements can be made

during exhala tion, when the gas flow is reversed.

In the moni tor (Fig . 23.9), concentra tions of carbon dioxide, oxygen, and anes thetic

agents are determined. The moni tor u ti l izes the gas composi tion data to

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compensate for changes due to densi ty and v iscosity (7 ). A correct ion factor must

be applied if helium is in the respira tory mixture (26). From the derived f lows (f low

rate , peak f low) and measured pressures (end-expiratory, plateau, minimum, and

maximum), the inspiratory and expiratory t idal and minute volumes , compliance,

and resistance are ca lcula ted and displayed, and f low-volume and pressure-volume

loops are displayed. Inspired and expired gas concentrat ions are also d isplayed.

View Figure

Figure 23.9 The monitor utilizes the gas composition data to compensate for changes due to density and viscosity. From the derived flows and measured pressures, the inspiratory and expiratory tidal and minute volumes, compliance, and resistance are calculated and displayed, and flow-volume and pressure-volume loops are displayed. Inspired and expired gas concentrations are also displayed.

Alarms inc lude a h igh PEEP alarm wi th a default value of 10 cm H2O. High pressure

and low inspira tory pressure alarms have defau lt settings of 40 and 0 cm H2O.

There are high and low expira tory minute vo lume a larms as wel l as messages for

leak, disconnect ion, and obstruction .

Calibration The sensor needs to be ca librated a t least every 6 months (27). One indica tion that

ca libration needs to be performed is an open or overshoot pressure- and flow-

volume loop (Figs. 23 .61 and 23.62). The opera tion manua l should be consulted for

the complete procedure. It should be perfo rmed wi th the equ ipment in the

configuration tha t wil l be used wi th the next patient. Accessories, such as heat and

moisture exchangers, placed proximal to the sensor wil l no t af fec t calibra tion.

However, if the c linic ian wishes to p lace an accessory on the distal end of the

sensor, then the uni t should be cal ibrated wi th this in place. A dif ferent size of

tracheal tube or the omission of the connector could affect the ca libration value and

resul t in incorrec t volume measurements.

Evaluation The D-Li te is used to measure f lows in ranges common for adul ts and chi ld ren down

to 3 kg. Res istance to f low is 0.5 cm H2O at 30 L/minute. I ts volume is 9.5 mL.

Tidal volumes of 150 to 2000 mL and minute volumes of 2.5 to 30 L/minute can be

measured. The pedi-l i te sensor can measure t idal volumes in the range from 15 to

300 mL. The range of measurement for a irway pressure is -20 to +80 cm H2O. The

range of f low rates is -100 to +100 L/minute. I t over-reads when xenon is used (8).

Regular v isual inspec tion and water removal is required for troublefree

performance. I f used fo r extended periods with heavy humidif icat ion, condensed

water may occlude the pressure sensing or gas sampl ing tubes. The pressure and

sample l ine tubings shou ld be on the upper s ide of the sensor.

The D-Li te sensor has a simple and robust construct ion, l igh t weight, low dead

space, and no moving parts. I t is not posit ion dependent and al lows bidi rec tiona l

gas f low measurement. Small amounts of mucus and water d roplets do not affect

the measurements . Only one adaptor is needed for respirometry and gas sampling.

I t can be used with both c i rc le and Mapleson breathing sys tems. Another important

advantage is the abil i ty to moni tor f low-volume and pressure-volume loops.

View Figure

Figure 23.10 Novometrics sidestream sensor.

Novometrics Sidestream Sensor The Novometrics s idestream sensor (Fig. 23 .10) combines measurement of f low,

pressure , and carbon d ioxide. Flow is determined f rom a different ial pressure

measurement across a f ixed orif ice wi th a spl i t obs truct ion to one side of the optical

window used for mainstream carbon dioxide measurement. The sensor has a dead

space of less than 0 .8 mL, mak ing i t useful for the neonate (28).

Heated Wire Anemometer In the heated-wire anemometer (thermal diss ipat ion device) (Figs. 23.11, 23.12),

gas f lows around a th in wire (usually plat inum or a p lat inum al loy) that is heated to

a constant temperature (10). Heat is diss ipated when gas f lows past this wire. The

greater the volume of gas f lowing pas t per uni t time, the more heat wi l l be

diss ipa ted. The current level is usual ly low so that the outs ide of the sensor does

not become heated.

View Figure

Figure 23.11 Heated wire anemometer.

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View Figure

Figure 23.12 Heated wire anemometer. One wire is for measuring flow, and one is for reference.

The hot-wire sensor tends to be more accurate at low f low rates. Since it is

insensi tive to f low d irect ion, two heated wires are needed to determine the f low

di rection . The heat dissipa ted by the second wire is determined when there is no

gas f low (e.g., during inhala tion in the exhalat ion s ide of the breathing system). I t

under-reads in the presence of xenon and sl ightly over-reads wi th ni trous oxide (8).

Ultrasonic Flow Sensor An ul trasound sensor (Figs. 23.13, 23 .14, 23.15) measures the inf luence of gas

f low on the transmission times of pulses between two crystals. Sending and

receiv ing transducers are used to transmit s ignals through the f low. The s ignal

travels fas ter when moving wi th the f low stream rather than aga inst the f low stream.

The dif fe rence between the two transmission times is used to calculate the f low

rate .

The f low sensor has no mov ing parts, is easy to c lean, is autoclavable, and i ts

accuracy is independent of gas composit ion. The f low measurement range is 0 to

120 L /minute. The accuracy is specif ied as ±10% or 15 mL, whichever is greater.

Resis tance is less than 2 cm H2O at a f low of 60 L/minute.

Variable Orifice Flow Sensor The variable orif ice f low sensor is used wi th the Ohmeda 7900 series venti la tors

(Chapter 12) and the ci rcle system. Sensors a t both connect ions to the carbon

dioxide absorber a re used to measure insp iratory and expira tory f lows . These

sensors can be used to generate pressure- and f low-volume loops. The 7900

venti lato r ut i l izes the information f rom these sensors to a llow it to del iver accurate

t ida l volumes.

Construction Each sensor (F igs. 23.16 , 23.17) uses the princ iple of p ressure drop across an

orif ice. A plast ic flap that opens wi th increasing f lows is placed across the di rec tion

of gas f low. Two sensors and a transducer

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inside the anes thesia machine measure pressure proximal and distal to the f lap.

Volume is calculated from these f lows . The sensor on the inspiratory s ide is

connec ted to a pressure sensor so that b reathing system pressure is measured.

The information is used by the venti la tor to compensate for changes in fresh gas

f low.

View Figure

Figure 23.13 Ultrasonic flow sensor in breathing system.

View Figure

Figure 23.14 Inside of ultrasonic flow sensor.

Use Before use, the tubes should be checked to make certain that they are clear. The

pressure l ines should poin t up, and there should be no k inks , cracks , or other

problems. The use of f i l ters is recommended to protec t the sensors f rom

contamination. Calibrat ion by using a menu on the vent ilator is recommended on a

weekly basis.

View Figure

Figure 23.15 Diagram of ultrasonic flow sensor.

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View Figure

Figure 23.16 Variable orifice sensor. Gas flow causes the Milar flap to bend. There is a pressure drop across the flap. A transducer inside the ventilator converts the pressure drop into a flow.

Accuracy The sensor can measure f lows from 1 to 120 L/minute. There are no respiratory

rate l imi ts . The instrument wi l l read high with xenon and ni trous oxide (8).

Since both inspired and exhaled volumes are measured, the vent i lator can make

adjustments so that changes in f resh gas f low do not affect the del ivered volumes.

Since the sensors are loca ted at the absorber, they cannot compensate for gas

compression or expansion in the breath ing system. This is a small error un less very

compliant breathing tubes are used or the breathing system contains a large

volume of gas.

Evaluation The main advantage of this dev ice is that i t al lows the venti lator to automatica lly

compensate for changes in f resh gas f low. Disadvantages include the need for two

sensors and f il te rs . A break in one of the pressure l ines can cause a leak in the

breathing sys tem

P.740

(29). The sensor i tself may be the source of a leak (30,31,32,33). The sensors are

sensi tive to humidity (34,35).

View Figure

Figure 23.17 A: Variable orifice sensor. Tubings are attached on either side of the flap. B: Tubings from the sensors attached to the anesthesia machine. It is important that they are attached to the proper connection.

Fixed Orifice Flow Sensor This dev ice cons is ts of a restric tor and two pressure sensors, one on ei ther side of

the res tric to r. A zeroing valve compensates for pressure sensor drif t .

