Anesthesia machine basics

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Seminars in Anesthesia, Perioperative Medicine and Pain (2005) 24, 138-146

nesthesia machine basics

ames B. Eisenkraft, MD

rom the Department of Anesthesiology, Mount Sinai School of Medicine of New York University, New York, New York.

The anesthesia machine is that component of the anesthesia delivery system that receives medical gases(oxygen, nitrous oxide, air, heliox) under pressure and controls the flow of each gas individually. Itcreates a gas mixture of known composition at a known flow rate and delivers it to the common gasoutlet of the machine. From here, the fresh gas flow is conducted to the anesthesia circle breathingsystem. This review will discuss the storage of compressed oxygen and nitrous oxide and how thesegases arrive to a generic anesthesia machine. The paths taken by these gases as they flow through thegeneric machine will be described. The important components used to create the precisely controlledfresh gas mixture, as well as the safety features of the machine will also be described. The reader shouldgain an understanding of the machine basics that he/she can use to understand his/her particular modelof machine.© 2005 Elsevier Inc. All rights reserved.

KEYWORDS:Compressed medicalgases;Anesthesia machine;Patient safety

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The contemporary anesthesia workstation comprises thenesthesia machine, vaporizer, ventilator, monitors, andlarm systems. The workstation, breathing system, andaste gas scavenging system constitute the anesthesia de-

ivery system. The basic anesthesia machine, which is theubject of this review, receives compressed gases (oxygen,itrous oxide, sometimes air, and/or heliox) from tanknd/or pipeline supply sources, creates a controlled gasixture in terms of gas concentrations and total gas flow

ates, and delivers this mixture to the vaporizer, where theesired concentration of potent inhaled anesthetic may bedded. The resulting fresh gas mixture of known composi-ion and metered production rate is delivered to the patientircuit, which is most commonly a circle breathing system.he patient circuit is a mini-environment to which theatient’s lungs are exposed. The patient’s body will tend toquilibrate with the gas mixture in the breathing circuit toroduce the desired PaCO2, PaO2, and depth of anesthesia.

Presently in the United States, the two largest manufac-

Address reprints requests and correspondence: James B. Eisenkraft,D, Department of Anesthesiology, Box 1010, The Mount Sinai Hospital,

450 Madison Avenue, New York, NY 10029-6574.
cE-mail address: [email protected].
277-0326/$ -see front matter © 2005 Elsevier Inc.. All rights reserved.oi:10.1053/j.sane.2005.07.002

urers of anesthesia delivery systems are Dräger (Telford,A), and Datex-Ohmeda (a Division GE Health Care, Mad-

son, WI). A number of other companies also manufacturer distribute anesthesia machines in the United States (eg,atascope, Penlon, Blease). Since all anesthesia machineserform the same task, this review will discuss the essentialsf the compressed gases oxygen and nitrous oxide, how theyre supplied to the machine, and how gas flows are con-rolled to create the desired fresh gas mixture that will leavehe machine via the common gas outlet. Reference to thewo largest manufacturers’ systems will be made whereppropriate. This review will discuss the conventional ge-eric machine as opposed to the most recent electronicorkstations, but reference to the latter will be made where

elevant.

he basic anesthesia machine

he gas flow arrangements of a basic anesthesia machinere shown in Figure 1. The basic machine receives oxygennd nitrous oxide from two supply sources: a tank or cyl-nder source and a pipeline source. It is important to know
ertain properties of these gases.

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139Eisenkraft Anesthesia Machine Basics

xygen

Oxygen (O2) has a molecular weight of 32 AMU (atomicass units, also known as Daltons). The molecular weight

an be used to calculate the density (mass per unit volume)f the gas. Thus, by Avogadro’s Volume, 1 gram moleculareight (ie, the molecular weight in grams) of any gas orapor occupies 22.4 liters at standard temperature and pres-ure [STP: 0°C (which is 273 degrees Absolute or 273elvin) and 760 mm Hg pressure (or one atmosphere)].Gases expand when temperature increases. According to

harles’ Law, the volume of a fixed mass of gas is directlyroportional to Absolute temperature. Room temperature issually 20°C or 293 Kelvin (ie, 273 � 20). Therefore, 32rams of oxygen will occupy 22.4 � 293/273, or 24 L at0°C. Oxygen boils at a temperature of �183° C at onetmosphere pressure (760 mm Hg, which is the same as 14.7ounds per square inch absolute pressure or PSIA). Theoiling point of a liquid (in this case, the temperature at

igure 1 Schematic showing the gas flow arrangements of aheck-out: a guide for preoperative inspection of an anesthesia mf Anesthesiologists, Park Ridge, IL.

hich oxygen changes from liquid to gas phase) is related to c

mbient pressure such that as pressure increases so does theoiling point. However, a certain critical temperature iseached above which, no matter how much pressure ispplied, the liquid oxygen will boil into the gaseous form.he critical temperature for oxygen is �118°C, and theritical pressure, which must be applied at this temperatureo keep oxygen liquid, is 737 PSIA. Because room temper-ture is 20°C and therefore well above oxygen’s criticalemperature, oxygen can exist only as a gas at room tem-erature.

