Chapter 12. Anesthesia Ventilators

8/11/2019 Chapter 12. Anesthesia Ventilators

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Chapter 12

Anesthesia VentilatorsA ven ti la tor (brea thi ng mac hine ) is an auto ma tic dev ic e des ign ed to prov ide oraugment patient venti lat ion. Newer anesthesia venti lators are an integral part of the

anesthesia workstat ion. They are designed with more features and v enti latory

modes than earl ier models and have the abil i ty to venti late more dif f icult pat ients

and to allow ventilation to be tailored to the patient's needs.

Tradit ional anesthesia venti lators could not provide as high inspiratory pressures o r

f lows as their i ntensive care unit (ICU) counterparts (1,2,3,4). As a result , some

ICU venti lators needed to be adapted for use during surgery in order to care for

patients who were dif f icult to venti late. I f posit ive end-expiratory pressure (PEEP)

were needed, often the anesthesia provider had to add a PEEP valve to the

anesthesia breathing system. Some of these valv es were imprecise, not v ariable,

and could be misconnected (Chapter 7). On some older ventilators, the user had to

manually enable the low pressure alarm when the ventilator was turned ON. Also, it

may have been necessary to close the adjus table pressure l imit ing (APL) valve

and/or turn the bag/ventilator switch when turning on the ventilator. Another

drawback of older venti lators was that separate models or dif ferent bellows

assemblies were required for adult and pediatric patients. The delivered t idal

volume was affected by fresh gas f low and breathing system compliance. Finally,

older venti lators offered only volume control v enti lat ion.

The demand for performance equivalent to ICU venti lators has led to a number of

improvements in anesthesia venti lators. High inspiratory pressures and f lows can

be delivered. Newer anesthesia venti lators have an integral PEEP valve, and many

have several venti latory modes. Another improvement is improved f lexibil i ty so that

the venti lator can deliver volumes for a wide range of patients from the smallest

child to the largest adult . The new venti lators are designed to overcome the effects

of f resh gas, breathing system compliance and gas compression on t idal volume.

Turning the venti lator ON involves fewer steps and automatically enables the low

airway pressure alarm.

Venti lators used in anesthesia are c overed by international and U.S. s tandards

(5,6 ,7).

This chapter wil l cover a number of venti lators available at the t ime of this writ ing.

I t is impossible to provide all of the details that need to be mastered to safely use a

part icular venti lator. Software updates and upgrades occur frequently. I t is


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important that the user manual be studied before using a venti lator that is

unfamiliar to the anesthesia provider.

Definitions

Barotrauma : Injury result ing from high airway pressure.

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Compliance: Ratio of a change in volume to a change in pressure. I t is a

measure of distensibil i ty and is usually expressed in mil l i l i ters per centimeter

of water (L or mL/cm H 2O). Most commonly, compliance is used in reference

to the lungs and chest wall. Breathing system components, especially

breathing tubes and the reservoir bag, also have compliance.

Continuous Posit ive Airway Pressure (CPAP): Airway pressure maintained

above ambient. This term is commonly used in reference to spontaneous

venti lat ion.

Exhaust Valve : Valve in a venti lator with a bellows that when open allowsdriving gas to exit the bellows housing.

Expiratory Flow Time : Time between the beginning and end of expiratory

flow.

Expiratory Pause Time : Time from the end of expiratory f low to the start of

inspiratory flow.

Expiratory Phase Time : Time between the start of expiratory f low and the

start of inspiratory f low. I t is the sum of the expiratory f low and expiratory

pause times.

Fresh Gas Compensation : A means to prevent the fresh gas f low from

affect ing the t idal volume by measuring the actual t idal v olume and using this

information to change the volume of gas delivered by the v enti lator.

Fresh Gas Decoupling: A means to prevent the fresh gas f low from affect ing

the t idal volume by isolat ing the fresh g as f low so that it doesn't enter the

breathing system during inspiration.

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

flow.


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Inspiratory Pause Time : That portion of the inspiratory phase time during

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

during which the inspiratory phase has zero f low). I t i s also c alled the

inspiratory hold, inf lat ion hold, and inspiratory plateau . The inspiratory pause

time may be expressed as a percentage of the inspiratory phase t ime.

Inspiratory Phase Time : Time between the start of inspiratory flow and the

beginning of expiratory f low. I t is the sum of the inspiratory f low and

inspiratory pause times.

Inspiratory: Expiratory Phase Time Ratio (I:E ratio): Ratio of the inspiratory

phase time to the expiratory phase time.

Inspiratory Flow Rate: Rate at which gas f lows to the patient expressed as

volume per unit of t ime.

Inverse Ratio Venti lat ion : Venti lat ion in which the inspiratory phase t ime is

longer than the expiratory phase time.

Minute Volume : Sum of all t idal volumes within one minute.

Peak Pressure : Maximum pressure during the inspiratory phase t ime.

Plateau Pressure : Resting pressure during the inspiratory pause. Airway

pressure usually fal ls when there is an inspiratory pause. This lower

pressure is called the pl atea u pres sure .

Posit ive End-expiratory Pressure (PEEP): Airway pressure above ambient at

the end of exhalat ion. This term is commonly used in reference to controlled

venti lat ion.

Resistance: Ratio of the change in driving pressure to the change in f low

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

(cm H2O/L/second).

Sigh : Deliberate increase in t idal v olume for one or more breaths.

Solenoid: A component that controls pneumatic f low by means of an

electronic signal. Spil l Valve : The valve in an anesthesia venti lator that allows excess gases i n

the breathing system to be sent to the scavenging system after the bellows

or piston has become fully f i l l ed during exhalat ion.

Tidal Volume : Volume of gas entering or leaving the p atient during the

inspiratory or expiratory phase t ime.

Venti latory (Respiratory) Rate or Frequency: Number of respiratory cycles

per minute.


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Volutrauma : Injury due to overdistention of the lungs.

Work of Breathing: Energy expended by the patient and/or ventilator to move

gas in and out of the lungs. I t is expressed as the rat io of work to volume

moved, commonly as joules per liter. It includes the work needed to

overcome the elast ic and f low-resist ive forces of the both the respiratory

system and apparatus.

Relationship of the Ventilator to the Breathing System

A ven ti la tor repl ac es the res erv oir bag in the bre ath ing sys tem. I t ma y be

connected to the breathing system by a bag/venti lator selector valve (Chapter 9).

On some newer workstat ions, turning the bag/venti lator selector switch to theventi lator posit ion or a mode select ion switch turns ON the venti lator. On other

venti lators, there is an ON-OFF switch.

Most anesthesia venti lators have a bellows in a box (bag in a bott le, double circuit)

design (Fig. 12.1). The bellows is housed in a pressure chamber, and the inside of

the bellows is co nnected to the breathing system. The bellows acts as an interface

between the breathing system and the venti lator driving gas, just as the reservoir

bag acts as an interface between the breathing system and the anesthesia

provider's hand. I t separates

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breathing system gas from driving gas. The pressure of the anesthesia provider's

hand is replaced by the driving gas pressure that compresses the bellows.


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View Figure

Figure 12.1.Functioning of the bellows-in-box ventilator.A:Beginning of inspiration. Driving gas begins to bedelivered into the space between the bellows and itshousing. The exhaust valve (which connects the driving gas

pathway with atmosphere) is closed. The spill valve (whichvents excess breathing system gases to the scavenging

system) is also closed. B:Middle of inspiration. As drivinggas continues to flow into the space around the bellows, its

pressure increases, exerting a force that causes the bellowsto be compressed. This pushes the gas inside the bellows

toward the breathing system. The exhaust and spill valvesremain closed. If the pressure of the driving gas exceeds the

opening pressure of the safety relief valve, the valve willopen and vent driving gas to atmosphere. C:End of

inspiration. The bellows is fully compressed. The exhaustand spill valves remain closed. D:Beginning of expiration.

Breathing system (exhaled and fresh) gases flow into thebellows, which begins to expand. The expanding bellowsdisplaces driving gas from the interior of the housing. Theexhaust valve opens, and driving gas flows through it toatmosphere. The spill valve remains closed. E:Middle ofexpiration. The bellows is nearly fully expanded. Drivinggas continues to flow to atmosphere. The spill valveremains closed. F:End of expiration. Continued flow of gasinto the bellows after it is fully expanded creates a positive

pressure that causes the spill valve at the base of the bellowsto open. Breathing system gases are vented through the spillvalve into the scavenging system.

During inspirat ion, driving gas is delivered into the space between the bellows and

its housing. This causes the bellows to be compressed so that gas f lows into the

breathing system. At the same t ime, the spil l v alve (which vents excess gases to

the scavenging system) and exhaust valve (which vents driving gas) are closed.

During exhalat ion, the bellows re-expands as breathing system gases and fresh gas

flow into it . Driving gas is v ented to atmosphere through the exhaust valve. After

the bellows is ful ly expanded, excess gas from the breathing system is vented to

the scavenging system through the spil l valve.

Instead of a bellows in a box, some venti lators have an electrically driven piston.