Respirometer Position in the Breathing System Figure 23.18 shows possib le locat ions for a respirometer in the c irc le sys tem. From

the standpoin t of accuracy, the most desirable locat ion is be tween the breathing

system and the patient (posit ion C). In this loca tion, read ings are not affected by

breathing sys tem leaks , expansion of breath ing system components, o r gas

compression. Both inspired and expired volumes can be measured. Placing the

sensor a t th is si te wil l increase the dead space, and water condensation may be a

problem. Th is posi tion may resul t in inc reased l ikelihood of damage, disconnect ion,

or tracheal tube k ink ing.

A disadvantage of posi tion C is that if a disconnection occurs between the sensor

and the breathing system during spontaneous venti lat ion, the spirometry

measurements wil l no t be af fec ted. I f the sensor is loca ted a t posit ion B , the

disconnection wi l l be detected by the change in volume.

A common pract ice is to locate the respirometer in the exhalation l imb upstream or

downstream of the unidirect ional valve (posi tions A and B). An advantage of these

posit ions is tha t if the respirometer can sense reverse f low, a malfunc tioning

unidi rect ional valve can be detec ted. If a d isconnection that prevents exhaled

gases from passing down the exhalation tubing occurs, the resp irometer wi l l not

sense a gas f low, and an apnea message and a larm wil l be ac tivated. A

respirometer in this locat ion wi l l usually read accura tely during spontaneous

respira tion, but during control led respirat ion, i t wil l usual ly g ive erroneously high

readings

P.741

(36,37). This is due to expansion of components of the breathing system and gas

compression. If a venti lato r with a hang ing bellows is used, a respirometer in this

posit ion may st il l indicate f low when a disconnection occurs (38).

View Figure

Figure 23.18 Possible sites for a respirometer in the circle system. (See text for details.) PEEP, positive end-expiratory pressure; APL, adjustable pressure limiting.

I f the resp irometer is located downstream of the absorber (posi tion E), the volume

of gas measured wi l l be dec reased by the amount of carbon dioxide absorbed in the

absorber.

Another possible loca tion fo r the respirometer is on the inspiratory s ide of the

system (pos it ion D). In this locat ion, the respirometer wil l disp lay erroneously high

readings , as gas that does not inf late the patient 's lungs wil l also pass th rough it .

During contro lled venti lation , a disconnect ion may not be detected.

I t is common to loca te pressure-f low sensors at bo th posi tions B and D. This allows

both the inspiratory and exhalation volumes and pressures to be measured. This

provides the info rmation to produce a f low-volume or pressure-volume loop.

Sensors in both these posi tions are used wi th the Datex-Ohmeda 7900 venti lato r to

compensate for changes in t idal volume due to fresh gas f low or leaks .

When one o f these devices is used in a Map leson system wi th an adul t pat ient, it

should be placed between the patient connect ion port and the patien t. In smal le r

patients, i t should be placed in the expiratory l imb to avoid an increase in dead

space (11,15).

Airway Pressure Monitoring Airway pressure moni tors (vent i lator or respiratory monitors or alarms; pressure

alarms; pressure alarm systems; anesthesia, pat ient, or breathing c i rcui t moni to rs;

venti lato r moni to ring alarms ; breathing gas inte rrupt ion moni tors; d isconnect

moni tors ; breathing pressure moni to rs) are used to warn of high- or low-pressure

condi tions in the breathing system (39,40).

High- o r low-pressure condi tions in the breath ing system have been a major cause

of anesthesia mortali ty and morb id ity. A device tha t responds to pressure changes

wi th in the breath ing system and provides warning of a problem is s trongly

recommended. Other parameters such as exhaled carbon dioxide and exha led

volumes may remain rela tively normal in the presence of dangerously abnormal

ai rway pressures.

Continuous airway pressure monitoring is now the norm in both the opera ting room

and in crit ical care areas . It is a s imple and noninvasive technique tha t helps in

assess ing the patient 's mechanical and spontaneous venti lation and determining

the presence of PEEP.

P.742

View Figure

Figure 23.19 Virtual pressure gauge on anesthesia machine display. This can be displayed on demand.

Equipment An airway pressure moni to r may be frees tanding or incorporated in to a venti la to r or

an anesthesia machine. Most of these devices are inexpensive, robust, easy to use,

and rel iable . They may be powered ei ther from the main electrica l system wi th

battery backup or by a batte ry wi th ba ttery test capabi li ty .

Most new anes thesia machines have a buil t- in da ta sc reen where information

including ai rway pressure is avai lable . Usually, there wi l l be an airway pressure

versus time waveform displayed on th is screen. A v i rtual electron ic pressure gauge

may be disp layed on the moni tor screen (F ig. 23.19). Many machines in use are

st i l l equipped wi th a mechanica l pressure gauge on the absorber. Th is does not

al low e lectronic recording or the data to be integra ted wi th o ther parameters to

al low compl iance calcula tions . Alarms were not assoc ia ted wi th these manometers,

so they had to be repeatedly scanned by the anesthesia provider.

Alarms Most pressure alarms have an aud io pause (delay, mute, s ilencing) control that wil l

delay the aud ib le s ignal fo r some or a ll functions. This pause should not prevent

the v isual s ignal f rom func tioning. Some units ' aud ib le alarms can be comple tely

turned OFF (41). Other features on some devices include automatic act ivation when

a pressure is detected, the abil ity to de tect impending battery fai lure , and

protect ion f rom accidental inactivation or power fai lure (42,43). Some airway

pressure monitors have the abi l i ty to display pressure waveforms .

Low Peak Inspiratory Pressure A low peak inspira tory pressure (minimum airway pressure, low airway pressure ,

venti lation failure, apnea, cycling, pressure fa ilure, disconnect, vent i lator

disconnect, minimum venti la tory, venti lat ion pressure, threshold pressure , low-

pressure , peak a irway, fai l -to-cycle, low pressure, low c i rcui t pressure) alarm is

ac tivated i f the pressure detected does not exceed a preset min imum within a f ixed

t ime.

Bas ic mon itoring s tandards adopted by the ASA and the American Association of

Nurse Anesthetis ts (AANA) state that when venti lation is controlled by a mechanical

venti lato r, there sha ll be a means of de tecting disconnection of breath ing system

components in cont inuous use. Such an alarm has been recommended by other

responsible bodies around the world (44). The low peak insp iratory pressure alarm

is one of the means to fu lf i l l th is requirement. However, the pressure moni tor is not

foolproof . Under certain c i rcumstances, i t may fail to de tect anesthetic ci rcui t

disconnections. Pressure moni toring wi l l fail to detect a disconnect ion that occurs

during spontaneous vent ilat ion (45).

The airway pressure mus t exceed a threshold value (l imi t) to prevent an alarm. If

this l imit is not reached over a period of time, usual ly around 15 seconds, the alarm

is act ivated. The thresho ld needs to be set at a value s l igh tly below the peak

ai rway pressure. This value varies, depending on the c linical s i tuat ion. The l imi t

may be se t manua lly or au tomatically around a value s light ly lower than the peak

pressure determined by several successive breaths. When the peak ai rway

pressure becomes higher or lower, the th reshold may be automatical ly al tered or

there may be a means to move the thresho ld c loser to the new peak pressure (Fig.

23.20). On some machines , there wi l l be a v isual informationa l signa l that the l imit

is set too low.

The low peak pressure ala rm is enabled when the venti lato r is turned ON. I t is

inact ive when the venti la tor is no t being used.

Condit ions that can cause a low peak pressure alarm inc lude a disconnec tion or

major leak in the breathing system; an obs truct ion upstream of the pressure sensor;

inadequate fresh gas f low (disconnection of the f resh gas l ine, an inte rna l mach ine

obstruct ion, o r loss or reduc tion of pipe line pressure); the bag/venti lator selec tor

P.743

valve in the bag pos it ion; a leaking tracheal tube cuf f ; ex tubat ion; a faulty, poorly

se t, o r unconnec ted vent ilator; fai lure of the gas or power supply to the venti lator; a

malfunct ioning scavenging system; increased compl iance; reduced resistance; and

a suction dev ice mistakenly placed wi th in the gas f low pathway (46 ,47). Low

pressure alarms are of l i tt le o r no use during spontaneous breath ing when the

pressure in the system does not rise and fall appreciably (45).

View Figure

Figure 23.20 The pressure (top waveform) exceeds the threshold (dotted line) by a small amount. At the bottom is a touch control marked “AUTO PRESSURE THRESHOLD.” If the threshold is too high or too low, it can be altered by using the control or by touching the threshold on the screen and moving it to the desired location.

The alarm threshold should be set jus t below the minimum peak pressure expec ted

during inspira tion (41,42,48,49). This peak pressure wi l l vary not only from patient

to patien t but also during a given case. Of ten, the th reshold is se t lower in an

attempt to prevent false-pos itive ala rms. If the alarm limi t is set too low, a fa lse

negat ive may occur (50 ,51,52). I t has been sugges ted that a pressure threshold of

less than 8 to 10 cm H2O is unacceptab le (53). On most modern de livery sys tems,

the ci rcu it p ressure waveform and the low-pressure alarm threshold can be

displayed, mak ing i t easy fo r the operator to ad just the threshold properly (Fig .