In many hospitals, the oxygen pipeline is supplied frombulk liquid source. Liquid oxygen is kept at or below its

oiling point (generally at �160°C) in a storage vessel thatesembles a large vacuum flask. The vacuum between thenner and outer layers of the bulk liquid oxygen storageessel prevents loss or gain of heat by conduction, convec-ion, and radiation. Above the liquid oxygen is oxygen gas.ince the bulk oxygen storage vessel is outside the building,

t is subjected to extremes of temperature. The vessel in-

generic anesthesia machine. See text for details. Adapted from, ASA 1987. Reproduced by permission of the American Society

basicachine

orporates a safety relief valve that permits oxygen gas to be

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140 Seminars in Anesthesia, Perioperative Medicine and Pain, Vol 24, No 3, September 2005

eleased into the atmosphere if there is an increase in am-ient temperature causing the pressure in the vessel toxceed a certain threshold.

During normal use, when gaseous oxygen flows from theop of the storage vessel en route to the oxygen pipelineystem, it is very cold. Therefore, it passes through a heatingoil, and then through a pressure regulator that maintains theospital’s pipeline pressure at 50-55 pounds per square inchauge pressure (PSIG).

Note that a pressure gauge reads 0 PSIG when exposedo the atmosphere but, as explained above, atmosphericressure is 14.7 PSIA. Thus, 50 PSIG is the same as 64.7SIA (ie, 50 � 14.7). Alarms and safety devices, includingelief valves and shut-off valves, as well as back-up liquidxygen storage vessels, ensure the safe functioning of theipeline. A recent report described failure of the connectionetween the liquid oxygen vessel and the pipeline, whichesulted in release of 8000 gallons of liquid oxygen into thetmosphere.1

The oxygen pipeline serves to deliver oxygen gas toxygen-specific wall outlets in the operating rooms andhroughout the hospital. A green hose is used to conductxygen from the wall outlet to the oxygen-inlet hose con-ection on the back of the machine. The connection betweenhe oxygen hose and the wall outlet is usually one that cane easily connected or disconnected; the so-called “quick-onnect.” Quick connect systems are gas-specific accordingo manufacturer, thus an Ohmeda oxygen wall outlet willot accept an oxygen hose fitting made by a different man-facturer, such as Schraeder. There are a number of differ-nt manufacturers of quick connect systems, and in onenstitution there may be systems from more than one man-facturer. In such institutions, it is important to ensure thathe necessary quick connect fittings are present, particularlyn emergency equipment such as the Sanders injector sys-em that may be needed for transtracheal jet ventilation.

The oxygen hose connection on the back of the machines not a quick connect but is gas-specific by diameter, usinghe nationally standardized diameter-indexed safety systemDISS). The diameters of the connections for oxygen, air,itrous oxide, and vacuum all are of unique sizes. Theseonnections are threaded and a wrench is required to tightenhem. In some institutions, DISS fittings are used on theall gas outlets. The machine must be disconnected from

he wall oxygen supply in order to check the pressure in thexygen tanks. The DISS connections therefore lack theonvenience of the quick connects when it comes to check-ng the pressure in the oxygen tanks.

All anesthesia machines have a back-up supply of oxy-en stored in a tank (also known as a cylinder) in case ofipeline failure. Medical gas cylinders are available in dif-erent sizes, each size being assigned a letter. Most ma-hines are equipped with one or two E cylinders of oxygenhat hang on oxygen-specific yokes. A smaller size (D
ylinder) is also available for mounting on the machine. m
Oxygen tanks are filled to a pressure of about 1900 PSIGt room temperature (almost the same as 1914.7 PSIA).nce filled, they contain a fixed number of gas molecules

hat obey Boyle’s law, which states that, for a fixed mass ofas at constant temperature:

Pressure � Volume � a Constant, or P1 � V1 � P2 � V2.

A full E cylinder at a pressure of 1900 PSIA will evolve60 L of gaseous oxygen at an atmospheric pressure of 14.7SIA (0 PSIG; 760 mm Hg). The internal volume (V1) of ancylinder is therefore approximately 5 L because

900 � V1 � 14.7 � 660;

therefore, V1 � (14.7 � 660) ⁄ 1900 � 5L.

Because oxygen is a gas at room temperature and itbeys Boyle’s law, the tank pressure gauge can be used tostimate how much oxygen gas remains to be evolved fromhat tank. If an E cylinder oxygen tank pressure gauge reads000 PSIG, then the tank is (1000/1900), or 52% full, andill generate only (660 � 52%) or 340 L of oxygen. If suchtank was being drained at a rate of 6 L/minute, it would

mpty in just under 1 hour (actually 340/6 � 57 minutes).t is important to understand these principles when oxygenylinders are being used to supply the machine or duringransport of a patient. It should be obvious that if theachine is equipped with two oxygen tanks, only one

hould be on at any time so that both are not emptiedimultaneously!