By eliminating the need for a drive gas circuit (an addit ional source of compressible

volume), a stable f low delivery can be provided. In piston venti lator systems that

are presently available, the reservoir bag is not isolated from the breathing system


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during the exhalat ion phase of automatic venti lat ion and acts to modulate pressure

increases in the system. During inspirat ion, when the piston forces gases into the

breathing system, the bag is isolated from the breathing system and collects the

fresh gas f low entering the breathing system. On some venti lators, the bag can be

seen to expand and contract with respirat ion even though the piston is ac tually

venti lat ing the patient. A problem with piston venti lators may be air entrainment

with a disconnection

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(8,9). In this case, the machine may not alarm and the patient wil l c ontinued to be

venti lated, but air wil l be entrained, result ing in lower concentrat ions of oxygen and

anesthetic agents.

Factors That Affect the Delivered Tidal Volume

F r e sh Gas F low

With older v enti lators, the delivered t idal and minute volumes changed when the

fresh gas f low, I :E rat io, or respiratory rate was altered despite the bellows

excursion remaining unchanged. I f the fresh gas f low increased, the t idal and

minute volumes increased (10 ,11 ,12,13 ). I f the fresh gas f low decreased, the t idal

and minute volumes decreased. Since fresh gas was added to the inspired t idal

volume only during inspirat ion, venti lator sett ings that prolonged the inspiratory

time (and thereby increased the I:E ratio) would cause an increased tidal volume.

Lower I :E rat ios decrease the t idal v olume. As respiratory rate i ncreased, the

increase in t idal v olume from fresh gas f low was less, a lthough the effect on minute

volume remained the same. Slowing the respiratory rate had the opposite effect.

Manufacturers have re-engineered their venti lators to eliminate the fresh gas effect

on the inspired volume. One method is to measure the inspired fresh gas f low and

compensate for it by altering the bellows excursion (fresh gas compensation).Another metho d is to pre ven t the f resh gas f rom en te ring the brea thing sys tem

during inspirat ion by using a valve that diverts the fresh gas into a reservoir bag

during inspirat ion (fresh gas decoupling).

Com p l i an c e an d Com p r e s s i o n Vo l ume s

Decreases in compliance in the breathing system can be accompanied by

decreases in t idal volume as more of the inspiratory f low is expended by expanding

the components. Gas compression losses depend on the volume of the breathing


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system and the pressure d uring inspirat ion. Advanced t echnology now all ows the

venti lator to compensate for changes in breathing system compliance by altering

the volume delivered. Breathing system compliance is determined during the

checkout procedure before use. For accurate compliance compensation, the

breathing system must be in the configurat ion that is to be used when the checkout

procedure is performed. Changes in the circuit configuration (such as lengthening

the breathing tubes or adding components) wil l c ause the compensation to be

inaccurate (14).

Other venti lators measure inspired volumes at the patient connection and adjust the

venti lator excursions accordingly.

Lea k sA le ak aro und the trac hea l tub e or sup rag lot ti c dev ic e wi l l cau se a decrease in tida l

volume that is not taken into account by the ventilator. Sidestream gas monitors

may decrease the volume delivered to the patient.

Components

D r iv i n g Gas Sup p l y

Most currently available anesthesia v enti lators are pneumatically p owered but

electrically controlled. The driving (drive, power) gas is either oxygen, air, or a

mixture of air and oxygen. I t is usually less expensive to power the venti lator with

air. Some venti lators can switch between driving gases so that if there is a loss of

pressure in the primary driving gas supply, the other gas can be used.

Some venti lators use a device called an injector (Venturi mechanism) to increase

the driving gas f low. An injector is shown in Figure 12.2. As the gas flow meets a

restrict ion, its lateral pressure drops (Bernoull i principle). When the lateral

pressure drops below atmospheric, air wil l be entrained. The result is an increase

in the total gas f low leaving the injector, and a decreased consumption of driving

gas.

A sign if icant fl ow of gas is neces sary to driv e a bellows (15 ,16 ,17). The amount will

vary, depending on the venti lator and the sett ings. The use of a gas cylinder to

power a venti lator may quickly deplete the gas supply.

Con t r o l s

The venti lator controls regulate the f low, volume, t iming, and pressure of the

bellows compression or piston movement.


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A l a rm s

The venti lator and workstat ion standards (6,7) group alarms into three categories:

high, medium, and low priority, depending on whether the condition requires

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immediate ac t ion, prompt act ion, or operator awareness but not necessari ly act ion

(Chapter 26).

View Figure

Figure 12.2.Injector (Venturi). Gas flows through theconstricted area at a high velocity. The pressure around it

drops below atmospheric, and air is entrained. The net result

is an increase in total gas flow leaving the outlet of theinjector.

The venti lator standard (6) mandates an alarm that indicates that the pressure in

the breathing system has exceeded a set limit (high-pressure alarm). On modern

venti lators, this threshold is adjustable by the user, usually with a default around

50 cm H2O. There must be an alarm to indicate that the pressure in the breathing

system has not reached a minimum value within a certain time period (low airway

pressure alarm).

P r es s u r e -l im i t in g Me c h an i smA pressure -limiting mechan ism (p res sure -l im it in g val ve, ma x im um limi te d pressure

mechanism, driving gas pressure relief valve, pressure l imitat ion mechanism,

maximum working pressure control, pressure l imit controller, inspiratory pressure

limit, adjustable pressure relief valve, high pressure safety relief valve,

overpressure release) is designed to l imit the inspiratory pressure. The anesthesia

workstat ion standard (7) mandates that this be adjustable. An adjustable

mechanism carries the hazard of operator error. If set too low, insufficient pressure


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for venti lat ion may be generated; if set too high, excessive airway pressure may

occur. Setting the pressure limit 10 cm H 2O above the peak pressure achieved with

the desired t idal volume and f low rate wil l avoid most barotrauma (18 ).

Pressure-l imiting devices work in one of two ways. When the maximum pressure is

reached, one type holds the pressure at that level unti l the start of exhalat ion, at

which t ime the pressure decreases. The other type terminates inspirat ion when the

pressure l imit is reached so that the pressure drops immediately.

Be l l o w s A s s emb l y

Bellows

The bellows is an accordionlike device that is attached at either the top or bottom

of the bellows assembly. Latex-free bellows are available. There are two types of

bellows, distinguished by their motion during exhalation: ascending (standing,

upright, f loating) and descending (hanging, inverted). Ventilators with descending

bellows were common until the mid 1980s. After that, most new ventilators had

ascending bellows, but descending bellows are used by a number of more recent

venti lators.

With an ascending bellows (Figs. 12.36, 12.44), the bellows is attached at the base

of the assembly, and the bellows is compressed downward during inspirat ion.

During exhalat ion, the bellows expands upward. These venti lators impose a slight

resistance at the end of exhalat ion, at which t ime the pressure in the bellows rises

enough (2 to 4 cm H2O) to open the spil l valve. The t idal volume may be set direct ly

by adjust ing the inspiratory t ime and f low or b y a p late that l imits upward excursion

of the bellows. With a disconnection or leak in the breathing system, the bellows

wil l collapse to the bottom or fai l to expand fully. The venti lator may continue to

deliver small t idal volumes (19).

To deliver the entire t idal volume, the bellows mus t descend to the proper level or,

depending on the venti lator, be fully compressed at the end of the inspiratory

phase. I f the inspiratory f low is insuff icient to ful ly compress the bellows or achieve

the desired t idal volume, a l ower t idal volume wil l be delivered.

With a descending bellows (Fig. 12.50), the bellows is attached at its top and is

compressed upward during i nspirat ion. There is usually a weight in the dependent

port ion of the bellows that facil i tates downward re-expansion during exhalat ion. As

the weight descends, it can cause a small negative pressure in the bellows and

breathing system. With a leak or disconnection in the breathing system, the weight

in the bellows wil l cause the bellows to expand, and room air wil l enter the


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breathing system. All or part of the next inspirat ion wil l then be lost into the room.

Newer ventilators with hanging bellows employ sophisticated software to detect

disconnections or leaks (20 ,21). The software analyzes sensor outputs and tr iggers

appropriate alarms. A negative pressure relief valve prevents the patient from being

exposed to negative pressure.

Housing

The bellows is surrounded by a clear plast ic c ylinder (canister, bellows chamber or

cylinder, pressure dome) that allows the bellows movement to be observed. A scale

on the side of the housing provides a rough approximation of the t idal volume being

delivered. The housing for piston venti lators usually has a scale that can be

observed.

Exhau s t Va lv e

The exhaust valve (exhalat ion valve, v enti lator relief valve, compressed gas

exhaust, bellows control valve) communicates with the i nside of the bellows

housing on pneumatically powered v enti lators. I t is closed during inspirat ion.

During exhalat ion, it opens to allow driving gas inside the housing to be exhausted

to atmosphere. With a piston venti lator, there is no need for an exhaust valve.