23.2). On some moni tors , an adv isory s ignal wi l l be act ivated if the thresho ld is set

a certain amount be low the peak pressure. Some units automatically set the

threshold based on the pressure sensed during prev ious breaths. There may be a

manual threshold reset contro l.

Problems wi th these moni tors have been reported. A disconnection or leak may not

be detec ted if the alarm is not swi tched ON (54) or the th reshold is se t too low. A

false-negat ive condi tion may occur if the end-exp iratory pressure is above the

threshold pressure. Other condit ions that may produce a pressure high enough to

exceed the th reshold when a disconnec tion occurs inc lude the breathing system

connec tor's obstruction by a pi l low, sheet, or surgical drape; a high-resistance

component such as a heat and moisture exchanger, capnometer cuvette, or

humidif ier; ai r entrainment into the breathing system (especial ly wi th a venti lato r

bellows descending during expirat ion); partial extubat ion; compression of an empty

venti lato r bel lows; and a Mapleson system wi th a high resistance (55,56,57,58,59).

I f a vent i lator that uses a ram of oxygen to produce inspira tion is used wi th a T-

piece system, a disconnec tion at the common gas out let may not be detected due

to the high resistance of the f resh gas tubing (60). Those devices opera ting on

batteries wi l l not a larm if the batte ries fail . I t is essential tha t the a larm be checked

before use by mak ing a disconnection at the patient connector whi le the venti lator

is cycling (Chapter 33) (50). Unfortunately, s tudies show tha t this test is not

performed rout inely or correct ly (48). It is importan t tha t another means of

detec ting a disconnec tion (such as a capnograph or volume or f low monitor) be

used.

Sustained Elevated Pressure A sus tained (continuous, continuing) pressure monitor ac tivates an alarm if the

pressure does not fall be low a certain level during part of the respiratory cyc le.

Most are always enabled. Some incorporate a valve tha t opens to relieve the

pressure af ter a certain t ime.

Severa l mechanisms can produce a sus tained e levated pressure: accidental

ac tivat ion of the oxygen flush valve; occlusion or obstruc tion of the expiratory l imb;

an improperly ad justed adjustable pressure l imi ting (APL) valve ; occlusion of the

scavenging sys tem; a malfunct ioning venti la to r; or a malfunctioning or incorrect ly

se t PEEP valve (42,43,55).

High Pressure A high-pressure alarm is act ivated if the pressure exceeds a certain limit . On some

devices, the threshold

P.744

is fixed (usua lly 50 to 80 cm H2O); on o thers, it is adjus table (41). Some

instruments automatica lly set the alarm thresho ld at a set amount above the

average peak pressure for several p rev ious breaths. Mos t of these a la rms are

always enabled. There shou ld be no delay on the high-pressure a larm. Some

anesthesia delivery sys tems are f i tted wi th pressure-l imi t ing valves that vent gas

f rom the breath ing system when a high pressure is detected (41).

Possible causes of high pressure include airway obs truction, reduced compl iance,

increased res is tance, oxygen flush ac tivat ion during the inspiratory phase, a

punctured venti lator be llows , occlusion or obs truct ion of the expira tory l imb of the

breathing sys tem, scavenger malfunct ion , or the pat ient coughing or s training

(42,61). Even in the presence of complete obstruct ion , th is alarm wi l l not be

ac tivated i f the peak inspiratory pressure does not reach the set l imi t (49). High

compliance, low res is tance, leaks, low inspira tory flow rates, high respira tory ra tes ,

low I:E rat ios, low tidal volumes, and low f resh gas f lows can al l dec rease the peak

inspiratory pressure so that there is no ala rm cond it ion (2,62). During pressure

control venti lat ion, the inspira tory ai rway pressure is preset and thus cannot act as

a warning of tracheal tube occlusion (63).

Subambient Pressure A subambient (subatmospheric) pressure alarm is act ivated by a pressure that fal ls

below atmospheric pressure by a predetermined amount. Subatmospheric pressure

can be generated by a pat ient attempting to inhale against a collapsed reservoir

bag or increased resistance; a blocked inspiratory l imb (during the venti lator's

expiratory phase); a malfunct ioning active closed scavenging sys tem; suct ion

applied to a nasogastric tube placed in the tracheobronchial tree or to the work ing

channe l of an endoscope passed into the airway; a sidestream gas analyzer; or the

ref i l l ing of a hanging bel lows venti la tor bel lows (14,42,49,64,65,66).

Monitoring Site The location where pressure is sensed wil l af fect i ts usefulness. Figure 23.21

shows possible s i tes. Idea lly, the s i te should be c lose to the patient 's airway

(posi tion C). Many disposab le breathing systems have a smal l port a t the Y-piece

that can serve as the connect ion s ite fo r tub ing that transmits the pressure to a

moni toring device (57). Pressures during both inspirat ion and exha la tion can be

measured at this s ite. The D-Li te and Novometrics sensors discussed earlier in this

chapter

P.745

are placed at this s i te . Pressure- and f low-volume loops can a lso be generated from

pressures sensed at this s ite. While p lacement between the pat ient and the

breathing sys tem is best f rom this standpoin t, in practice i t may present problems

wi th dead space, disconnect ions, tracheal tube kinking, and water bui ldup in the

pi lot l ine . The l ines must be connected for every case.

View Figure

Figure 23.21 Possible sites for monitoring airway pressure in the circle system. (See text for details.) PEEP, positive end-expiratory pressure; APL, adjustable pressure limiting.

The more distant the measurement s i te is from the patien t, the less usefu l it is as

an es timate of ai rway pressure (2,67). Breathing sys tem resistance and

compliance, leaks, obstructions , and other mechan ical factors may cause the

measured pressure to be qu ite different f rom the pressure in the patient 's ai rway

(56).

Frequently, the monitoring s ite is in the breathing system (posit ions A, B, and D).

An occlusion in the breathing system wil l cause a low-pressure state distal to the

obstruct ion and a high-pressure state proximal to i t, so certain types of problems

may be missed (57). I f PEEP is used, i t wi l l not be indicated on a pressure moni tor

located at posi tion B. Posi tions A and D are frequently used to moni tor pressure

during inspira tion and exhalat ion. These locat ions may be used to provide

pressures for p ressure-volume loops.

In the past, the sensor was sometimes located in the venti lato r (posi tion E). This is

unsatis fac tory because under certain c i rcumstances, suff ic ient back pressure to

inhibi t the minimum pressure alarm may be genera ted a t the bellows even when

there is a disconnec tion (55,56). Placing the sensing point in the vent i lator may

also resul t in fai lure to detect an incorrect ly set bag/vent i lator selector valve .

Spirometry Loops

A loop is a graphic representat ion of the dynamic relationship between two

variables (p ressure and volume or f low and volume) during both insp iration and

exhalation (6,23,68). The f low, volume, and pressure curves i l lustrated in Figure

23.1 are the bases of spi rometry loops.

Pressure- and f low-volume loops are available on certain physiologic moni tors as

an option. Later-generat ion anes thesia machines and most physiologic moni tors

now offer these loops , usually as an option. The authors bel ieve that the

info rmation provided just if ies the extra cos t.

Illustrative Loops The Pressure-volume Loop The pressure-volume (compliance) loop shows volume on the vertica l ax is and

ai rway pressure on the horizonta l ax is (F ig . 23.22). With controlled vent i lat ion, the

pressure in the breathing system increases during inspirat ion. At the same time, the

inspired volume of gas inc reases . The t idal volume is the poin t on the vertical ax is

that corresponds to the highest poin t on the loop. The peak pressure is the highest

value on the horizontal axis. The shape of the inspiratory phase is determined by

the type of respira tion being monitored.

View Figure

Figure 23.22 Pressure-volume loop. The pressure-volume relationship reflects pulmonary and tracheal tube mechanics. During controlled ventilation, a line drawn from the zero point through the point of end inspiration represents the compliance, which is determined by dividing the tidal volume by the pressure at end inspiration. With good compliance, that line forms an angle of 45 degrees or less with the volume scale. A loop that becomes more horizontal indicates a decrease in compliance.

A line drawn from the zero po in t th rough the po in t of end inspirat ion during

control led vent ilation (Fig. 23.22) represents the compl iance. With good

compliance, that l ine fo rms an angle of 45 degrees or less with the volume scale. A

loop that becomes more horizontal indica tes a decrease in compliance.

The port ion of the loop represent ing exhalation starts at the point of highest volume

and moves downward toward zero. The area ins ide the loop is related to the work of

breathing (1 ,69).