Only an oxygen tank can be mounted in an oxygenanger yoke on the anesthesia machine. There are two pinsn the hanger yoke that mate with two corresponding holesn the oxygen tank valve. This is the nationally standardized

edical gas pin index system. For each medical gas theres a specific configuration of the two pins in the hangeroke, and corresponding two holes in the tank valve for thatas. This system should prevent a nitrous oxide tank fromeing mounted in a hanger yoke for oxygen.

Oxygen that enters the machine from the pipeline or tankupply enters the machine’s high pressure system for oxy-en. The high pressure system for oxygen is defined as allhose components upstream of the oxygen flow controlusually a needle) valve that is used to control the flow ofxygen from the flowmeters (see below). While the pipe-ine supply of oxygen enters the machine at a pressure of 50SIG, the tank supply enters the hanger yoke at pressures ofp to 2200 PSIG. The pressure of the oxygen coming fromhe tank source is therefore down-regulated (ie, oxygenasses through a regulator valve) and enters the machineigh pressure system at a nominal pressure of 45 PSIGFigure 1). A pressure regulator valve is a device thateduces a variable high input pressure to a constant lowutput pressure for the gas whose pressure is being regu-ated. The tank supply serves as a back-up in case theipeline fails, and once the tank has been checked, it shoulde turned OFF. If the oxygen tank is left open and the
achine is being supplied from the pipeline, oxygen is

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rawn preferentially from the pipeline (50 PSIG) becausehe regulator that controls flow from the oxygen tank sourceill only permit flow from the tank when the pressure in theachine high pressure system falls below about 45 PSIG.owever, the pipeline pressure may at times fluctuate toelow 45 PSIG, in which case oxygen would be drawn fromn oxygen tank that had been left open. Thus, if the machines being supplied from the oxygen pipeline, the oxygenanks on the machine should be checked and then turnedFF. This is (1) to prevent the tank oxygen supply beingrawn down and unintentionally depleted, and (2) if theonnection between the tank and its hanger yoke was notas-tight, tank oxygen could leak to the atmosphere and note available when needed.

Having entered the machine high pressure system forxygen at a pressure of 45 PSIG (from tank) or 50 PSIGfrom pipeline), oxygen may flow in seven directions (Fig-re 1):

1. It provides the power source for a pneumaticallyriven anesthesia ventilator. Most anesthesia ventilatorseg, those on Datex-Ohmeda Aestiva and Excel machines,rager Narkomed 2, Narkomed 3, and Narkomed 4) use

ompressed oxygen as the driving gas. The driving gas issed to compress the ventilator bellows during the inspira-ory phase of positive pressure ventilation. It is important toealize this because if the machine and therefore ventilatorre being supplied from the tank rather than the pipeline, theank will be depleted much more rapidly. In general, thexygen used to drive the ventilator is at least the minuteentilation set to be delivered (eg, TV 500, RR 10/min givesV of 5 L). Thus, if the pipeline oxygen supply fails during

se of the ventilator and one switches to the tank supply,ne should consider ways to limit the rate of use of the tankxygen by ventilating the lungs using the reservoir bag,aving the patient breathe spontaneously if possible, andsing the lowest flow of oxygen necessary at the oxygenowmeter.

2. It supplies the auxiliary oxygen flowmeter that isresent on most contemporary machines. This is the sepa-ate flowmeter that is used to supply a nasal cannula.

3. It supplies oxygen to an auxiliary oxygen DISS fit-ing. This may be used to power a Sanders injector designf jet ventilator, or a venturi design vacuum/suction system.

4. If the oxygen flush control (valve) is opened byressing the oxygen flush control button, oxygen flows tohe common gas outlet of the machine at a rate of 35-75/min and potentially at a pressure of 50 PSIG. The flush

herefore must not be activated if a patient is connected tohe breathing system and there is no means for pressureelief. For example, activation of the flush during the in-piratory phase of positive pressure ventilation, duringhich gas can enter the breathing system but cannot leave,as the potential to cause positive pressure barotrauma.

5. It pressurizes an oxygen supply pressure failurelarm system such that if oxygen pressure in the high
ressure system falls (usually below 30 PSIG) an audible t
larm sounds. On modern machines, a pressure-operatedlectrical switch ensures a continuous audible (and visual)larm when the oxygen supply pressure falls below 30SIG. This will alert to a possible problem with the ma-hine’s oxygen supply pressure such as pipeline failure or ifhe tank in use is nearing empty.

6. It pressurizes and opens the “fail-safe” valve (Figure, item A). This is a pressure-sensitive valve that can de-rease or totally interrupt the supply of nitrous oxide andther gases (eg, heliox, air) to their flow control systems ifhe pressure of gas in the oxygen high pressure system fallselow a threshold level.

In the Datex-Ohmeda machines this valve is called theressure sensor shut-off valve (PSSV). These valves in-

errupt the gas supplies to the nitrous oxide and other gasowmeters when the oxygen pressure falls below a nominal6 PSIG. In the Datex-Ohmeda machines, the valves areither open or closed.