Sp i l l Va l ve

Because the APL valve is isolated from the breathing system during venti lator

operation, a spil l valve (vent valve, dump valve, overf low valve, expired gas outlet,

expiratory valve or port, safety dump valve, pop-off valve, relief v alve, f lapper

valve, pressure relief valve, overspil l

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valve, gas evacuation outlet valve, exhaust gas valve, gas evacuation or evacuator

valve, expiratory pressure relief valve) is used to direct excess respired gases into

the scavenging system. This valve is c losed during inspirat ion. During exhalat ion, it

remains closed unti l the bellows or piston is ful ly expanded, then opens to vent

excess breathing system gases. The scavenging transfer tubing connects the

exhalat ion port of the spil l v alve to the scavenging system interface (Chapter 13).

With an ascending bellows, the spil l v alve has a minimum opening pressure of 2 to

4 cm H2O (22). This enables the bellows to f i l l during exhalat ion. This amount of

PEEP is applied to the breathing system. I t is not applied with a piston or a hanging

bellows venti lator.


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With a pis ton venti lator, excess gas is vented through a spil l valv e, which may not

in the v enti lator, or through an electronically controlled APL valve, which acts as a

spil l valve.

Ven t i l at o r Ho s e Con n e c t i o n

The venti lator standard (6) requires that the fitt ing on the tubing connecting the

venti lator to the breathing system be a s tandard 22-mm male conical f i t t ing. A f i l ter

may be used on the tubing to lessen transmission of pathogens and part icles. In

most newer venti lators, a separate hose is not present, and the connections

between the venti lator and the breathing system are internal. This reduces the

likelihood of misconnections, disconnections, or kinked hoses (23 ).

Pos i t i v e End - ex p i r a t o r y P r e s s u r e Va lv e

PEEP valves are discussed in Chapter 7. Modern venti lators have integral

electrically operated PEEP valves. Some venti lators apply PEEP to the entire

system, while others apply i t only to the expiratory hose (23).

A sta nd ing causes a small amo unt (2 to 4 cm H2O) of PEEP. There is no unset

PEEP with hanging bellows or piston-driven ventilators.

Ventilation Modes

Anesthes ia ven ti la to rs offe r one or mo re ven ti la ti on modes (18 ). Many offer dual

modes to gain the advantages of both. Venti lator sett ings must be c arefully

individualized in each mode to avoid hypoventi lat ion, hyperventi lat ion, volutrauma,

or barotrauma. I t is important when switching from one mode to another to ensure

that the t idal volume, peak pressure, and alarm sett ings are appropriate.

A ven ti la tor can del iver gas by genera ting f low or pressure. With f low genera tors ,

the f low pattern can be constant (square wave) or nonconstant (accelerat ive or

decelerat ive). Pressure generators produce a c onstant or nonconstant pressure.

Inspiratory f low rate varies according to the preset p ressure and the patient 's

resistance and compliance.

The characterist ics of inspirat ion and exhalat ion related t o the venti lator sett ings,

compliance, and resistance are ref lected in the pressure and f low-volume loops.

These are discussed in detail in Chapter 23.

Features of some commonly used v enti latory modes are shown in Table 12.1 . The

terminology used to describe the way a v enti lator operates has not been universally

agreed on, and some manufacturers have coined new terms for their venti lators.

Vo l ume Con t r o l


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The most commonly used mode in the operating room is v olume control (volume-

controlled or volume) venti lat ion, in which a preset t idal v olume is delivered. The

tidal or minute volume and respiratory rate are set by the anesthesia provider and

delivered by the venti lator, independent of p atient effort. I t is t ime init iated, volume

limited, and cycled by volume or t ime.

Flow rate is f ixed at a c onstant value during inspirat ion. I f the inspiratory f low is too

low to provide the set tidal volume, the bellows or piston will not complete its

excursion. I f the f low is set at a faster rate than is needed to provide the t idal

volume, there wil l be an

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inspiratory pause. An excessively high peak inspiratory pressure may result f rom

sett ing the inspiratory f low rate too high (24). The inspiratory phase may be

terminated before the t idal volume has been delive red if the peak ai rway pressure

reaches the set pressure l imit.

TABLE 12.1 Ventilatory Modes

Mode I nitiation Limit Cycle

Volume control ventilation Time Volume Volume/Time

Pressure control ventilation Time Pressure Time

Intermittent mandatoryventilation

Time Volume Volume/Time

Synchronized intermittentmandatory ventilation

Time/Pressure Volume Volume/Time

Pressure support ventilation Pressure/Flow Pressure Flow/Time

Typically, a volume control waveform shows steadily increasing pressure during

inspirat ion. Changes in c ompliance or resistance are ref lected in changes in peak

inspiratory pressure and the difference between peak and plateau pressure ( 25).

For a g iven set t idal volume, the pressure in the breathing system is determined by


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the resistance and compliance of the breathing system and the patient. Plateau

pressure is a ref lect ion of compliance. Peak pressure is also inf luenced by

resistance. The pressure-volume and f low-volume loops associated with volume

control venti lat ion are seen in Figures 23.22and 23.23.

Adding PEEP dec rea ses the t id al volu me del ivered, wi th th e eff ec t gre ate r wi th

small t idal volumes (26 ,27). On newer venti lators with integral PEEP, venti lat ion

may be better maintained (4).

I f closed system suctioning is performed during volume control v enti lat ion, there

wil l be a signif icant r ise in airway pressure when the ca theter is inserted and low

airway pressure during suctioning (28 ,29,30).

P r es s u r e Con t r o lPressure control (pressure-l imited, pressure-controlled, pressure-preset control,

lung protect ive, or p ressure) venti lat ion is available on many anesthesia venti lators

(2,31,32,33). With this mode, the operator sets the inspiratory pressure at a level

above PEEP. The venti lator quickly increases the pressure to the set l evel at the

start of inspirat ion and maintains this pressure unti l exhalat ion begins.

Inspiratory gas f low is highest at the beginning of inspirat ion, then decreases.

Increased resistance may change the shape of the flow-versus-time waveform to a

f latter, more square-shaped pattern as t idal volume delivery shif ts into the latter

part of the inspirat ion (25). This allows the venti lator to preserve t idal volume with

increased resistance until resistance becomes severe. The pressure-volume and

flow-volume loops show special characterist ics seen with pressure-controlled

venti lat ion (Figs. 23.29, 23.30).

When pressure control venti lat ion is used, t idal v olume is determined by the rise

t ime and set pressure. Tidal volume is not set or constant but f luctuates with

changes in resistance and compliance and with p atient-venti lator asynchrony (25).

If resistance increases or compliance decreases, the tidal volume will decrease. It

has been postulated that a decrease in t idal volume with pressure control

venti lat ion would detect a part ial ly occluded tracheal tube, but it was f ound that

t idal volume was not decreased unti l the occlusion was nearly complete (25 ).

Unlike most ICU v enti lators, an anesthesia venti lator in the pressure control mode

operates with a preset I :E rat io, so increasing the respiratory rate shortens

inspiratory t ime and lowers t idal volume (2). An increase in PEEP causes a

reduction in t idal volume. Tidal volume is not affected by fresh gas f low because

excess gas is v ented through the spil l v alve.


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On some venti lators, the inspiratory f low is adjustable (Fig. 12.48). There may also

be a sett ing that controls the inspiratory r ise t ime. For patients with good

compliance, inspiratory f lo w should be high to ensure that the inspiratory pressure

is rapidly attained. Limit ing the maximum inspiratory f low is useful to avoid

overshooting the target pressure, especially when compliance is low.

In patients with lung injury or during single-lung venti lat ion, pressure control

venti lat ion may improve oxygenation and produce greater t idal volumes than

volume control venti lat ion because of the decelerat ing f low pa ttern that delivers gas

to the alveoli early during inspirat ion (31). I t is often used with supraglott ic devices

and patients with narrow or part ial ly obstructed tracheal tubes to provide venti lat ion

at relat ively low pressures (34 ,35 ). I t may be useful i f there is an airway leak (e.g.,

uncuffed tube, supraglott ic airway device, bronchopleural f istula). However, i f there

is a large leak, the cycling pressure l imit may not be reached, causing a prolonged

inspirat ion (18 ).

During closed system suctioning (Chapter 3), pressure control venti lat ion results in

less int r insic PEEP during catheter insert ion and less subatmospheric pressure

during suctioning than during volume control venti lat ion (28 ,29 ).

In t e rm i t t e n t Manda t o r y

With intermittent mandatory venti lat ion (IMV), the venti lator delivers mechanical

(mandatory, automatic) breaths at a preset rate and permits s pontaneous,

unassisted breaths of a c ontrollable inspiratory gas mixture between mechanical

breaths. The venti lator has a secondary source of gas f low for spontaneous

breaths. This ut i l izes either continuous gas f low within the c ircuit or a demand

valve that opens to allow gas to f low from a reservoir. Continuous gas f low at a rate

greater than peak inspiratory f low involves no addit ional work of breathing but

requires a large volume of f resh gas. The demand valve system, although more

eff icient in f resh gas use , can impose signif icant work of breathing on the patient.

This mode is often used for weaning patients from mechanical venti lat ion. The IMV

rate is gradually

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reduced, allowing increased t ime for the patient 's spontaneous breaths.