The Flow-volume Loop The f low-v olume (resistance) loop (Fig. 23.23) has volume on the horizontal axis

and f low on the vert ical ax is . The zero point for volume is to the righ t on the

horizontal axis, corresponding to func tional residual capacity. During inspiration ,

f low rate increases (plotted downward). The inspiratory f low drops to zero as

inspiration ends. The tidal volume is reached at the point where f low returns to zero

and the loop crosses the horizonta l ax is. The shape of this part of the loop depends

on the mechanism of respirat ion (e.g., vo lume controlled, pressure controlled,

manual , or spontaneous).

P.746

View Figure

Figure 23.23 Flow-volume loops with controlled ventilation.

Exhalation is represented by the part of the loop above the horizontal ax is. The

shape of this portion of the loop is determined by the rate of passive lung def lat ion,

which is in turn de termined by elastic recoil of the lung and chest wal l and by the

total flow resistance offered by the bronchial tree , ai rway device tube, expiratory

l imb of the breathing sys tem, and any addi tiona l equ ipment. With a normal loop, the

f low rate during exhalation increases rapidly at the beginning, quickly reaches a

peak, then slows and gradual ly returns to zero.

Figure 23.24 shows another way of i l lustrating a f low-volume loop that is used by

some manufacturers. The zero point is at the junction of the horizontal wi th the

vertical ax is . Inhalation is above the horizontal axis, and exha lat ion is below. Flow-

volume loops i l lustrated in this chapter employ the representation used by

pulmonologists . This loop may be presented in other ways, bu t the basic loop is the

same.

View Figure

Figure 23.24 Alternative method of displaying flow-volume loops. (See text for details.)

Representative Normal Loops Loops that are i l lustrated in this chapter do not come from actual patien ts bu t are

styl ized to i l lus trate certain aspects of respira tory mechanics. They are based on

ac tual loops as much as possible. The reader should not expect to see an exact

reproduc tion of these loops when moni toring a pat ient. Clinica l condit ions and

venti lato r function are rarely stra ightforward and usually inc lude a number of

factors . The loop represents a composi te of mechanical and phys io log ic factors. As

famil iari ty with the loops inc reases , the user can progress to the finer details of

inte rpretat ion .

Volume-controlled Ventilation with No Inspiratory Pause

Pressure-volume Loop The pressure-volume loop for volume-contro lled venti lation wi th no insp iratory

pause is shown in Figure 23.22. It begins at zero volume and near or at zero

pressure . During insp irat ion, both pressure and volume inc rease, so the loop moves

up and to the right. At the end of inspirat ion, both peak pressure and t ida l volume

are attained. The end of inspirat ion represents a fall in pressure that occurs before

exhalation can begin. The loop returns downward and toward the lef t to i ts original

s tarting posit ion.

Flow-volume Loop The f low-v olume loop seen wi th volume-control led vent i la tion wi th a constant f low

generator is shown in Figure 23.23. The flow quick ly r ises to a level tha t is

constant, producing a f la t inspira tory portion . At the end of insp iration, the f low

drops rapidly to zero. The tidal volume is reached as the loop crosses the volume

l ine.

As exhalation begins, there is a rapid ascent to a peak. Flow then dec reases , and

the loop falls smoothly toward zero f low and volume. The angle a t the top of the

loop is narrow.

Controlled Ventilation with Positive End-expiratory

Pressure

Pressure-volume Loop The addi tion of PEEP causes the starting po int of the pressure-volume loop to shif t

to the righ t by the amount of PEEP that is appl ied (F ig. 23.25). PEEP may cause an

increase in compliance and is represented by the loop becoming more vert ical wi th

a dec rease in peak pressure. If the loop tends more to the right, PEEP may not be

benef ic ial and may need to be wi thdrawn.

The loop disp lay makes i t easy to detect inadvertent PEEP. This may be due to the

PEEP valve having been inadvertently turned ON, a partial obstruc tion in the

breathing sys tem, a malfunc tioning exhalation un id irectional valve, fai lu re of the

posit ive-pressure re lief in the scavenging inte rface, or an incorrect ly set APL valve.

P.747

View Figure

Figure 23.25 With PEEP, the loop is shifted to the right. PEEP, positive end-expiratory pressure.

Flow-volume Loop The f low-v olume loop wi th PEEP during control led vent i la tion wi th a constant f low

venti lato r is shown in F igure 23.26. PEEP wil l decrease the exp iratory driv ing

pressure , producing lower f lows during exha la tion so that the loop appears f latter

(4).

Controlled Ventilation with an Inspiratory Pause

Pressure-volume Loop Figure 23.27 shows the pressure-volume loop during control led vent i lation with an

inspiratory pause. After the peak pressure is reached, the venti la to r pauses fo r a

short t ime wi th the lungs inf la ted. During this pause, the pressure in the breathing

system drops to a pla teau level , usually 2 to 5 cm H2O lower than peak pressure.

Fresh gas entering the breathing system from the anesthesia mach ine wi thout fresh

gas decoupl ing (Chapter 12) wi l l inc rease the inspired volume. During exhalat ion,

the pressure and volume wi l l drop in the expected manner.

Flow-volume Loop The f low-v olume loop seen wi th an insp iratory pause is shown in Figure 23.28.

There is a drop in f low near the end of inspira tion wi th a small increase in t ida l

volume during the pause resu lt ing from f resh gas f lowing into the breathing system

i f there is no fresh gas decoupl ing. This pattern should not be confused wi th a

spontaneous breath during control led vent i lat ion (Fig. 23.60). The increase in

volume due to fresh gas f low is s traight, f rom right to lef t , and s tays near the zero

f low l ine. Exhalation is s imi lar to the loop wi thout an inspiratory pause.

View Figure

Figure 23.26 PEEP produces a decrease in expiratory flow. PEEP, positive end-expiratory pressure.

P.748

View Figure

Figure 23.27 During an inspiratory pause, it is common for the airway pressure to decline 2 to 5 cm H2O. The lower pressure is called the plateau pressure. If the ventilator does not block fresh gas flow during inspiration (fresh gas decoupling) there will be an increase in tidal volume during the inspiratory pause.

Pressure-controlled Ventilation

Pressure-control led vent ilat ion differs f rom volume-control led respirat ion in that the

inspiratory f low is not constant. Pressure-control led vent i lation is d iscussed in

Chapter 12.

Pressure-volume Loop The pressure-volume loop is shown in Figure 23.29. The loop is wider than that

seen with volume-controlled venti lat ion and starts off wi th a greater pressure rise

than volume increase. As inspiration proceeds, volume rises faster than wi th

volume-control led vent i lat ion.

Flow-volume Loop The pressure-contro lled mode of vent i lat ion has an accelerating-decelera ting

inspiratory f low prof i le in contras t to the constant insp iratory f low seen wi th volume-

control led vent ilation (Fig. 23.30). The exhala tion part of the loop is s imi lar to that

wi th vo lume-control led vent i la tion.

View Figure

Figure 23.28 The blip near the end of inspiration represents the increase in tidal volume during the inspiratory pause due to fresh gas continuing to flow into the breathing system. This will not be seen if the ventilator has fresh gas decoupling.

P.749

View Figure

Figure 23.29 Pressure-volume loop with pressure-controlled ventilation. Pressure rises rapidly to the set pressure during inspiration.

Spontaneous Respiration without Positive End-expiratory

Pressure

Pressure-volume Loop With spontaneous respirat ion and no PEEP, the pressure-volume loop (Fig. 23 .31)

starts ou t at zero pressure and volume. During inspira tion ai rway, pressure is

negat ive, so the loop moves in a c lockwise di rection. At the end of inspirat ion, the

pressure returns to zero. At this point, the loop crosses the t idal volume point on

the ord inate . During exhalat ion, ai rway pressure becomes posit ive, and the loop

moves to the right. At the same t ime, the volume drops. At the end of exhala tion,

the pressure and volume return to zero. The shape of the loop is st il l double

convex, but i ts s lope is d if fe rent from tha t seen wi th contro lled venti lation.

Compl iance cannot be calcu lated f rom this loop because the insp iratory pressure is

negat ive. The area of the loop represents the work of breath ing.

Flow-volume Loop The f low-v olume loop wi th spontaneous resp iration is shown in Figure 23.32. The

f low rate during inspiration varies more than wi th mechan ica l venti lation. Peak f low

occurs near the middle of inspira tion. At the end of inspiration , f low becomes zero,

and the loop crosses the horizonta l line at a volume correspond ing to the t idal

volume. The f low during exhala tion is s imi lar to that found in other forms of

respira tion.

View Figure

Figure 23.30 Flow-volume loop with pressure-controlled ventilation. Flow is rapid at the beginning of inspiration, then decreases.