In the Drager Narkomed 2, Narkomed 3, and Narkomedmachines, the fail safe valve is called the oxygen failurerotection device (OFPD) and there is one interfacing theigh pressure system for oxygen with the high pressureystems for each of the other gases supplied to the machineeg, nitrous oxide). Unlike the Datex-Ohmeda PSSV “failafe” valves, the OFPDs gradually reduce the supply pres-ure to the nitrous oxide and other gas flowmeters as thexygen supply pressure decreases. The supply of otherases to their respective flowmeters is completely inter-upted when the oxygen supply pressure falls below 12 � 4SIG. In this way, a hypoxic mixture arising from oxygenupply problems to the flowmeters should be prevented.hus, in Drager Narkomed, Datex-Ohmeda and otherrands of machines, when the oxygen supply pressure isow, only 100% oxygen is delivered.

7. It passes to the oxygen flow control valve. This is theeedle valve that is connected to the oxygen flow knobhich is used to set the oxygen flow at the oxygen flow-eter.In Datex-Ohmeda machines (eg, Modulus II, Modulus II

lus, Modulus CD, and Excel models), to reach the oxygenow control, oxygen in the high pressure system must firstass through a second stage regulator valve where the pres-ure is down-regulated to about 16 PSIG. This second stageegulator (Figure 1, item B) ensures that the oxygen flow-eter is supplied at a constant pressure of 16 PSIG. Thus,

ven if the oxygen supply pressure to the machine falls toelow 45-50 PSIG, as long as it exceeds 16 PSIG, thexygen flow at the flowmeter will be maintained. Withouthis second stage regulator, if the oxygen supply pressure tohe machine were to fall, the oxygen flow would decrease athe flowmeter and, if nitrous oxide were being used also, aypoxic gas mixture might result at the level of the flow-eters. In summary, in Datex-Ohmeda machines, if the

xygen supply pressure falls below 30 PSIG, the low pres-ure supply alarm sounds (see #5 above); below 20 PSIG
he “fail safe” valve will interrupt the flow of other gases to

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heir flowmeters so that only oxygen can be delivered, andhe oxygen flow set on the oxygen flowmeter will notecrease until the oxygen supply pressure falls below 16SIG.

Drager Narkomed 2, Narkomed 3, and Narkomed 4 ma-hines do not use a second stage oxygen pressure regulatorFigure 1, item B is absent) to supply the oxygen flowmetert constant pressure. Instead they use OFPDs (see #6 above)o continually interface the pressure of oxygen in the highressure system with the pressure of nitrous oxide justpstream of the nitrous oxide flowmeter. A decrease in thexygen supply pressure causes a proportionate decrease inhe pressure of nitrous oxide supplied to its flowmeter.hus, as oxygen supply pressure decreases, the flow ofxygen and that of all other supplied gases will decrease inroportion to avoid creation of hypoxic gas mixture.

itrous oxide

Like oxygen, nitrous oxide is supplied from a pipelineystem to wall outlets in the operating room at 50 PSIG, orrom a back-up E cylinder tank supply on the machine.itrous oxide has a molecular weight of 44 AMU and boils

t �88°C at one atmosphere pressure (760 mm Hg or 14.7SIA). Because it has a critical temperature of 36.5°C (andritical pressure of 1054 PSIA), it can exist as a liquid atoom temperature (20°C). E cylinders of nitrous oxide arelled to 90-95% capacity with liquid nitrous oxide andbove the liquid is gaseous nitrous oxide. Because the liquidgent is in equilibrium with its gas phase, the pressurexerted by the gaseous nitrous oxide is the saturated vaporressure (SVP) at the ambient temperature. The SVP ofitrous oxide at 20°C is �750 PSIG, or 51 atmospheres.

A full E cylinder of nitrous oxide will provide 1600 L ofas at one atmosphere pressure (14.7 PSIA). As long asome liquid nitrous oxide is present in the tank and thembient temperature remains at 20°C, the pressure in theitrous oxide tank will remain at �750 PSIG. It is clear that,nlike with oxygen, one cannot determine the content of aitrous oxide tank from the nitrous oxide tank pressureauge. It can, however, be determined by weighing the tanknd subtracting the weight of the empty tank (TARE

EIGHT) to determine what weight of nitrous oxide re-ains. By Avogadro’s volume, 44 g of nitrous oxide occupy

2.4 L at standard temperature and pressure, or 24 L at0°C. Once all of the liquid nitrous oxide has been used upnd the tank contains only gaseous nitrous oxide, Boyle’saw may be applied to the gas remaining. In this situation,here the tank pressure is 750 PSIA (due to gas only) and

he internal volume of the E cylinder is approximately 5 Lsee oxygen section), one can calculate how much nitrousxide will be evolved at one atmosphere pressure (14.7SIA).

Therefore, (P1 � V1 � P2 � V2), 750 � 5 � 14. 7 � V2,r V2 or � 255 L.