Syn c h r o n i z ed In t e rm i t t en t Manda t o r y

Synchronized intermittent mandatory venti lat ion (SIMV) s ynchronizes venti lator-

delivered breaths with the patient's spontaneous breaths. If patient inspiratory


15/99

activity is detected, the venti lator synchronizes its mandatory breaths so that the

set respiratory frequency is achieved. Posit ive pressure (mandatory) breaths may

occur at irregular intervals.

The time between the end of each mandatory breath and the beginning of the next

is subdivided into a spontaneous breathing t ime and a tr igger t ime. During the

trigger t ime, the venti lator checks whether the airway pressure has dropped a

minimum amount below the pressure measured at the end of the expiratory phase.

If a drop is not sensed, the venti lator delivers a breath. The tr igger window may be

adjustable (Fig. 12.49).

A ma nd ato ry ti da l vol um e an d a minim um mechanical ven ti la ti on ra te must be

selected. This determines the minimum minute ventilation. When setting the

venti lator rate, the patient 's spontaneous rate must be considered. I f the SIMV rate

is set too high, the patient may become apneic. Sett ing an I:E rat io is not required

in SIMV. The I:E rat io wil l change as the patient 's respiratory rate and rhythm

changes.

SIMV is used to facil i tate emergence from anesthesia as the patient transit ions

from controlled to spontaneous venti lat ion. I t ensures a minimal amount of

venti lat ion while freeing the anesthesia provider from periodically venti lat ing the

patient by hand. It reduces the incidence of patient-ventilator disharmony where the

patient tr ies to f ight the venti lator and the need for sedation or narcosis for t he

patient to tolerate mechanical venti lat ion. During anesthesia, SIMV may be used to

provide backup mechanical ventilation for spontaneously breathing patients. SIMV

can be c ombined with pressure support venti lat ion (PSV).

Manda t o r y M i n u t e

Mandatory minute ventilation (MMV) is a method of mechanical ventilation in which

the amount of venti latory support is automatically adjusted to f luctuations in

spontaneous venti lat ion so that a preset minute venti lat ion is delivered. The

venti lator circuitry monitors spontaneous expired volume and, if i t fal ls below a

predetermined level, provides the dif ference between the selected and actual

minute volume.

P r es s u r e Sup p o r t

PSV (pressure-assisted or assisted spontaneous venti lat ion) has been a feature of

ICU venti lators for years and is now on many anesthesia venti lators (36 ,37,38,39).

I t is designed to augment the patient 's spontaneous breathing by applying posit ive

pressure to the airway in response to patient-init iated breaths. A disadvantage of


16/99

this mode of venti lat ion is that if the patient fai ls to make any respiratory effort, no

pressure-supported breaths wil l be init iated. To avoid this potential ly disastrous

situation, most venti lators have a backup or apneic SIMV rate in case that the

patient 's spontaneous respirat ion ceases (assist/control venti lat ion).

A suppo rted brea th ma y be pressure or f low ini t ia ted. Flo w t ri ggeri ng imp oses les s

inspiratory workload than pressure tr iggering and is used more frequently (40 ).

When the user-selected f low or s ub-baseline pressure caused by a spontaneous

breath is reached, f low from the venti lator begins and the set pressure is quickly

reached. The venti lator then modulates the f low to maintain that pressure. The f low

decreases unti l i t fal ls below a predetermined fract ion of the init ial rate (usually 5%

or 25%) or a f ixed f l ow (usually 5 L/minute) or after a specif ic durat ion as a backup

(41). At this p oint, f low is terminated and exhalat ion begins. Because the PSV level

is reached early in inspirat ion and is maintained throughout the inspiratory phase,

the pressure waveform has a square, flat-topped shape. PEEP may be added if

needed.

The anesthesia provider must set the tr igger sensit ivity and the inspiratory pressure

(usually from 5 to 10 cm H 2O). The tr iggering sensit ivity should be set so that it wil l

respond to inspiratory effort without auto-cycling in response to art ifactual changes

in airway pressures. The init ial inspiratory f low is usually nonadjustable but can be

changed on some venti lators by adjust ing the inspiratory r ise t ime (Fig. 12.48). The

optimal init ial inspiratory f low is highest in patients with low compliance, high

resistance, and most act ive v enti latory drive. On some venti lators, the tr igger

window can be changed (Fig. 12.49).

Tidal volume is de termined by the pressure support level, lung characterist ics, and

patient effort. The desired t idal volume should be calculated and the pressure

support level adjusted so that the desired volume is delivered. I f the exhaled

volume is inadequate, the inspiratory pressure should b e increased or inspiratory

rise t ime decreased (if adjustable). PEEP may cause an increase in t idal volume

(42). Very high inspiratory f low (due to a high set pressure) may dec rease t idal

volume by prematurely terminating inspirat ion (37). As the patient's effort

increases, the level of inspiratory pressure can be reduced. Undesired

hyperventi lat ion can be treated by adjust ing the tr igger sensit ivity, pressure level,

or trigger window or, if these seem adequate, additional sedation.

PSV can be used to reduce the patient 's work of spontaneous breathing (38 ,43 ,44).

In addit ion, it c an increase the functional residual capacity. I t may be useful for

preoxygenating obese patients by improving the eff iciency


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of spontaneous venti lat ion and during weaning from mechanical venti lat ion. I t can

be useful with a supraglott ic airway device to keep the airway pressure lower than

the supraglott ic device leak pressure (38,42,45). If there is a leak around the

device, PSV will be able to compensate for the leak to some extent, as the airway

pressure is maintained irrespective of the volume.

An advan tage of PSV is th e synchro ny between th e pat ient and the ven ti lator. The

patient controls rate, volume, and inspiratory t ime. This may increase patient

comfort. Breath stacking and f ight ing the ven ti lator are decreased. Even patients

who are init ial ly tachy-pneic may be s uccessfully managed in this mode, as the

pressure support can be set suff icient ly high to augment t idal v olume and hence

reduce the respiratory rate. Peak and mean airway pressures are lower than with

volume control venti lat ion, reducing the risk of barotrauma (42 ).

Too high an i nspiratory f low may cause patient discomfort (33). PSV wil l deliver a

variable minute volume in a patient with a changing respiratory drive. Inappropriate

venti lator tr iggering can occur with PSV (40 ,46,47 ). This may be caused by a leak

or a decrease in airway pressure caused by cardiac contract ions.

With closed system suctioning, PSV results in a lower airway pressure during

catheter insert ion and higher end-expiratory pressure during s uctioning than either

volume control or pressure control venti lat ion (28,48 ).

Specific Ventilators

Drage r A V2+

The AV2+ is the successor to the AVE and AV2 ven-t i lators.

Description

The AV2+ is shown in Figures 12.3and 12.4. I t has an ascending bellows. Ti dal

volume is adjusted by using the kn ob above the bellows assembly, which raises or

lowers a plate at the top of the bellows. A scale on the bellows housing provides a

rough indication of the tidal volume delivered.

Most venti lator controls are located across the top of the venti lator. To the left of

the t idal volume control and above the bellows is the inspiratory pressure l imit

control. On the left above the pressure limit and tidal volume controls is the

frequency control with a digital

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readout to the left. To the right of the frequency control are the control and display

for the I:E ratio. In order to set an inverse ratio, an extended range button below

the display and control must be depressed.

View Figure

Figure 12.3.Drager AV2+ ventilator. I:E,inspiratory:expiratory.

View Figure

Figure 12.4.Drager AV2+ ventilator. (Courtesy of Drager

Medical.)

To the right of the I:E rat io control are the inspiratory f low control and gauge. The

scale on the gauge is divided into low, medium, and high f low.

To the right is t he venti lator ON/OFF switch. A green l ight next to the switch

indicates that the ventilator is turned ON. The ventilator may be turned ON at this

switch or by turning the Manual/Automatic switch on the absorber to the automatic


19/99

posit ion. The venti lator control switch can only be turned ON with the

Manual/Automatic switch in the Automatic posit ion. I f the ON/OFF switch is turned

ON with the bag/vent selector switch in the bag posit ion, a fault l ight to the left of

the ON/OFF switch wil l b e i l luminated.

The spil l valve (Fig. 12.5) is at the base of the bellows assembly. A pilot line from

the canister connects through the top of the spil l valve to a balloon diaphragm.

Pressure in the canister causes the balloon diaphragm to be i nf lated, closing the

opening to the scavenging system. The ball in the spil l valve ensures that a certain

pressure must be present to allow gas f low through the spil l valve, even if the

balloon diaphragm is deflated. This results in approximately 2 cm H 2O PEEP in the

breathing system.

The internal construct ion is shown in Figures 12.6 to 12.9. The inspiratory pressure

regulator reduces the gas from approximately 50 psig to the value indicated on the

inspiratory f low gauge. A solenoid in the oxygen l ine l inks the pneumatic and

electronic port ions of the venti lator. When the solenoid is energized, it al lows gas

to f low through the control valve in the oxygen l ine to the Venturi mechanism.

A smal l-diameter tub e carr ies oxygen from th e ins pi rato ry f lo w regulator to the top

of the auto-ranging valve, which controls the amount of ambient air entrained at any

given inspiratory sett ing. The auto-ranging valve contains a diaphragm that is

depressed when pressure is applied. The plunger moves downward, controlling the

opening through which ambient air is entrained during inspirat ion.