P.750

View Figure

Figure 23.31 With spontaneous ventilation, the shape of the loop is double convex, but the slope is different from that seen with controlled ventilation. PEEP, positive end-expiratory pressure.

Spontaneous Respiration with Positive End-expiratory

Pressure

Pressure-volume Loop

I f PEEP is appl ied during spontaneous venti la tion, the pressure-volume loop wi l l

s tart out a t the PEEP value and move to the lef t (F ig. 23.33). Inspira tion cannot

begin un ti l the pressure has become negative. At this point, the t idal volume

increases rap id ly. During exhalat ion, pressure increases rap id ly, and the loop

moves toward the right and down ward to the po in t of origin . The loop has a

rectangu lar shape. The larger internal area of the loop indicates the inc reased work

of b reathing .

Flow-volume Loop The corresponding f low-vo lume loop during spontaneous respiration with PEEP is

shown in Figure 23.34. Both the inspira tory and exhalation port ions of the loop are

f lat tened. The exhalation port ion is more rounded than when PEEP is not p resent.

This conf igurat ion is s imilar to the loop demons trat ing a f ixed inspiratory and

expiratory obstruction (Fig . 23.52).

Face Mask Positive-pressure Ventilation

Pressure-volume Loop The basic shape of the pressure-volume loop is s ti l l doub le convex (Fig. 23.35).

The insp iratory port ion of the loop is more rounded. If there is a signif icant leak

around the mask, an open loop may be seen.

Flow-volume Loop The f low-v olume loop wi th mask venti lation (Fig . 23.36) is more rounded during

both inspira tion and exhalat ion than that seen wi th intubat ion. Th is can vary with

the way in wh ich the anesthesia provider squeezes the bag.

Intermittent Mandatory Ventilation Inte rmittent mandatory vent i lat ion produces a combination of loops representing

both spontaneous and control led breaths .

View Figure

Figure 23.32 With spontaneous ventilation, the flow rate during inspiration varies more than with mechanical ventilation. Inspiration and exhalation tend to mirror each other. Tidal volume during spontaneous ventilation is usually lower than with controlled ventilation.

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View Figure

Figure 23.33 With spontaneous ventilation and positive end-expiratory pressure, the loop is shifted leftward and becomes rectangular. PEEP, positive end-expiratory pressure.

Pressure-volume Loop The pressure-volume loop (Fig. 23 .37) shows both a spontaneous breath (solid l ine)

and a control led breath (dashed line). Mos t moni to rs wi l l display these loops

consecutively and not on the same sc reen as il lustrated un less one of the loops has

been previously saved.

Flow-volume Loop The f low-v olume loop (Fig. 23 .38) shows a spontaneous breath (solid l ine) and a

control led breath (dashed line). Each has the characteris tics of the normal loop for

this type of respiration.

Patient-triggered Ventilation I f the venti lator is in a tr iggering mode (pressure support vent ilat ion), a

spontaneous breath wi l l be necessary to in itia te a pos it ive-pressure respirat ion.

The pressure-volume loop wil l s tart out negat ively, but as the vent i la tor is engaged,

the loop becomes rap idly posi tive fo r

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the duration of inspiration (Fig. 23.39). The exhala tion port ion of the loop wi l l be

s imilar to that found in controlled venti lation . The f low-vo lume loop associated wi th

pressure support vent i lat ion is shown in Figure 23.40.

View Figure

Figure 23.34 PEEP during spontaneous respiration results in lower flows during both inspiration and exhalation. PEEP, positive end-expiratory pressure.

View Figure

Figure 23.35 With mask ventilation, the pressure rises more slowly during inspiration. During expiration, the absence of the tracheal tube decreases resistance to flow and volume and pressure drops rapidly. The shape of the upstroke will vary.

Spontaneous-assisted Ventilation I f the spontaneous ly breathing pat ient is not p roducing a satis fac tory t idal volume,

respira tion is often manually assisted. Figure 23.41 shows one of the conf igurat ions

of the pressure-volume loop that may be seen. As the patien t init ia tes the

venti lation , there wi l l f i rs t be a negative pressure . As the bag is squeezed, p ressure

and t idal volume inc rease.

Loops Representative of Patient Factors Variations f rom a normal loop can be caused by anyth ing that affects the way gas

moves past the sensor during inspirat ion or exp iration . This may be a pat ient

factor, a c ircui t variable, o r a venti lator variable.

Changes in Compliance

Pressure-volume Loop A major advantage of p ressure-volume loops is the ir ab il i ty to detect changes in

compliance. If the lungs or chest wa ll become s ti ffer, increased pressure wil l be

necessary to del iver the same t ida l volume. This causes the pressure-volume loop

to be

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displaced c lockwise (Fig. 23 .42). I f PEEP is introduced, there may or may not be an

increase in compliance (Fig. 23 .43). I f PEEP does not increase compl iance or

makes the s ituat ion worse, this can be determined by subsequent loops .

View Figure

Figure 23.36 Mask ventilation. The inspiratory flow is more variable when ventilation is manually controlled than when a ventilator is used. During exhalation, the lower resistance due to the absence of a tracheal tube results in higher flow.

View Figure

Figure 23.37 Pressure-volume loop with intermittent mandatory ventilation.

Flow-volume Loop Decreases in compl iance wi l l affec t the f low-v olume loop (Fig. 23 .44). F low wi l l be

increased during exhalat ion, wi th a higher peak and a s teeper s lope.

Decreases in compl iance can result f rom inadequate muscle relaxation; a ir

embolism; d iseases and tumors that invade la rge areas of the lung or al ter i ts

dis tens ibi l i ty; narcotics ; bronchial in tubat ion ; bronchoconstric tion ; pneumothorax;

reduc tion pneumoplasty; lateral decubi tus, l i tho tomy, or Trendelenburg posit ions;

ex ternal pressure on the ches t or abdomen; abdomina l re tractors or packing;

abdominal enlargement; curvature of the spine; obesi ty; prone posi tion;

pressuriza tion in the peri toneal cavi ty during laparoscopic surgery; or adul t

respira tory distress syndrome (ARDS) (6,68,70,71,72,73,74,75,76,77,78,79,80).

Dec reases in compl iance can be found during part ial coronary bypass. The loop

returns to the prebypass s ta te af te r the bypass is discont inued. Compliance is

lower in chi ldren than in adul ts (Fig. 23 .45) (80).

Fac tors that increase compl iance include PEEP, emphysema, and resolution of the

factors that decrease compl iance. Since changes in compliance often occur

gradually, they may not be recognized unless the change is la rge . It is useful,

therefore, to store a loop f rom the beginning of a case for comparison.

View Figure

Figure 23.38 Flow-volume loop with intermittent mandatory ventilation.

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View Figure

Figure 23.39 Patient-triggered ventilation. As the patient takes a spontaneous breath, the loop becomes positive for the duration of inspiration.

Changes in Resistance An increase in res is tance may be caused by tracheal tube obs truct ion (kinking ,

dislodgment, or secre tions ), b ronchoconstric t ion, airway col lapse f rom loss of

elast ic recoil or by obstruc tion in a large airway caused by secretions, blood,

fore ign body, neoplasm, inf lammation, or using a trachea l tube that is too small .

Whi le mi ld bronchospasm causes on ly s l ight changes in the f low-volume loop, as it

increases there wi l l be changes in both the inspiratory and exhalat ion portions .

With severe expiratory res is tance, expira tory f low may stop abruptly before the next

mechanical inf lat ion . The effects of treatment for b ronchospasm can be assessed

by observ ing the loop after trea tment.

Pressure-volume Loop During contro lled venti lation , increased resistance means that higher inspira tory

pressures wil l be required to del iver a given f low. Tidal volume may be reduced. As

shown in Figure 23.46 (sol id l ine), the pressure-volume loop is shif ted to the right

and downward with a large internal area. The pressure falls rapid ly af ter inspira tion

is complete. The loop may be open if there is air trapping . With spontaneous

venti lation , the inspiratory l imb is displaced leftward (Fig. 23.47).

View Figure

Figure 23.40 Pressure support ventilation.

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View Figure

Figure 23.41 Spontaneous-assisted ventilation. As the patient begins to inspire, a negative pressure is seen. Then the bag is squeezed and the pressure becomes positive. The shape of the inspiratory portion will depend on how the bag is squeezed.

Flow-volume Loop I f res is tance is inc reased, the f low-v olume loop wi l l show decreased f low

throughout exhalation (4,21) (Fig . 23.48). As resistance increases fu rther (Fig.

23.49), there wil l be changes in bo th the inspiratory and exhalation port ions, and

the tidal volume may be dec reased. With severe exp iratory resistance, expiratory

f low may stop abruptly before the nex t mechanical inf lation.

Chronic Obstructive Lung Disease Emphysema is charac terized by a progressive loss of elastic t issue in the lung.