At this point, the nitrous oxide tank would be (255/
600), or 16% full. A tank with a pressure of 400 PSIA g
ould evolve [(400/750) � 255], or 136 L of nitrous oxideas. (As a side note, when applying Boyle’s law, one mustse PSIA and not PSIG, particularly for atmospheric pres-ure.)

The nitrous oxide pipeline is supplied from large storageessels containing liquid nitrous oxide at a pressure of750 PSIG. The gas evolved passes though a pressure

egulator that ensures that the nitrous oxide wall outlets areupplied at a pressure of 50 PSIG.

Analogous to oxygen, the machine high pressure sys-em for nitrous oxide consists of those parts upstream ofhe nitrous oxide flow control needle valve (see Figure 1).itrous oxide from the tank supply enters the nitrous oxideanger yoke at pressures of �750 PSIG and then passeshrough a nitrous oxide regulator that reduces the pressureo 45 PSIG. As with oxygen, there is a pressure differentialetween the pipeline supply pressure and that from theitrous oxide tank so that if both sources are available andhe nitrous oxide tank has been left open, the pipeline sourceill preferentially be used.Having entered the anesthesia machine high pressure

ystem for nitrous oxide, the gas must flow past the “failafe” valve (OFPD or PSSV) before reaching the nitrousxide flow control valve.

In the Datex-Ohmeda Modulus anesthesia machines thatave the Link-25 Proportion Limiting System (see below),second-stage nitrous oxide regulator (Figure 1, item C)

urther reduces gas pressure so that nitrous oxide is suppliedo its flowmeter at a nominal pressure of 26 PSIG. (Thectual downstream pressure of this regulator is adjusted athe factory or by a field service representative to ensureorrect functioning of the proportioning system.)

Oxygen and nitrous oxide clearly serve different func-ions, and it is essential that the correct gas be supplied tohe correct inlet of the anesthesia machine. All gas fittings inhe OR and on the anesthesia machine are indexed and areon-interchangeable among specific medical gases. Speci-city of tank connections is ensured by the pin-index safetyystem in the hanger yoke, and for piped gases there arepecific diameter-indexed safety system (DISS) and propri-tary (manufacturer-specific) quick-connect fittings. Fit-ings that do not connect readily should never be forcedogether.

as flow control system (knobs, needlealves, rotameters, etc.)

he anesthesia machine is used to adjust the proportions ofxygen and nitrous oxide, as well as total gas flows deliv-red to the patient. For each gas (oxygen, nitrous oxide, etc.)his is achieved by means of:

1. A knob that is connected to a needle valve wherebyas flow is set and adjusted. Turning the knob counter-lockwise opens the valve wider and thereby permits a
reater flow of gas. The flow control knob for oxygen is

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arger than those for the other gases, and is fluted rather thannurled so that it is touch-coded. Thus, the oxygen knobeels different from the knobs for the other gases.

2. A gas flow measuring system. Traditionally, gas flowsn the conventional anesthesia machine are measured bysing the rotameter flowmeter. There may be one rotameterr two rotameters in tandem for each gas. If two are presentor each gas, the first permits accurate measurement of lowows (usually up to 1 L/min) and the other of flows of 10-12/minute. In North America, the oxygen rotameter(s) is(are)ositioned on the right side of the rotameter bank. Theotameter is a constant pressure, variable orifice flowmeter,ased on the Thorpe tube principle. Each rotameter com-rises a vertical tapered glass tube that is of small diametert the bottom, wider at the top, and contains a ball, float, orobbin. The cross-sectional area between the outside of theoat and the inside of the tapered glass tube represents theariable orifice. A certain pressure difference across theobbin is required to “float” the bobbin in the verticallypward flowing gas stream. As the orifice widens, greaternd greater flows are required to create the same pressureifference across the bobbin, which floats at a higher leveln the tapered glass tube. At low gas flow rates, flow isssentially laminar and Poiseuille’s law applies:

Flow �� P � r4

8 � � � l

here P is the pressure drop across the bobbin,r is the radius of the tube� is the viscosity of the gas, andl is the length of the bobbin or float.When the orificial area (r2) is larger and flows are

reater, flow becomes turbulent in which case:Flow is proportional to: �P, r2, length�1, and den-

ity �1.Rotameter flowmeters are precision instruments. Flow

ubes are manufactured for specific gases, calibrated with anique float, and for use within a certain range of temper-tures and pressures. Flowmeters are not interchangeablemong gases, and if a gas was passed through a rotameteror which it was not calibrated, the flows shown wouldikely be incorrect. Theoretical exceptions to this would behat, at low (laminar) flows, the flow rates of gases withimilar viscosities would be read identically (eg, oxygen andelium have viscosities 202 and 194 micropoise, respec-ively), and at high flows, gases of similar densities (eg,itrous oxide and carbon dioxide which both have a molec-lar weight of 44 AMU) would be read identically. Again,t is emphasized that flowmeters are not interchangeablemong medical gases and are now manufactured such thathey cannot be interchanged.