View Figure

Figure 12.5.Spill valve on Drager AV2+ ventilator.(Courtesy of Drager Medical.)


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P.321

The control valve allows gas to f low through it when pressure is applied. The

Venturi receives oxygen from the control valve and air f rom the auto-ranging valve

and combines them to form the drive gas that pushes the bellows downward during

inspirat ion.

The pilot actuator, which controls the opening of the exhaust valve, operates in

response to oxygen pressure that enters at the top. When sufficient pressure is

applied, the valve moves downward against the spring, which closes the valve when

no pressure is applied. During inspiration, the pressure of the driving gas inside the

bellows housing pushes the bellows downward. When no pressure is applied to the

pilot actuator, the exhaust valve opens, and the driving gas f lows to atmosphere

through the exhaust valve.

Controls

The venti lator can deliver t idal v olumes f rom 20 to 1500 mL. Respiratory rate can

be set from 1 to 99 breaths per minute (bpm). Inspiratory f low can be set between

10 and 100 L/minute. The I:E rat io can be set from 1:4.5 to 4:1. The inspiratory

pressure l imit range is 15 to 120 cm H 2O.

Alarms


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The AV2+ alarms are associated with the anesthesia machine and are not part of

the ventilator. Alarm messages are displayed on the anesthesia machine monitoring

screen. Warnings are accompanied by a three-pulse pattern that is ini t ial ly

repeated ev ery few seconds in a series o f descending volume and then constantly

at ful l v olume unti l the alarm condit ion is resolved. Cautions are accompanied by a

three-pulse tone pattern that is repeated every 30 s econds. Advisories ut i l ize a

single tone or no sound, depending on the advisory. The highest priority currently

act ive alarm condit ion is annunciated. Audio signals for lower-priority alarm

condit ions are temporari ly suppressed to minimize confusion caused by

simultaneous alarms.

Ventilation ModesVolume control is the only v enti latory mode on this venti lator. The v enti lator is t ime

cycled and v olume preset.

I n s p i r a t i o n

During inspirat ion (Fig. 12.6), the controller energizes the solenoid and pressurizes

the control valve, causing it to open. This allows oxygen from the pressure

regulator to f low to the bellows assembly. A small port ion of the oxygen is diverted

to the pilot actuator. This causes the pilot actuator to move downward, sealing the

exhaust valve and preventing drive gas from escaping to atmosphere. A smallport ion of the regulated oxygen also f lows to the auto-ranging valve and opens i t in

proport ion to the sett ing on the inspiratory f low regulator.

Oxygen f lows through the Venturi, entraining room air. The drive gas, consist ing of

oxygen and entrained air, then pressurizes the space between the bellows and the

canister. This causes the bellows to b e compressed and gases inside the bellows

flow to the breathing system.

The spil l v alve prevents gases from entering the scavenging system during

inspirat ion. Drive gas f lows through the pilot l ine and inf lates a balloon diaphragm

that blocks the outlet between the inside of the bellows and the scavenging system.

This valve remains closed unti l the bellows has reached its l imit of expansion

during exhalation.

In s p i r a t o r y P a u s e

During the inspiratory pause (Fig. 12.7), the controller continues to energize the

solenoid. As long as oxygen flows to the bellows assembly, pressure on the pilot

actuator is maintained, and the exhaust valve remains closed. Since the bellows is


22/99

completely compressed, no addit ional gas can enter the bellows housing, and no

more air is entrained by the Venturi. Excess oxygen is vented to atmosphere

through the air entrainment port. The pilot line and the balloon diaphragm

P.322

in the sp il l valve remain pressurized, so gas f low to the scavenging system remains

blocked.

View Figure

Figure 12.6Drager AV2+ ventilator. Inspiration. I:E,inspiratory:expiratory. (Redrawn courtesy of DragerMedical.)

E x h a l a t i o n

During exhalat ion (Fig. 12.8), the controller de-energizes the solenoid, which s tops

the f low of ox ygen through it . The oxygen in the tubing between the solenoid and

the supply valve is vented to atmosphere via a small exhaust tube at the top of the

solenoid. Once this oxygen is v ented, the control valve closes and stops the f low of

oxygen to the Venturi. This also allows the pilot actuator to depressurize. With the

pilot actuator depressurized, the spring forces the plunger upward, opening the

exhaust port. Exhaled gases push the bellows up ward. Drive gas vents to

atmosphere through the exhaust port. As the pressure in the canister dec reases,

the pressure within the pilot l ine for the spil l v alve also decreases, and the balloon

diaphragm deflates. The ball check v alve below the balloon diaphragm presents

more resistance to the f low of ex haled gas than does the bellows, so exhaled gases


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P.324

continue to f i l l the bellows. Expiratory f low ends when the bellows reaches the plate

at the top.

View Figure

Figure 12.7.Drager AV2+ ventilator. Inspiratory pause.I : E, inspiratory : expiratory. (Redrawn courtesy of DragerMedical.)


24/99

View Figure

Figure 12.8.Drager AV2+ ventilator. Exhalation. I:E,inspiratory:expiratory. (Redrawn courtesy of DragerMedical.)

E x p i r a t o r y P a u s e

Af ter the bellows ha s reached maximum expans io n (Fig. 12.9), the expiratory pause

time begins. The solenoid and the supply valve remain closed. The pressure in the

pilot l ine to the spil l valve decreases to atmospheric, and the balloon diaphragm

deflates. When pressure from gas in the bellows exceeds the resistance created by

the weight of the ball in the spil l v alve, the ball is l i f ted, and gases can f low into the

scavenging sys tem.

Special Features

A saf ety re l ief val ve vents dri ve ga s to atm os ph ere if th e dri ve gas pressure

exceeds 120 cm H2O.

In the event of mains power failure, a fully charged battery wil l power the v enti lator

for approximately 20 minutes. There are yellow indicators to signify that there is

alternating current (AC) power failure and that the battery power is low. I f the

machine has switched to

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battery power, a three-pulse tone sounds every 30 seconds. A battery test button is

present on the machine. A green battery test indicator signifies that the battery

power is s at isfactory. Battery messages are displayed on the monitor screen.


25/99

View Figure

Figure 12.9.Drager AV2+ ventilator. End exhalation. I:E,inspiratory:expiratory. (Redrawn courtesy of DragerMedical.)

Hazards

The problems discussed below were reported with the predecessor AV-E venti lator.

Since the two venti lators are similar i n construct ion, there is a possibil i ty that

similar problems could occur with the AV2+ venti lator.

A cas e has bee n rep ort ed in wh ich the mu f fl er plac ed ov er the driv in g gas exhaus t

became saturated with water and obstructed the flow from the bellows chamber

(49). This resulted in high airway pressures as gas continued to f low into the

venti lator.

In another reported case, the control valve malfunctioned, result ing in continuous

driving gas f low to the bellows (50). High airway pressures resulted.

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Prolongation of the inspiratory phase owing to i nsuff icient parts lubricat ion has

been reported (51).

Ventilatory irregularit ies resulting from improper seating between the bellows and

its mount have been reported (52).

There has been a report of the spill valve becoming incompetent, resulting in

hypoventi lat ion (53 ). In other reported cases, the pilot l ine connecting the bellows


26/99

chamber to the spil l valve became kinked so that it was occluded (54 ,55). If the

pilot l ine becomes occluded during inspirat ion, hypoventi lat ion wil l result because

the spil l valve wil l be open. I f the occlusion occurs during exhalation, gas wil l be

unable to exit the circuit , and the pressure inside the ci rcuit wil l increase.

PEEP can result under certain circumstances. This was reported when some hoses

were draped over the spil l valve, part ial ly obstruct ing the pilot l ine (56 ,57).

Cleaning and Sterilization

Cleaning and disinfect ion or s teri l izat ion are c omplicated matters b eyond the scope

of this text. They are explained in detail in the operator's manual.

D rage r D i v an

The Divan venti lator (digital venti lator for anesthesia) is a component of the North

Ame ric an Dra ge r 6000 series ma chi nes .

Description

The venti lator control panel (Fig. 12.10) is situated at the front of the anesthesia

machine below the desktop. To alter a function, the key f or that function is pressed.

Changes are made by using the rotary knob at the right of the control panel and the

change confirmed by pressing the knob. I f the al tered value or mode is not

confirmed within 10 seconds, the venti lator returns to the previous value.

At the lef t s id e of th e con tro l panel is a Ma nu al / Sponta ne ou s key. Af ter pressin g

this key and confirming the sett ing, the patient can breathe spontaneously or be

venti lated manually by adjust ing the APL v alve.

Below the Manual/Spontaneous key is the Volume Mode key. When it is pressed

and its function confirmed, the venti lator goes into the volume control mode. Below

the volume mode key is the SIMV key. To the right of this key is the key for

pressure control venti lat ion (Pres Mode).

To the right of the Manual/Spontaneous key is a window with a bar graph that

indicates piston movement and displays the percent of the set t idal volume (0%

represents f ull exhalat ion, while 100% indicates inspirat ion to the set t idal volume).