These pat ients have no problem with in flating the lungs but must work to exha le .

During mechan ical venti lat ion , pat ients with airf low obstruction may develop

inadvertent PEEP (auto-PEEP, occult or in trinsic PEEP, dynamic hyperinf lat ion, a ir

t rapping) if there is not enough t ime for complete

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exhalation (81,82,83,84,85). The exhaled vo lume wi l l be less than the inspired

volume.

View Figure

Figure 23.42 Low compliance causes the loop to be moved closer to the horizontal axis. High compliance causes the loop to move closer to the vertical axis. The dotted line shows normal compliance. The solid line shows decreased compliance.

View Figure

Figure 23.43 The dotted line represents decreased compliance. With the addition of PEEP, the loop is moved to the left, and the increased compliance results in a more normal-looking loop. If the loop does not improve with PEEP, the PEEP may not be beneficial and may need to be removed. PEEP, positive end-expiratory pressure.

Pressure-volume Loop The pressure-volume loop seen wi th this condit ion is shown in Figure 23.50. A t the

beginning of inspiration, the pressure rises s lowly.

During exha lat ion, the pressure drops wi th l i tt le change in volume unt i l the end of

exhalation . An open loop may be seen.

Flow-volume Loop The corresponding f low-vo lume loop is shown in Figure 23.51. During expiration ,

there is a severe reduc tion in f low. The loop may be open if the patien t does not

have suff icient time to exhale completely. In terrupted expiratory f low may sugges t

the presence of intr insic PEEP (auto-PEEP).

Pat ients with obstructive a irway disease may not complete a ful l exhalation prio r to

the start of the next inhala tion, result ing in pers istent posi t ive pressure. Th is wi l l be

indicated by the absence of a period of zero f low before the next inhalation (Fig .

23.63).

Airway Obstruction Flow-volume loops may be he lpful in identi fying ai rway obs tructions (86,87,88). The

inspiratory l imb of the loop

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is useful in diagnosing extra thorac ic ai rway obs truct ion, and the expiratory l imb is

sensi tive to intra thoracic obstruc tion (87,89). When the c ross-sectional area of the

airway is decreased to a c ri tical level , characteris tic patterns of f low occur with

spontaneous venti la tion. Typical ly, the flow ra te wi l l pla teau. The value of the flow

rate at this plateau wi l l depend on the cross-sectional a rea of the f low-l imi t ing

segment in the airway.

View Figure

Figure 23.44 The dotted line represents decreased compliance. Flow is greater at the beginning of exhalation due to the increased pressure.

View Figure

Figure 23.45 Pressure-volume loop in pediatric patient.

With a f ixed intrathorac ic or thorac ic obstruct ion and spontaneous venti la tion (F ig .

23.52), bo th the inspira tory and expiratory l imbs of the f low-volume curve are

f lat tened (86,87,90).

A variable extrathoracic obs truct ion (Fig. 23.53) wi l l af fect inspirat ion as the

negat ive pressure causes the obstruction to inc rease. During exhalation, posi t ive

pressure in the a irway wi l l keep the trachea open at the si te of the les ion, leaving

the expiratory curve unaffec ted (87).

A variable intra thoracic obstruc tion (F ig. 23.54) wi l l show a normal inspiratory

curve as the negat ive intrathorac ic pressure wi l l keep the ai rway open. During

expirat ion, the in trathoracic pressure becomes pos itive

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and thus decreases the ai rway diameter so that the exp iratory f low is reduced.

View Figure

Figure 23.46 With an increase in resistance, a higher pressure is needed to deliver the same volume (solid curve). Tidal volume may be reduced.

View Figure

Figure 23.47 Spontaneous respiration with increased resistance. The normal loop is shown with dotted lines. With increased resistance, greater pressure (more negative during inspiration, more positive during exhalation) will be needed to move the same volume of gas.

Restrictive Disease The increase in elastic recoil with restric tive defects increases the force driv ing

expiratory f low. Thus, the flow-volume loop usual ly shows a high expiratory f low

associated wi th a steep descending l imb (Fig. 23.55).

Secretions Secret ions in the tracheal tube wil l cause a sawtooth pattern in the pressure- and

f low-vo lume loops (83,91) (Figs. 23.56, 23.57).

Pediatric Patients Pediatric pa tients require a different scale for p ressure- and f low-v olume loops.

Smal l pa tients require re la tively high ai rway pressures because of the small

diameter of the trachea l tube. An example is found in Figure 23.45.

Spontaneous Breathing during Controlled Ventilation I t is possible for a nonparalyzed pat ient to b reathe spontaneously during controlled

venti lation . This can occur at any time during the resp iratory cycle. The

spontaneous breath usual ly has a lower inspiratory f low than the mechanical

breath.

View Figure

Figure 23.48 Flow-volume loop with increased resistance. The dotted line represents the curve with normal resistance. With increased resistance, there is diminished expiratory flow. The convex configuration of the expiratory limb reflects uneven lung emptying.

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View Figure

Figure 23.49 With a severe increase in resistance, the ventilator cannot fully compensate, and inspiratory flow will be diminished. Tidal volume may be decreased. Expiratory flow is also severely decreased. The dotted line represents the normal curve.

Pressure-volume Loop Figure 23.58 shows a pressure-volume loop with a spontaneous breath during

exhalation . As the spontaneous breath occurs , the pressure drops below the

expected level whi le the volume rises above the usual curve. As the spontaneous

breath is exhaled, the pressure increases brie fly and the volume drops rapidly. The

remainder of the loop follows the expec ted shape.

Figure 23.59 shows a series of pressure-volume loops wi th the sol id l ine (loop 1)

representing a normal loop produced wi th mechanical vent i lation. Loops 2 and 3

show the patient b reathing against the venti lato r. The loop moves toward the

negat ive s ide of the pressure axis and then changes to posi t ive. The pressures

generated are qui te h igh and the t idal volume is dec reased s ince the patient is

exhal ing during insp iration .

Flow-volume Loop Figure 23.60 dep icts a f low-volume loop wi th a spontaneous breath near the end of

inspiration . There is a sudden increase in inspira tory flow and volume. As the

breath is exhaled, the f low and volume move toward zero .

View Figure

Figure 23.50 With severe COPD, resistance during expiration is greatly increased. The patient may have difficulty exhaling completely before the next inspiration, producing an open loop. COPD, chronic obstructive pulmonary disease.

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View Figure

Figure 23.51 With severe COPD, expiratory flow is severely reduced. COPD, chronic obstructive pulmonary disease.

View Figure

Figure 23.52 Flow-volume loop with fixed intra- or extrathoracic obstruction.

View Figure

Figure 23.53 A variable obstruction located outside the thorax will cause a plateau during inspiration. The expiratory portion of the curve is close to normal. The dotted line shows a normal loop.

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View Figure

Figure 23.54 With a variable intrathoracic obstruction (such as a tumor in the trachea or a mediastinal mass), inspiratory flow may be relatively normal, but during expiration, flow rises to a plateau instead of the usual rise to and descent from peak flow. The dotted line shows a normal loop.

View Figure

Figure 23.55 With a restrictive defect, the increase in elastic recoil is associated with higher expiratory flows. As the process becomes more severe and lung volumes are decreased, the flow-volume curve becomes tall and narrow. The dotted line shows a normal loop.

View Figure

Figure 23.56 Pressure-volume loop with secretions in the tracheal tube.

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View Figure

Figure 23.57 Flow-volume loop with secretions in the tracheal tube.

Open Loop A loop should return to i ts s ta rt ing point at the end of the resp iratory cycle. An open

loop (F ig . 23.61) has a gap between the end and s tart ing points , indicat ing that the

exhaled volume is less than the inhaled volume. Whi le the pressure-volume loop

appears to c lose, i t actual ly returns to zero pressure along the vert ical ax is rather

than at the starting point. The amount of gas loss can be read between zero and

the return point of the loop.

An open loop means that more gas has passed the sensor during inspirat ion than

returns during exhalation . Most often, this is because of a leak distal to the sensor.

Incorrect ca libration should also be considered. An open loop is of ten seen wi th

mask anesthesia , an uncuffed tracheal tube, or a supraglottic ai rway device. A

double-lumen tube may impose suff ic ient res istance to resul t in the lungs not

comple tely emptying, even in patients wi th normal ai rways (22 ,92).

Leaks of ten occur after lung reduc tion or o ther thoracic surgery (93). The extent of

the leak and changes in the volume of gas los t can be tracked wi th the f low-volume

loop.

An open loop may be due to incomple te exhalat ion caused by chronic obstruct ive

pulmonary disease (COPD), inc reased resistance caused by apparatus, a tension

pneumothorax, lung retrac tion, or a flap-valve obstruction in a large ai rway or a

double-lumen tube (94).