As machines evolve and new technologies are intro-uced, some of the traditional mechanical components areeing replaced by more sophisticated devices. For instance,n one contemporary anesthesia workstation (Datex-
hmeda S5/ADU), gas flows are still controlled by needle d
alves, but flows are measured using electronic flow sensorsased on the principle of the pneumotachograph. Essen-ially, the pressure difference is measured across a laminarow resistor through which the gas flows. Reference to theoiseuille formula above shows that if r, �, and l areonstant, P can be used to measure flow. Flow is theneasured by using a differential pressure transducer, and

isplayed on a screen in the form of a virtual graduatedowmeter, together with a digital display.

Some anesthesia machines offer, as an option, an oxygenow that cannot be discontinued completely because eitherstop is provided on the oxygen flow control valve to

nsure a minimum oxygen flow of 200-300 ml/min past theeedle valve (Datex-Ohmeda machines), or a gas flow re-istor is provided (Drager Narkomed 2, Narkomed 3, andarkomed 4 machines), which permits a similar flow of00-300 ml/min to bypass a totally closed oxygen flowontrol needle valve. In the Drager Narkomed machines, theinimum oxygen flow feature functions only in the “O2/

2O” mode but not in the “ALL GASES” mode.

xygen ratio monitoring and proportioningystems

major consideration in the design of modern anesthesiaachines has been the prevention of the delivery of hypoxic

as mixtures. The “fail safe” systems only serve to interruptDatex-Ohmeda PSSV) or proportionately reduce and ulti-ately interrupt (Drager Narkomed ORMC) the nitrous

xide and other gases (eg, helium) supplied if there is aecrease in oxygen supply pressure to the machine. It isot a flow-sensitive system and it cannot prevent the deliv-ry of hypoxic mixtures. The term “fail safe” therefore isomewhat of a misnomer.

In contemporary machines, the flows of oxygen anditrous oxide are interlinked so that a fresh gas mixtureontaining at least 25% oxygen is created at the level of theowmeters whenever oxygen and nitrous oxide are beingsed.

Datex-Ohmeda anesthesia machines use the Link-25roportion Limiting Control System to ensure an adequateercentage of oxygen in the gas mixture created. In thisystem, a gear with 14 teeth is fixed on the nitrous oxideow control valve spindle, while a gear with 29 teeth canotate or “float” on a threaded oxygen flow control valvepindle, rather like a nut rotating on a bolt. The two gearsre connected together by a precision stainless steel linkhain. For every 2.07 revolutions of the nitrous oxide flowontrol knob, the oxygen knob and spindle set to the lowestxygen flow will rotate once because on the 29:14 ratio ofear teeth. Because the gear on the oxygen flow control isounted (like a nut on a bolt) so that it can “rotate” on the

xygen control valve spindle (rather than being integralith the spindle), oxygen flow can be adjusted indepen-
ently of nitrous oxide flow. However, regardless of the

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xygen flow set, if the flow of nitrous oxide is increasedufficiently, the counter-clockwise rotation of the nitrousxide gear via the link chain causes the gear on the oxygenpindle also to rotate counter-clockwise. With sufficientotation, the oxygen gear will engage with the oxygen flowontrol knob, causing it to turn and thereby oxygen flow toncrease in proportion to that of nitrous oxide. The gears andhain thus serve to proportion the needle valve openings ofhe nitrous oxide and oxygen flow control needle valves.he system thus far described would be supply-pressureependent. The 75% nitrous oxide:25% oxygen proportion-ng is completed because the nitrous oxide needle valve isupplied via a second stage gas regulator that reduces ni-rous oxide pressure to a nominal 26 PSIG (adjusted asreviously described) before it reaches the flow controlalve, whereas the oxygen flow control valve is suppliedith oxygen at a pressure of 16 PSIG from a second stagexygen regulator. This arrangement allows the nitrous oxidend oxygen flow control valves to be set independently ofne another, but whenever what would be a nitrous oxideoncentration of more than 75% is unknowingly set, thexygen flow is automatically increased to maintain at least5% oxygen in the resulting gas mixture. This system in-reases the oxygen flow according to the nitrous oxideow that has been set. It should be noted that the Link-25ystem only interconnects the nitrous oxide and oxygenow control valves. If the anesthesia machine has flowontrols for other gases, such as helium or air, a gas mixtureontaining less than 25% oxygen could potentially be cre-ted at the flowmeter level.

In the Drager Narkomed machines, the oxygen ratioonitor (ORM) provides an audible alarm upon the setting

and delivery) of low concentration oxygen mixtures. Thexygen ratio monitor controller (ORMc) provides an ad-itional pneumatic interlock by means of a slave controlalve to maintain the delivery of at least 25% oxygen at theowmeter level when nitrous oxide and oxygen are beingsed. At oxygen flow rates of less than 1 L/min, even higheroncentrations of oxygen are delivered.