Below the bar graph window is a numeric display window that displays the values

for the keys below it . At the left is the sett ing for maximum allowable pressure

(Pmax) in the volume and SIMV modes or preset airway pressure (Pset) in the

pressure control mode. In the middle is the sett ing for t idal volume in the SIMV and

volume control modes. At the right is the respiratory rate sett ing. To alter a

parameter, the key under it is pressed and the rotary knob rotated to increase or


27/99

decrease the sett ing. The new value is displayed in the window above the key.

When the proper

P.327

value is displayed, it is confirmed by pushing the rotary knob.

View Figure

Figure 12.10.Control panel of Divan ventilator. (Redrawncourtesy of Drager Medical.)

To the right of the numeric display is an alphanumeric display. This prompts the

operator to take certain act ions during the checkout procedure and updates the

progress of the checkout. After a change in v enti latory mode or parameter is made,

there wil l be a prompt to confirm the change. Other messages report ing status and

faults are displayed here.

Under the alphanumeric display are addit ional keys. The I:E key sets the I:E rat io.

The % I.P./Flow key sets the rat io of i nspiratory pause t ime to inspirat ion phase

time in the volume control and SIMV modes and the inspiratory flow rate in the

pressure control mode. The PEEP key is used to set the PEEP in all modes. The

SIMV Rate key sets the minimum ventilatory rate in the SIMV mode.

At the ri gh t s ide of the control pa ne l is a s ta ndby key. In th is mo de, driv e gas us e is

minimized, and inspection or repairs can be performed.

Above th e s tandb y key is a te st key. Th is causes the venti lato r to measure sys tem

compliance and leakage. This can only be init iated when the venti lator is in the

standby mode.

Below the tabletop is an electrically powered piston (Fig. 12.11). The manufacturer

offers an optional top cover with a transparent window that allows th e user to see


28/99

piston movement. I f there is inadequate gas in the breathing system to allow the

piston to retract ful ly, the piston wil l stop and alert the anesthesia provider. This

wil l prevent a negative pressure from being exerted (23).

A hea te r is incorpo rated in to th e absorb er he ad to min imize mo istu re condensati on.

The internal construct ion of the venti lator and breathing system is shown in Figures

12.12to 12.17. V1 is the fresh gas decoupling valve. V2 is the surplus gas valve.

V3 is the valv e that controls gas f low to the APL and gas relief valves. Gas is

aspirated from near the patient port, analyzed, and returned to the circuit

downstream of the expiratory unidirect ional v alve. An ultrasonic f low sensor

(Chapter 23) and PEEP valve are located in the expi ratory l imb.

ControlsThe venti lator cannot be set to values result ing in an inspiratory f low greater than

75 L/minute, a minute volume greater than 25 L/minute, or an expiratory t ime of

less than 400 ms.

PEEP can be set from 0 to 20 (default 0) cm H 2O. PEEP is not available in the

Man/Spont or SIMV modes.

The peak airway pressure (Pmax/Pset) can be set from 10 to 80 (default 25) cm

H2O in the volume control and SIMV modes and 10 to 70 (default 10) cm H 2O in the

pressure control mode. When the maximum allowable pressure is reached, f low is

adjusted so that the pressure remains constant through the end of inspirat ion. In

this situat ion, the full t i dal volume may not be delivered.

Figure 12.11.Piston ventilator of Divan ventilator.

(Courtesy of Drager Medical.)


29/99

View Figure

The minimum difference between Pmax and PEEP is 5 cm H 2O. If the pressure

increases by more than 5 cm H2O above Pmax, inspiration is immediately stopped

and expirat ion begins.

The % inspiratory pause/flow (% I.P./Flow) sets the length of the inspiratory pause

in the v olume control and SIMV modes or the inspiratory f low in the pressure

control mode. The pause range is 0% to 60% (default 10%). During pressure control

venti lat ion, the inspiratory f low rate can be set from 5 to 75 (default 50) L/minute.

The available t idal volumes are 10 to 19 mL, 20 to 100 mL, and 110 to 1400

(default 600) mL.

The respiratory rate range is 6 to 80 (default 12) bpm in the v olume and pressure

control modes and 3 to 8 0 (default 12) bpm in the SIMV mode.

The range of available I :E rat ios is 1:3 to 2:1 (default 1:2). I f an inverse I:E rat io is

set and co nfirmed, a message is displayed.


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View Figure

Figure 12.12.Divan ventilator. Spontaneous inspiration.APL, adjustable pressure limiting; PEEP, positive end-expiratory pressure. (Redrawn courtesy of Drager Medical.)

P.328

Alarms

Alarm limits , wh ich dep end on th e ven ti la to ry mode and the pati en t , are presen te din an alarm window on the machine monitor screen. If an alarm limit is exceeded or

not reached, the alarm limits menu is displayed, and the value is highlighted.

Related alarms are combined. A message that indicates the computer analysis of

the problem wil l appear.

The alarm silence key i n the alarm window allows the alarms to be audio paused

(silenced) for 60 seconds if pressed once and for 120 seconds if pressed twice.

Alarms can be sus pended by press ing a key on the bo ttom of the scree n.

Warnings are announced by a three-tone sequence of h igh, high, low. The tones

are also in a sequence of dif ferent volumes with the f irst and fourth sequence being

at ful l v olume. Alarm messages are displayed on a f lashing red background with

white text. The f lashing stops when the alarm silence button is depressed. Flashing

resumes when the audio pause (si lence) period has ended.


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View Figure

Figure 12.13.Divan ventilator. Spontaneous exhalation.APL, adjustable pressure limiting; PEEP, positive end-expiratory pressure. (Redrawn courtesy of Drager Medical.)

P.329

Cautions are displayed on a f lashing yellow background in black text. The

messages are announced by a three-tone burst of low, low, high. Announcements

occur every 30 seconds.

Adv is ori es are displ aye d on a wh ite backgro und wi th blac k tex t tha t does not f la sh.

A sing le ton e ma y sou nd.

An al arm log can be ac ces sed. It wi l l al lo w the cl in ic ian to observ e all a la rm ev en ts

that have occurred during the case. I t c an store up to 500 events.

Ventilation Modes

The desired mode (Volume or Pressure Control or SIMV) is selected by pushing the

key for that mode and confirming that choice.

In the pressure control mode, inspiratory f low rate can be set independent of airway

pressure (Pset). However, Pset may not be achieved if the inspiratory f low rate is

too low. In this case, an alarm message wil l be displayed.


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View Figure

Figure 12.14.Divan ventilator. Inspiration during manualventilation. APL, adjustable pressure limiting; PEEP,

positive end-expiratory pressure. (Redrawn courtesy ofDrager Medical.)

P.330

During SIMV, the time between each mandatory respiration and the beginning of

the next is subdivided into a spontaneous breathing t ime (Tspont) and a tr igger

t ime (Ttrigger). During the tr igger t ime, the system checks whether the airway

pressure has dropped at least 0.5 cm H2O below the pressure measured at the end

of expirat ion. I f this has not occurred, the venti lator delivers a breath.

S p o n t a n eo u s B r e at h i n g

To allow spontaneous breathing, the MANUAL/SPONTANEOUS key is pressed and

the APL valve set to SPONT. Valves V1 and V3 are open, while V2 is closed. When

the patient inspires (Fig. 12.12), the inspiratory valve opens, and gas f lows f rom

the reservoir bag. During exhalat ion (Fig. 12.13), the expiratory valve opens, and

exhaled gases pass through the absorber and into the bag. During late exhalat ion,

the pressure rises,

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and excess gas f lows through V3 and the gas relief valve to the scavenging system.


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View Figure

Figure 12.15.Divan ventilator. Inspiration duringmechanical ventilation. APL, adjustable pressure limiting;PEEP, positive end-expiratory pressure. (Redrawn courtesyof Drager Medical.)

Manu a l

For manual ventilation, the Manual/Spontaneous key is pressed, and the APL valve

is set to MAN. The pressure during inspirat ion wil l be l imited by the APL valve

sett ing. When the pressure l imit is reached, excess gas wil l f low through V3 and the

APL valv e to th e scav eng ing system through the gas rel ie f val ve (Fig. 12.14).During exhalation, exhaled gases flow through the absorber into the reservoir bag.

Mech a n i c a l

When mechanical venti lat ion is s elected, the APL valve is closed. The bag

functions as a reservoir for fresh gas.

During inspirat ion (Fig. 12.15), valves V1, V2, and V3 are closed. Piston movement

produces gas f low through the inspiratory valve to the patient port. Fresh gas

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continues to enter the reservoir bag but does not affect the t idal volume, because

valve V1 is closed (fresh gas decoupling).


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View Figure

Figure 12.16.Divan ventilator. Mid exhalation duringmechanical ventilation. APL, adjustable pressure limiting;PEEP, positive end-expiratory pressure. (Redrawn courtesyof Drager Medical.)