Overshoot Loop An overshoot loop ind ica tes that the exhaled volume is greater than the inspired

volume. This usually indicates

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that the moni to r needs to be recal ibra ted . It can also resul t if pressure on the

thorax causes some of the func tiona l residual volume to be added to the tida l

volume.

View Figure

Figure 23.58 This spontaneous breath occurs during expiration.

View Figure

Figure 23.59 Loop 1 represents the normal loop. Loop 2 shows a spontaneous breath during inspiration. The pressure drops, and the volume increases briefly. There is a decrease in compliance caused by an increase in tension in the chest wall muscles. In loop 3, the patient inhales at the beginning of the respiratory cycle, so the loop moves to the left of the vertical axis. There is a further decrease in compliance.

Pressure-volume Loop The pressure-volume overshoot loop (Fig . 23.62) can appear as a normal loop, but

the beginning poin t is to the right of the endpoint. The dif ference between the

beginning and endpoint is the amount o f excess gas that is exhaled.

Flow-volume Loop On the overshoot f low-vo lume loop (Fig . 23.63), the endpoin t is to the right of the

zero volume po int. The d is tance to the right represents the excess gas that has

been exha led.

Intrinsic Positive End-expiratory Pressure and Air Trapping Intr insic (au to, occu lt) PEEP (PEEPi) resul ts f rom a difference between the ac tual

expiratory t ime and the

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expiratory t ime required fo r complete exhala tion of the tida l volume so that some

air is trapped in the lungs (Fig. 23.63). It may be generated by a very short

expiratory t ime and/or s low exp iration due to high resistance or abnormal ly high

compliance. Air trapping is l ikely to occur in patien ts wi th ai rf low limitat ion, inverse

ratio venti la tion, or when us ing a h igh respiratory rate .

View Figure

Figure 23.60 The spontaneous breath occurs near the end of inspiration. The thin, dotted line represents the normal loop. Instead of returning to zero at the end of inspiration, the flow increases. There is a small increase in volume as well.

View Figure

Figure 23.61 There is a leak, so exhaled volume is approximately 150 mL less than the inhaled volume. This produces open pressure-volume and flow-volume loops.

Loops Representing Equipment Problems Tubing Misconnection Some older sensors had a long and a short nipple des igned to connec t to the short

and long lumens of the pressure tubing, respectively. It was possib le to connect the

tubings long to long and short to short. If this were done, the moni tor would sense

exhalation as inha la tion and v ice versa. The loops would be drawn backward and

upside down (F ig. 23.64). It would take two respira tory cycles to produce a full

loop, and the moni tor would be unable to compute the compliance. Extremely high

PEEP values would be recorded. Newer sensors wi th male and female connections

make tubing misconnec tions less l ikely (Fig. 23 .8).

Disconnection between the Sensor and the Breathing

System I f there is a d isconnection between the spirometry sensor and the breathing

system, there wi l l be no f low

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through the sensor during mechanical vent ilat ion and a loop wil l no t be generated

(Fig . 23.65).

View Figure

Figure 23.62 The exhaled volume exceeds the inhaled volume, producing an overshoot loop.

View Figure

Figure 23.63 Intrinsic positive end-expiratory pressure and air trapping. The gap in the flow-volume loop indicates that there was still expiratory flow when the next inspiration commenced. PEEP, positive end-expiratory pressure.

I f the patien t breathes spontaneously with a d isconnection between the sensor and

the breathing system, loops seen wi th spontaneous respira tion wi l l be generated. I f

a c i rc le breathing sys tem also conta ins a f low sensor mounted on the exhala tion

s ide near the absorber, this sensor wi l l not indicate f low during spontaneous

venti lation and wil l help to localize the disconnec tion.

Leak between the Sensor and the Breathing System

Pressure-volume Loop

I f there is a leak or partial d isconnection between the sensor and the breathing

system, some gas wil l be lost. The pressure-v olume loop (Fig. 23 .66) wi l l be normal

in shape but wi l l show a dec rease in t idal vo lume and a decrease in peak a irway

pressure . This differs f rom the leak associated with the open loop in that the

inhaled volume is dec reased as a resul t of the leak, but the amount of gas exha led

is equal to the amount of gas inhaled.

Flow-volume Loop The f low-v olume loop during a part ial disconnection (Fig. 23.66) wi l l show a

decreased t idal volume and a decrease in peak expira tory f low. The loop wil l no t be

open because both inspired and exhaled tidal volumes wi l l be the same.

View Figure

Figure 23.64 Misconnection of the tubings will cause the loop to be drawn backward and upside down.

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View Figure

Figure 23.65 A disconnection between the sensor and the breathing system will result in no flow through the sensor.

Disconnection between the Sensor and the Patient With a disconnection between the sensor and the patient, during controlled

venti lation there wil l be f low during inspiration but not exhalat ion (Fig. 23 .67). The

pressure-volume loop wi l l show a high tida l volume but almost no pressure. Since

there wi l l be no return gas f low, there wi l l be only half of a p ressure- or f low-v olume

loop.

Bronchial Intubation Bronch ia l in tubat ion can occur anytime a trachea l tube is in place. It is the most

f requently occurring problem leading to hypox ia (95). If this happens , the

compliance decreases and the pressure needed to deliver a set t idal volume rises

(21,96,97). Bronchial intubation is discussed in Chapter 19 . It is most l ikely to

occur wi th a change in the patient 's posi tion. If a sudden change occurs in the

pressure-volume loop at this t ime, bronchial intubation should be suspected.

Withdrawing the tracheal tube s l ight ly wi l l remedy the prob lem and wi l l be

demonstra ted wi th the nex t loop. Pressure-volume loops may be among the best

ways of detect ing this problem.

Pressure-volume Loop The pressure-volume loop wil l show a decrease in compl iance wi th a rightward and

downward shif t and a h igh peak pressure (Fig. 23.68, sol id l ine).

View Figure

Figure 23.66 With a leak between the sensor and the breathing system, the loop will have a normal shape but the tidal volume, peak pressure, and expiratory flows will be decreased. The dotted line represents the normal loop, and the solid line represents the loop with a leak.

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View Figure

Figure 23.67 A disconnection distal to the sensor will result in flow during inspiration but none during exhalation.

Flow-volume Loop The f low-v olume loop associa ted wi th bronchial intubat ion is shown in F igure 23.69

(sol id l ine). The increase in peak pressure wil l increase the peak f low during

exhalation .

Double-lumen Bronchial Tube Problem

Double-lumen bronchial tubes are of ten placed incorrec tly or may be displaced

during pat ient positioning or surgery (6,21,22,98,99,100). Continuous spirometric

moni toring can help to detect the problem. Basel ine flow-volume and pressure-

volume loops should be es tablished and recorded for each pat ien t whi le in the

supine posi t ion during two-lung vent i lat ion to al low comparison with la ter loops.

Pressure-volume Loop When one-lung venti lat ion is begun, the pressure-volume loop should show a s light

sh if t of the s lope to the right, ref lect ing decreased compl iance (98,99,100). With

surgical handling of the nondependent lung, compliance may decrease further

(100).

I f a doub le-lumen tube is placed too deeply, there wil l appear to be a dec rease in

compliance (Fig. 23.70). Tidal volume may be decreased. Incomplete lung emptying

wi l l result in an open loop (22).

The loops shown in Figure 23.71 resu lted f rom the t ip of the bronchial lumen

impinging on the wa ll of the bronchus. Loop 1 (solid l ine) is the normal loop. In loop

3 (dotted l ine), the tube t ip has impinged on the wal l of the bronchus . Pressure

rises rap idly wi th l i t tle increase in volume unt i l suff icien t pressure has been

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exerted to move the t ip away from the wal l . At this point, the volume rises rapidly.

There wi l l be an increase in peak pressure and/or a decrease in compliance. If

there is a bal l-valve action , during exhalat ion the pressure wil l dec rease rapidly

wi th l i t t le change in volume unt i l the pressure in the bronchial tube has fallen

suff ic iently to allow exhala tion.

View Figure

Figure 23.68 Bronchial intubation (solid loop) will result in a decrease in compliance.

View Figure

Figure 23.69 Bronchial intubation. The solid loop shows an increase in expiratory flow, especially during the early part of expiration.

View Figure

Figure 23.70 The dotted line represents the loop when the double-lumen tube is correctly positioned. If the bronchial lumen is inserted too deeply, a severe reduction in compliance and tidal volume will be seen.

View Figure

Figure 23.71 Impingement of the end of the tube on the bronchial wall has created a ball-valve obstruction. The pressure rises rapidly with little increase in volume until the pressure is sufficient to overcome the obstruction. The volume then increases, and the pressure drops. If the pressure drops low enough, there will again be obstruction to flow, creating another notch on the upswing of the loop.