Using a system of gas flow resistors and diaphragms, theRMc limits the flow of nitrous oxide according to the

xygen flow set to prevent the delivery of a hypoxic gasixture. In addition, an electrical alarm is activated when

he ORMc is acting to prevent a hypoxic mixture when therager Narkomed machine is used in the N2O/O2 mode. In

he Drager Narkomed (models 2, 3, and 4) machines de-igned to supply 3 or 4 gases, the mode may be changed by

switch from “N2O/O2” only to “ALL GASES.” Whensed in the “ALL GASES” mode, the ORMc continues toroportion the flows of N2O and O2, but the audible alarm,hich in the “N2O/O2” mode would sound when the ORMcas acting to prevent a hypoxic mixtures, is disabled. In

ddition, when in the “ALL GASES” mode, any minimumxygen flow provision (eg, 200 ml/min) is non-operational.

It should be noted that the Drager ORM and ORMc
ystems differ from the Datex-Ohmeda Link-25 proportion- t
ng system in a number of ways. First, the Drager systemoes not require second stage oxygen and nitrous oxideegulators. Second, the Drager ORMc serves to limit theow of nitrous oxide according to the set flow of oxygen,hereas the Ohmeda system increases the flow of oxygens the nitrous oxide flow is increased. Like the Datex-hmeda system, the Drager ORMc only functions betweenitrous oxide and oxygen, and there is no interlinking ofxygen with other gases such as air or helium that mightlso be supplied by the machine. An oxygen analyzer inhe patient circuit is therefore essential if a hypoxic gasixture is to be detected and thereby prevented.

resh gas flow, outlet check valve, lowressure system relief valve

he oxygen and nitrous oxide flows, arriving to the tops ofheir respective rotameters, merge in a common manifoldnd pass to anesthetic agent vaporizer where a potent in-aled volatile agent may be added. Vaporizer function isiscussed elsewhere. The resulting gas mixture then flows tohe machine’s common gas outlet. Between the vaporizer(s)nd the common gas outlet, some models of Datex-Ohmedaachines (Modulus I, Modulus II, and Excel) have (1) an

utlet check valve (Figure 1, item E), and (2) a pressureelief that opens when the pressure in the low pressureystem of the machine exceeds 135 � 15 mm Hg (2.6 PSIG)Figure 1, item D). The pressure relief valve, as its nameuggests, prevents the build-up of excessive pressures up-tream of the common gas outlet. These components areocated upstream from where the oxygen flush flow wouldoin to pass to the common gas outlet (Figure 1).

The purpose of the outlet check valve, where presentFigure 1, item E; Datex-Ohmeda Modulus I, Modulus II,xcel models, but not Modulus II Plus or Modulus CD), is

o prevent reverse gas flow. This situation could cause gaso go back into the vaporizer (if the latter did not have itswn outlet check valve or specialized design to prevent apumping effect”) and might result in increased vaporizerutput concentrations. Drager Narkomed machines are de-igned so as not to require an outlet check valve; anyumping effect is limited by a special vaporizer design. Theatex-Ohmeda Modulus II Plus machine does not use anutlet check valve. This machine is equipped with vaporiz-rs that incorporate a baffle system and specially designedanifold to prevent the pumping effect, making an outlet

heck valve unnecessary. The Modulus II Plus does have aressure relief valve, however. Drager Narkomed machineesign does not require a separate pressure relief valve. Inhese machines, pressure relief, if required, takes placehrough the specially designed Drager Vapor vaporizers.he presence or absence of an outlet check valve andressure relief valve is significant when it comes to leak-
esting the low pressure system of the anesthesia machine.

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ommon gas outlet

The anesthesia machine “ends” at the common gas out-et. This is where the gas mixture created at the flowmeters,lus any potent inhaled anesthetic agent added by the va-orizer, exits the machine and enters the fresh gas hose thatonducts it to the circle breathing system. The common gasutlet has an internal diameter of 15 mm (same as theracheal tube connector). A disconnection at this locationould result in anesthesia gases being spilled into the op-

rating room atmosphere, and would create a significanteak in the circle breathing system leading to collapse of theeservoir bag or ventilator standing bellows. To preventccidental disconnection of the fresh gas hose connectorrom the common gas outlet, all contemporary machines arequipped with a manufacturer-specific retaining device.

etection of leaks in the machine low pressureystem

The pre-use checkout of the anesthesia machine includeserforming a leak test of the low pressure system. A leak inhose parts of the machine between the flow control needlealves and the common gas outlet may have a number ofauses including a cracked rotameter tube, failure of aubing connection, and a leak in the anesthesia vaporizer. Aeak in the oxygen rotameter tube might cause a hypoxic gasixture. A leak in an anesthesia vaporizer might result in

atient awareness. For details of how to perform a check onspecific model of machine, the reader should consult theanufacturer’s Operation and Maintenance (O&M) Manual

or that machine. It is important to understand the principlesnderlying the leak checking of the low pressure system.

A positive pressure leak check may be used on aachine that does not have an outlet check valve. With gasows and all vaporizers off, a known amount of positiveressure (usually �50 cm H2O) is applied via the commonas outlet. The absence of an outlet check valve permits thisositive pressure to be applied to the low pressure system.he manufacturer (Drager for their Narkomed 2, 3, and 4odel machines) specifies how much of a decrease in pres-

ure over a certain time period is acceptable. The test is thenepeated with each vaporizer turned ON in turn. This isecause in the OFF position the vaporizer is excluded fromhe positive pressure leak check.