When exhalat ion begins, the expiratory valve opens, allowing exhaled gases to f low

through the absorber and into the retracting piston and to the reservoir bag through

V1, which opens. Valves V2 and V3 remain closed. Fresh gas f lowing in to the

system mixes with some of the exhaled gases in the piston v enti lator. Mid

exhalat ion is depicted in Figure 12.16. The piston retracts, allowing the cylinder to

fill with gas from the reservoir bag and fresh gas. During the later part of exhalation

(Fig. 12.17), V2 opens, and gases are vented to the scavenging system through the

gas relief (spil l) valve.

Special Features

The Divan venti lator decouples fresh gas f low from t idal volume. Fresh gas entering

the circuit during inspirat ion

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is isolated from the patient circuit and accumulates in the reservoir bag. I f the

oxygen f lush is act ivated during inspirat ion, the gas wil l not be added to the t idal

volume but wil l enter the reservoir bag (23). The reservoir bag will inflate and

deflate during mechanical v enti lat ion.


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View Figure

Figure 12.17.Divan ventilator. Late exhalation duringmechanical ventilation. APL, adjustable pressure limiting;PEEP, positive end-expiratory pressure. (Redrawn courtesyof Drager Medical.)

This venti lator compensates for breathing system compliance and gas c ompression

so that the patient receives the set t idal volume. Information that makes t idal

volume compensation possible is gathered during the automated checkout. The Y-

piece must be occluded and fresh gas f low s et at a minimum to perform these

measurements.

Small-diameter breathing tubes are recommended for pediatric patients where tidal

volumes are less than 200 mL. After switching to the pediatric tubes, the leak and

compliance test should be performed before the patient is connected to the

venti lator. I f a low-compliance circuit such as a pediatric circuit were added without

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conducting a compliance test, the v enti lator could deliver excessive volumes. To

prevent this from occurring when a t idal volume of less than 200 mL is selected, the

venti lator wil l use the measured circuit compliance only if i t is 0.8 mL/cm H2O or

less (58 ). I f the measured circuit compliance is higher, a default value of 0.6 mL/cm

H2O is used.

The breathing system and pis ton assembly are designed to minimize circuit v olume

and the t ime that it takes the system to respond to changes in fresh gas

composit ion. A low-f low wizard helps the clinician to assess the fresh gas surplus.

I t provides graphical information of fresh gas surplus, a message report, and a help


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key. The message area gives recommendations for use with low f lows, including

bag size and venti lator sett ings.

When the anesthesia machine is turned ON, an automated checkout process that

requires about 5 minutes is set in motion. Other than a few prompts reminding the

anesthesia provider to s et a p ressure at the APL valve and to occlude the Y-piece

on the breathing system, this checkout is ful ly automatic. The checkout allows the

computer to determine information about gas compression, l eaks, and compliance

of the breathing system.

If the v enti lator detects an internal fault that might affect patient safety during

mechanical venti lat ion, it init iates a safe s tate in which venti lat ion can be continued

in the Manual/Spontaneous mode. When the v enti lator enters the safe state, the

clinician is alerted by a display reading Equipment Fault , and an audible tone

sounds. The ventilator now performs as if it were in the manual/spontaneous mode.

The venti lator override button is on the machine near the absorber head. I t i s

provided in the event there is an unforeseen condit ion that the software does not

recognize. Activat ing this override removes power from the v enti lator and allows

manual or spontaneous v enti lat ion.

The Narkomed 6000 has battery backup that will power the machine and ventilator

for at least 30 minutes. An alarm indicates when the battery has only another 10

minutes. After the batteries are exhausted, the machine can continue to be used

with manual venti lat ion o r spontaneous breathing.

Respitone is an option on the 6400 anesthesia machine. I t is a venti lat ion sound

composed of two distinct tones. One tone annunciates when the pressure waveform

crosses the apnea threshold during inhalation. Another tone annunciates on the

rising edge of a carbon d ioxide waveform corresponding to exhalat ion.

Evaluation

A compari son between a Divan and an AV 2+ venti lato r was ma de duri ng s imul ated

venti lat ion of pediatric patients (59 ). The Divan offered advantages in the low t idal

volume range during v olume control venti lat ion.

The Divan and an ICU venti lator were compared by using both an infant lung model

and infants with congenital heart disease (60 ,61 ). Both v enti lators provided

adequate venti lat ion in the volume control mode.

In comparison with an ICU v enti lator and an anesthesia venti lator with a gas-

powered bellows during pressure control venti lat ion, the Divan maintained t idal


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volume with increasing respiratory rates better than the other anesthesia venti lator

but not as well as the ICU venti lator (2).

HazardsA high ne ga tive pressure applie d to the airway can ex cee d the abi l ity of the

venti lator's negative pressure relief valve, causing the piston to lock (62 ,63 ). The

problem can be remedied by opening the ventilator cover and removing the piston

to break the negative seal.

The venti lator override button is in a rather inconspicuous place (64).

A cas e of powe r supp ly failu re th at interr up te d ven ti lati on ha s been re po rte d

(64,65). The l inkage to the backup batteries prevented them from kicking in.

Cleaning and DisinfectionMost of the v enti lator parts, including those exposed to breathing gases, can be

steam autoclaved. See the operation manual for specific disassembly and

steri l izing instructions.

D rage r Fab i u s GS

Description

On the Fabius machine, the venti latory module, which includes a piston, is located

behind a door on the left side of the machine (Fig. 12.18). The piston is inside ametal case that wil l swing out when the door is opened (Fig. 12.19). A window

allows the operator to view piston movement.

The piston assembly is shown in Figures 12.20and 12.21. Electrical power is used

to raise and lower the piston. The motor is n ear the bottom of the cylinder that

holds the piston. There are two roll i ng diaphragms that seal the p iston and prevent

mixing of ambient and respired gases. The upper diaphragm is attached at the top

and f its over the upper end of the piston. The lower part of upper diaphragm rolls

upward and downward as the piston moves upward and downward (Fig. 12.20). The

lower diaphragm is connected between the piston wall and the inside of the

cylinder. As the piston moves downward, the space above the upper diaphragm

increases, allowing exhaled gases to enter that space. There are high pressure and

negative pressure relief valves on the top of the piston, connecting with the space

for respired gases.


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View Figure

Figure 12.18.Drager Fabius GS ventilator. A windowallows the operator to view piston movement.

P.335

The display screen is shown in Figure 12.22(see page 338). At the left side are

keys that determine the venti latory mode (volume control, pressure control,

manual/spontaneous). A rotary mouse is at the bottom right of the screen. Once a

parameter is selected, the value is altered by turning the rotary mouse and is

confirmed by depressing it . The Standby key is to the right of the rotary mouse. To

the right of the rotary mouse and above the Standby key is the Mains Power l ight-

emitting diode (LED), which, when lit, confirms that the machine is connected to a

functioning electrical system.

To the right of the screen are three keys. The bottom one is the Home key. I t

causes the main screen to be displayed. The Setup key is above the Home key.

When pressed, the displayed window enables the operator to view and change

venti lat ion and to review sett ings.

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Above th e Setu p key is the Al arms key. Whe n pressed , al arm limits are shown on

the right side of the screen. To the right of the Alarms key is the Alarm Silence key.

Pushing this causes act ive alarms to be audio paused for 2 minutes.


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View Figure

Figure 12.19.Drager Fabius GS ventilator. When the dooris opened, the piston ventilator, which is inside a metal case,will swing out.

View Figure

Figure 12.20.Piston assembly. As the piston movesdownward, the upper diaphragm moves downward with it,creating a space for respired gases.

To the right of the Alarms key are two LED lamps that indicate the urgency of the

alarm message. A status bar near the top of the screen displays the venti latory

mode being used. I t also d isplays alarm silence status, battery power level , and the

time.

To the left side of the screen are virtual f lowmeters for air, oxygen, and nitrous

oxide. To the right of the f lowmeters in the upper third of the screen is an alarm

window. This displays up to four of the highest priority


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P.337

alarms. To the right of this window, the inspired oxygen concentrat ion and alarm

limits for oxygen concentrat ion are displayed. The respiratory volume monitor

window is the middle window to the right of the flowmeter window. It displays

respiratory rate and tidal and minute volumes. Below the respiratory volume window

is the breathing pressure monitor window. I t displays PEEP values and peak and

mean inspiratory pressures. Below the flowmeters and the breathing pressure

window is the breathing pressure waveform window. Below this are six windows

associated with venti lator parameters. Below these windows are keys fo r the

associated parameters.

View Figure

Figure 12.21.As the piston moves upward, gases are forcedout of the space at the top.

Controls

The range for the maximum venti lat ion pressure (PMA X ) is 10 to 70 (default 40) cm

H2O. Other controls are discussed under the individual venti lat ion modes.With volume control venti lat ion frequency can be set from 4 to 60 (default 12) bpm.

The t ime rat io between the inspiratory and expiratory t ime phases (Ti:Te) range is

4:1 to 1:4 (default 1:2). The inspiratory pause can be s et from 0% to 5 0% (default

10%). PEEP can be set from 0 to 20 (default 0) cm H 2O. The range for tidal volume

is 20 to 1400 (default 600) mL.