Flow-volume Loop The f low-v olume loop during proper doub le-lumen tube placement wi l l show a

s light ly decreased expira tory f low ra te. If the tube is p laced too deeply into the

bronchus, the loop wi l l show dimin ished inspiratory and expiratory f lows . If there is

a ball -va lve obstruction to the t ip of the double-lumen tube, the f low-volume loop

wi l l be irregular (F ig. 23.72).

The f low-v olume loop that would be generated if there were a disconnect ion of one

l imb of a double-lumen tube or a leaking bronchial cuff is shown in Figure 23.73

(sol id l ine). The inspira tory portion is normal. Because much of the insp ired tida l

volume is lost through the leak, the loop is open. In addi t ion, there wi l l be

decreased f low during exhalation .

View Figure

Figure 23.72 The loop represents repeated ball-valve obstruction to flow during both inspiration and expiration.

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View Figure

Figure 23.73 With a leak in the bronchial cuff or a disconnection of one limb (solid loop), there will be an open loop, with the inhaled volume exceeding the exhaled volume. Exhaled flows will be decreased.

Esophageal Intubation

With esophageal intubat ion, the pressure-vo lume loop wi l l usually show a decrease

in compliance (Fig . 23.74, solid l ine), although compliance may be inc reased or

normal (21). Gas that enters the stomach and is not returned wi l l c reate an open

loop. The f low-volume loop wi l l be dis to rted and show small inspiratory and

expiratory volumes, often wi th an open loop.

Obstructed Tube A nearly completely obstructed tracheal tube or an incorrect ly p laced suprag lo tt ic

ai rway device wi l l resul t in a pressure-vo lume loop that shows a high pressure wi th

l i t tle or no t idal volume (Fig. 23 .75). This is usually a s ignal tha t the device should

be removed and reinserted.

Advantages of Loop Technology Pressure- and f low-volume loops prov ide the c l inic ian with real-time information .

There are many problems that can occur unexpectedly during a case (e.g., k inked

tracheal tube, disconnection , migrat ion of a tracheal tube into a bronchus). Without

loops, these problems might no t be recogn ized promptly and correct ive act ion may

be de layed. Whi le many of the problems that occur during respira tion could be

determined by watching

P.771

pressure , tidal volume, and f low moni to rs, loop technology offers a graphic

representation tha t integra tes that information , making changes more obv ious.

View Figure

Figure 23.74 The loops associated with esophageal intubation may vary greatly. Compliance may be increased, decreased, or normal.

View Figure

Figure 23.75 With a nearly obstructed tube, there will be a high pressure with little volume.

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P.773

Questions For the fol lowing quest ions, select the correct answer

1. Normal static compliance in an adult is

A. 15 mL/cm H2O

B. 35 to 100 mL/cm H2O

C. 20 to 35 mL/cm H2O

D. 35 to 50 mL/cm H2O

E. 75 to 125 mL/cm H2O

View Answer2. For greatest accuracy, the gas flow sensor should be placed

A. In the inspiratory l imb a t the absorber

B. Between the insp iratory l imb and the Y-piece

C. Between the Y-piece and the pat ient

D. Between the Y-piece and the expiratory l imb

E. On the absorber on the exha lat ion s ide

View Answer3. The preferred site for monitoring a irway pressure is

A. At the vent ilator

B. In the inspiratory l imb a t the canister

C. At the connec tion between the patient and the breathing system

D. On the expiratory l imb at the canister

E. At the bag mount connection

View AnswerFor the following quest ions, answer

• i f A, B, and C are correct

• i f A and C are correct

• i f B and D are correct

• is D is correct

• i f A, B, C, and D are correct.

4. What does the difference between the peak and plateau pressure measure? A. Plateau pressure

B. Total ai rway resistance

C. Compliance

D. Insp iratory f low

E. Resistance in the breathing system

View Answer5. Use of muscle relaxants

A. May cause the compliance of the lungs to increase

B. May cause the compliance of the ches t wal l to increase

C. May cause a decrease in res istance

D. May resu lt in a decrease in plateau pressure

View Answer6. A rise in peak airway pressure may occur as a result of A. An increase in t ida l volume

B. An increase in inspiratory f low rate

C. An increase in res istance

D. An increase in compliance

View Answer7. With inadvertent bronchial intubation, A. There wi l l be a decrease in compliance

B. The pressure-volume loop wil l be shif ted to the right and downward

C. The peak pressure wi l l be increased

D. The f low-volume loop wil l show an increase in peak expiratory f low

View Answer8. Increased resistance to breathing during controlled respiration can be overcome by A. Adding or increasing PEEP

B. Increasing the driv ing pressure

C. Increas ing the inspired oxygen

D. Decreasing inspira tory f low

View Answer9. Tota l res istance during inspiration and expiration is influenced by A. The pat ien t's ai rway

B. The exha lat ion portion of the breathing sys tem

C. The s ize and charac teris t ics of the tracheal tube

D. The amount of PEEP added to the breathing system

View Answer10. Means to detect a leak around a tracheal tube include A. Airway pressure moni tors

B. Respiratory breath sound monitoring

C. Respiratory volume measurement

D. Capnography

View Answer11. Respiratory volume monitoring may fa il to detect

A. Occlusion of the ai rway

B. A disconnec tion during spontaneous breathing

C. Esophageal intubation

D. Apnea

View Answer12. Which factors are components of total compliance? A. Elas tic properties of the lungs

B. Elas tic properties of the thorax

C. Elastic properties of the abdomen

D. Elastic properties of the breathing sys tem

View Answer13. Conditions that can be detected by using a minimum pressure alarm include

A. An unconnected venti lato r

B. A major leak in the breathing system

C. A malfunct ioning scavenging system

D. An increase in res istance

View Answer14. The minimum airway pressure alarm should be set A. At the lowes t setting

B. At dif ferent sett ings during a case

C. Greater than 6 cm H2O

D. Slight ly less than the peak pressure during inspirat ion

View Answer15. Conditions that may prevent activation of the minimum pressure alarm with a disconnection include

A. Obstruct ion of a breathing sys tem connec tor

B. Part ia l extubation

C. High resistance of components of the breathing system

D. Air en trained in to the breathing system

View Answer16. A sustained pressure may be caused by A. Occlusion of the scaveng ing system

B. Ac tivat ion of the oxygen f lush valve

C. Improper adjustment of the APL valve

D. Occlusion of the inspira tory l imb of the breathing system

View Answer17. An excessively high pressure in the breathing system may be caused by A. The pat ien t coughing or straining

B. Use o f the oxygen f lush during the inspiratory phase of the vent ilator cycle

C. A punctured venti lator be llows

D. Increased compl iance

View AnswerP.774

18. Subambient pressure in the breathing system may be caused by A. A nasogastric tube p laced in the trachea and attached to suc tion

B. A blocked expiratory l imb

C. Insp irat ion wi th an empty reservoir bag

D. Refi l l ing of a venti lato r with a standing bel lows wi th low f resh gas f lows

View Answer19. On a pressure-volume loop, A. The farthest poin t to the righ t on the horizontal axis represents the tidal volume

B. The slope of the inspira tory portion is determined by the resistance

C. The h ighest point on the curve represents the peak pressure

D. The curve can s lope to the right or lef t during inspirat ion

View Answer20. Concerning a flow-volume loop,

A. The portion below the horizonta l l ine represents insp irat ion using the

representation used by pulmonologists

B. The portion above the horizontal l ine represents the passive def lat ion as

determined by the e lastic recoi l of the lungs and chest wal l in the representa tion

used by pulmonologis ts

C. The t idal volume is that poin t where the loop crosses the horizontal l ine

D. This loop is known as a compl iance loop

View Answer21. If a double-lumen tube is placed too deeply, A. The pressure-volume loop wil l be shif ted to the right and downward

B. There wi l l be a decrease in compliance

C. An overshoot loop may resul t

D. Resis tance wi l l inc rease

View Answer22. Possible causes of inadvertent PEEP include

A. Part ia l obs truct ion of the breath ing system

B. Malfunct ioning scavenging device

C. Obs tructive ai rway d isease

D. Airf low obstruct ion

View Answer23. Possible causes of an open loop include A. Improper cal ibrat ion o f the monitor

B. An uncuffed tracheal tube

C. A leak distal to the sensor

D. Tension pneumothorax

View Answer24. Decreases in compliance may be caused by

A. Inadequate muscle relaxat ion

B. Bronchial intubat ion

C. Obesi ty

D. Reverse Trendelenburg posi tion

View Answer25. With restrictive lung disease,

A. Peak exp iratory f low wi l l be increased

B. The f low-volume loop may become tall and narrow

C. The f low-volume loop may have a rela tively normal shape but may appear

smal ler in all dimensions

D. Resis tance is inc reased

View Answer