A negative pressure leak check is used in a machinehat has an outlet check valve (ie, some of the Datex-hmeda models described above). With gas flows and allaporizers off, a negative pressure leak check device isvacuated and then connected to the common gas outlet via15 mm diameter connector. This device is a squeeze bulb

hat, when evacuated, applies a negative pressure (�65 cm

2O) to the low pressure system. If there is a leak in the lowressure system, the squeeze bulb will reinflate. The pres-nce of an outlet check valve does not affect the perfor-
ance of this check. The negative pressure leak check is q
hen repeated with each vaporizer turned ON in turn to testor a leak in the vaporizer. The manufacturer’s manualhould be consulted for details as what is considered ac-eptable in terms of how rapidly the bulb may reinflateusually �30 seconds). Since the bulb has a volume of 15l, reinflation in 30 seconds would indicate a leak of 30l/minute.Reference to Figure 1 makes it apparent that the negative

ressure leak check could be used on machines that have oro not have an outlet check valve. The 1993 FDA’s pre-useheckout of the anesthesia machine2 describes the use of aegative pressure leak check device on all machines (ie,ith or without an outlet check valve) and that the bulb

hould not inflate in 10 seconds.

revention of hypoxic mixtures

revention of delivery of a hypoxic mixture to the commonas outlet is one of the major considerations in the design ofodern machines. A number of the safeguards have been

escribed above. Gas delivery systems may be compro-ised. Pipeline crossovers, erroneously filled tanks, and

nauthorized modifications to the machine could result in aas other than oxygen reaching those parts of the machineesigned to receive only oxygen. The systems that interrupthe supplies of other gases to their flowmeters when the gasupply pressure in the oxygen parts of the machine is de-reased (fail safe, pressure sensor shut-off valves, OFPDs)re pressure-sensitive. They do not qualitatively identify asxygen the gas on the oxygen side of the safety device. Asong as this pressure is adequate, flow of the other gases isossible. Contemporary proportioning systems may fail torevent a hypoxic mixture if a 3 or 4 gas machine is beingsed, since at present they proportion only N2O and O2.

All anesthesia delivery systems must have a functioningxygen analyzer that is equipped with an audible alarm tolert to low concentrations of oxygen. On most machines,he oxygen sensor is a fuel cell located in the proximity ofhe inspiratory unidirectional valve in the anesthesia circlereathing system. In principle, this analyzer is a battery, theutput of which is related to the oxygen tension (PO2) in theas mixture to which it is exposed. It is qualitative forxygen and is not affected by other gases. Although it isalibrated to read 21% on exposure to room air, it is actuallyeing calibrated to the PO2 in room air, which at sea levels 159 mm Hg (760 mm Hg � 21% � 159 mm Hg).uppose that an oxygen fuel cell analyzer were exposed tooom air at sea level, calibrated to 21%, and then used atltitude where the barometric pressure was less than 760m Hg. The reading shown would be less than 21%, even

hough oxygen constitutes 21% of the atmosphere at thatltitude, because the ambient PO2 would be less than 159m Hg.The oxygen analyzer is the only device that confirms

ualitatively the presence of oxygen, and does so in the gas

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ixture that flows to the patient’s airway via the inspiratoryimb of the circle breathing system. There is no analyzer onhe machine to confirm that the gas that enters the oxygenigh pressure system, or leaves via the auxiliary oxygenowmeter or other oxygen take-off (to drive the anesthesiaentilator or to power a Sanders injector jet ventilator), isctually oxygen. Thus, in the event of a pipeline crossoveretween nitrous oxide and oxygen, nitrous oxide wouldow from the auxiliary oxygen flowmeter!

onclusion

his review has briefly described how oxygen and nitrousxide are stored, how they enter the machine’s high pressureystem, the components that control their flows in the pathshat they take, and how they exit the machine at the commonas outlet.

When it comes to patient safety in regard to the machine,
emember that the most important item on the machine
re-use checkout is to ensure that there is immediatelyvailable and functioning an alternative means for ven-ilating the patient’s lungs. If you believe that there maye a problem with the machine or breathing system, alwaysonsider disconnecting the patient from the machine andreathing system and ventilate the lungs using a previouslyested self-inflating resuscitation bag (eg, Ambu bag). Inhis way, a machine failure (which is rare) or breathingystem misconnect or obstruction (which is less rare) shouldot result in an adverse outcome for the patient.

eferences

. Schumacher SD, Brockwell RC, Andrews JJ, et al: Bulk liquid oxygensupply failure. Anesthesiology 100:186-189, 2004

. U.S. Food and Drug Administration: Anesthesia apparatus checkoutrecommendations 1993. Available at: http://www.fda.gov/cdrh/humfac/
anesckot.pdf. Accessed June 16, 2005.
http://www.fda.gov/cdrh/humfac/anesckot.pdf

http://www.fda.gov/cdrh/humfac/anesckot.pdf

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Anesthesia machine basics