In the pressure control mode, the inspiratory pressure (P INSP ) can be set from 5 to

60 (default 15) cm H2O,

P.338


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and the inspiratory f low can be set from 10 to 75 (default 30) L/minute. PEEP can

be set from 2 to 20 (default 0) cm H2O. Venti lat ion frequency can be s et from 4 to

60 (default 12) bpm.

View Figure

Figure 12.22.Display screen for the Drager Fabius GSventilator.

Alarms

Alarms are auto ma tical ly ena bled when the venti lator is swi tche d to a ven ti la ti on

mode. Alarm messages are displayed in the alarm box in the center of the top of

the data screen. The text displays are followed by exclamation marks (!). There are

three marks (! ! !) for warnings, two (! !) for caution, and one (!) for advisories. The

LEDs to the right of the alarm silence key indicate the urgency of the alarm

condit ion. A warning is signaled by a blinking red LED. A caution is expressed by a

blinking yellow LED. An advisory is indicated by a continuous yellow LED. Warning

tones are continuous. Caution tones enunciate every 30 sec onds. An advisory has

a single or no t one.

Ventilation Modes

The Fabius offers volume control and pressure control venti lat ion. PSV can be

added. When the venti lat ion mode is changed, the function is displayed across the

bottom of the data screen and above the appropriate key.

S t a n d b y

In the Standby mode, the ventilator stops, and the monitoring and alarms are

turned OFF. I f gas f low is detected, a Gas Sti l l Flowing message appears in the


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alarm window. If the machine is in the Standby mode for 5 minutes and there is no

user input, the machine goes into the Sleep mode, and a screen saver appears.

Man u a l / S p o n t a n e o u s

In Man/Spon mode, the piston in the venti lator is moved to its topmost posit ion to

minimize system compliance. The APL bypass valve is closed, direct ing excess gas

through the APL valve.

For spontaneous v enti lat ion, the APL valve is put in the SPONT posit ion, in which it

is ful ly open. During inspirat ion (Fig. 12.23), gas from the bag f lows through the

fresh gas decoupling valve and the inspiratory unidirect ional valve to the Y-piece.

During exhalat ion (Fig. 12.24), exhaled gases f low through the expiratory

unidirect ional valve and the absorber. The reservoir bag f i l ls with a c ombination offresh gas and gas that has passed through the absorber. Excess gas exits through

the APL valve.

During manual venti lat ion, the APL valve is set to the MAN posit ion. The opening

pressure can be adjusted from 5 to 70 cm H2O. As the bag is compressed (Fig.

12.25, see page 341), the gas in the bag f lows through the fresh gas decoupling

valve, the inspi ratory unidirect ional valve, and the Y-piece. Some gas f lows

retrograde through the absorber and the APL valve, which is adjusted to provide

the proper pressure. During exhalat ion, exhaled gases f low through the expiratory

unidirect ional valve and the absorber. The reservoir bag f i l ls with a c ombination of

fresh gas and gas that has passed through the absorber.

Mech a n i c a l

When the Fabius is in automatic mode, the APL bypass v alve is held open. Fresh

gas decoupling is accomplished by using a decoupling valve between the fresh gas

inlet and the breathing system. The reservoir bag wil l inf late and deflate during

mechanical venti lat ion. I f the oxygen f lush is act ivated during inspirat ion, the gas

wil l not be added to the t idal volume but wil l enter the reservoir bag (23 ).

During inspirat ion (Fig. 12.26, see page 342), the pressure generated by the piston

closes the fresh gas decoupling valve. Fresh gas f lows retrograde through the

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absorber and enters the reservoir bag. The piston pushes gas through the

inspiratory unidirect ional valve and the inspiratory hose to the Y-piece. I f the

pressure exceeds the pressure l imit, the Pmax valve opens.


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View Figure

Figure 12.23.Drager Fabius GS ventilator. Inspirationduring spontaneous breathing. PEEP, positive end-expiratory pressure; APL, adjustable pressure limiting.

During exhalat ion (Fig. 12.27, see page 343), exhaled gas f lows through the

expiratory unidirect ional valve and into the reservoir bag, where fresh gas has been

collect ing during inspirat ion. The piston retracts, drawing in gas. Excess gas f lows

through the APL bypass valve and the exhaust valve to the scavenging system.

Special Features

A high pre ssure an d a negat iv e pressure relief val ve are lo cated at the to p of th e

venti lator piston (Figs. 12.20, 12.21). The high pressure relief valve opens at 75 5

cm H2O. The negative pressure safety relief valve lets in air at -2 to -5 cm H2O.

If a fault in the venti lator is not corrected and the anesthesia provider cannot switch

to manual venti lat ion by using the Man/Spont mode, manual venti lat ion is st i l l

possible. To do this, the ON/OFF system power s witch on the rear panel is switched

OFF, then ON.

The Fabius GS has battery backup that wil l power the machine and venti lator for at

least 45 minutes if the batteries are fully charged. I f the power fails, the mains LED

will go out, a message wil l appear, and a battery symbol wil l appear in the status

bar. After the battery is exhausted, the patient can be v enti lated in the

manual/spontaneous mode.

Breathing system compliance is determined during the checkout procedure. For

accurate compliance compensation, the breathing system must be in the

configurat ion in which it is to be used for the patient when the checkout procedure

is performed.


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The Fabius GS is equipped with fresh gas decoupling. During mechanical

inspirat ion, the fresh gas decoupling valve closes. This directs the fresh gas to the

reservoir bag, thereby stopping it from being added to the inspired t idal volume.

During exhalat ion, the decoupling valve

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opens, allowing exhaled gas and the accumulated fresh gas to f i l l the piston.

View Figure

Figure 12.24.Drager Fabius GS ventilator. Exhalationduring spontaneous breathing. PEEP, positive end-expiratory pressure; APL, adjustable pressure limiting.

Hazards

Ai r can be entr aine d du ri ng mecha nical ven ti lat io n if the re is a disconne cti on or

leak or the fresh gas f low is d irected to the wrong circuit (9,66,67 ). This could lead

to patient awareness and hypoxia. Such a problem should be discovered during the

checkout procedure but could occur later.

Venti lator fai lure result ing from worn parts of the motor that drives the bellows has

been reported (68,69). A ventilator failure warning was posted and manual

venti lat ion was possible after the venti lator was placed in the s tandby mode.

Cleaning and Sterilization

External parts of the ventilator may be cleaned with detergents and disinfectants.

The parts that are exposed to respiratory gases can be removed from the v enti lator.

The venti lator diaphragm, cover, and hoses can be steam autoclaved.

D r ag e r Apo l l o


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Description

The main screen of the Drager Apollo machine is shown i n Figure 12.28(see page

344). At the left on the bottom are virtual f lowmeters. Above this is gas-monitoring

information. To the right of this is the carbon dioxide waveform and pressure- and

flow-volume loops. Below the pressure-volume loop are bar graphs f or t idal volume

and airway pressure. To the right of the carbon dioxide waveform and f low-volume

loop are the values for venti latory parameters. Below this are the pipeline and

cylinder pressures. At the right are soft keys for various other functions.

Venti latory functions are controlled by using two sets of ke ys below the bottom of

the screen (Fig. 12.29, see page 345). The keys in the bottom row are used to

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set the ventilatory mode (manual/spontaneous, volume mode, pressure mode, or

pressure support). To s elect a mode, the key is pressed, and the knob to the right

is pressed to confirm the change. The right key is used to select the auxil iary

common gas outlet, which is an op tional feature.

View Figure

Figure 12.25.Drager Fabius GS ventilator. Inspiration

during manual ventilation. PEEP, positive end-expiratory

pressure; APL, adjustable pressure limiting.

Above th is row o f keys is an oth er row that is us ed to se t the ven til ati on para me ters .

To alter the sett ing, the key is pressed, and the knob is rotated to increase or

decrease the value shown above the soft key unti l the desired value is reached.

Then, the knob is pressed to confirm that sett ing.

To the right of the knob (not shown in Fig. 12.29) is the standby key, which is used

to switch between operating and s tandby modes.


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The internal construct ion of the Apollo venti lator is shown in Figures 12.30to 12.35

(see pages 345,346,347 ,348 ,349,350 ,35 1). Fresh gas enters the breathing system

and passes through the fresh gas decoupler valve. The venti lator, which has an

electrically driven piston, is connected to the inspiratory side of the circuit

downstream of the fresh gas decoupler valve. A f low sensor just downstream of the

unidirect ional valve monitors the inspiratory f low.

On the exhalat ion side of the circuit another f low sensor, a pressure gauge and a

PEEP valve are located upstream of the expiratory unidirect ional valve. The

reservoir bag and APL valve as well as an APL bypass valve leading to the exhaust

valve from the venti lator are between the expiratory unidirect ional valve and the

absorber.

Controls

The range for pressure l imitat ion (PM AX ) is 10 to 70 (default 40) cm H2O, with a

minimum of PEEP +10 cm H2O. The range for t idal volume is 20 to 1400 (default

600) mL. With PSV, the tidal volume range is 10 to 1400 mL. Respiratory frequency

can be set from 3 to 80 (default 12) bpm in volume control and pressure control

Documents

Chapter 12. Anesthesia Ventilators