Chapter 22. Gas Monitoring

8/11/2019 Chapter 22. Gas Monitoring

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View Figure

Figure 22.1Multipurpose monitor. Most gasmonitors are now part of a physiologic monitor thatincludes other monitoring such aselectrocardiograph, blood pressure, pulse oximetry,and the like. A gas monitor may also be part of theanesthesia machine. Newer anesthesia machines

have one or more screens to display monitoredfunctions, and the gas concentrations and waveforms

may also be displayed.

A nondiverting(mainstream, direct probe, f low through, in-l ine, on airway,

nonsampling monitor) monitormeasures the gas concentration at the

sampling site.

A diverting(sidestream, withdrawal, sampling, aspirat ing, snif fer, sampled

system monitor) transports a port ion of the gas being measured from the

sampling site through a sampling tube to the sensor, which is remote from

the sampling site.

The sampling site (sensing site) is the location from which gas is d iverted for

measurement in a divert ing monitor or the location of the sensor in a

nondiverting monitor.

The sampling tube ( inlet l ine, sample gas transport tube, sample capil lary

tube, sampling catheter or tube, transport tube, aspirat ing tube, sample l ine)

is the conduit for transferring gas from the sampling site to the sensor in a

divert ing gas monitor.

Gas levelis the concentrat ion of a gas in a gaseous mixture. I t may be

expressed ei ther as part ial pressure or volumes percent.

The part ial pressure of a gas is the pressure that a gas in a gas mixture

would exert i f i t alone occupied the volume of the mixture at the same

temperature.

The volumes percent(%, V/V, vol %) of a gas is the volume of a gas in a

mixture, expressed as a percentage of the total volume.


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Monitor Types

There are two general types of monitors in c l inical use: divert ing (sidestream) or

nondiverting (mainstream) (2,3). These refer to the measurement site of the gases

and not to the technology being used. Both can be integrated into a single module.

Nond i v e r t i n g

A nond iv ert ing gas mo ni tor meas ure s th e ga s by us ing a sensor loc ated direc t ly in

the gas stream. Only oxygen and carbon dioxide (CO2) can be measured by

nondivert ing monitors.

Carbon dioxide is measured by infrared technology with the sensor located between

the breathing system and the patient (Figs. 22.2, 22.3 ). A nondivert ing monitor is

available for the non-intubated patient, in which the sensor attaches to a

disposable oral and nasal adaptor.

The mainstream oxygen sensor uses electrochemical technology. I t is usually

placed in the breathing system inspiratory l imb. I f the technology is fast enough to

measure both inspired and exhaled oxygen, it should be placed between the patient

and the breathing system. Chapter 9discusses possible locations of the ox ygen

monitor sensor in the circle breathing system.

Advantages

Mainstream CO2 monitors have fast response times because there is no

delay t ime. The CO2 waveform generated has better fidelity than one

generated by a diverting monitor.

Because no gas is removed from the breathing system, it is not necessary to

scavenge these devices or to increase the fresh gas f low to compensate for

gas removed from the breathing system.

Water and secret ions are seldom a problem with this type of analyzer,

although secret ions on the windows of the cuvette used for mainstream CO2 monitoring can cause e rroneous readings. Water and secret ions

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are generally not a problem with oxygen sensors, as they are usually on the

inspiratory side of the breathing system.


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View Figure

Figure 22.2Nondiverting gas monitor. A:Thesensor is in position over the cuvette, which is

placed between the patient and the breathing system.The two clear tubings to the left are for spirometryloops (Chapter 23). B:The sensor is separated fromthe cuvette, which contains the window through

which the infrared light passes. C:Calibration cells.For convenience, they are attached to the cable to

the sensor. During calibration, the sensor is placedover each cell in sequence.

Sample contamination by fresh gas is less l ikel y than with a divert ing

monitor.

A s ta ndard gas is no t req uired fo r cal ibra ti on . Ox yg en sens ors are us ua ll y

calibrated by using room air.

These monitors use fewer disposable items than diverting monitors.

Disadvantages

To obtain accurate end-t idal CO2 values, the airway adaptor must be placed

near the patient. The sensor will add weight to the breathing system and may

cause traction on the airway device or breathing tubes.

The use of an adaptor between the patient and the breathing system will

increase dead space. However, studies show that end-t idal CO 2 values

obtained by using a mainstream analyzer with a pediatric adapter in healthy

neonates and i nfants are close to arterial values (4).

Leaks, disconnections, and circuit obstruct ions can occur ( 5,6 ,7 ,8).

With a mainstream CO2 monitor, condensed water, secret ions, or blood on

the windows of the cuvette wil l interfere with l ight transmission.

With a mainstream CO2 monitor, the sensor may become dislodged from the

cuvette. I f i t is completely dislodged, no waveform wil l be seen. I f i t is


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slight ly dislodged (Fig. 22.4), the reading may be incorrect although the

waveform will appear normal (9 ,10 ,11).

The expensive optical sensor for CO2 is vulnerable to costly damage.

At present, ma ins tream monito rs can meas ure only ox yg en and CO2.

The adaptor for the CO2 sensor must be cleaned and disinfected between

uses. There is potential for cross contamination between patients if this is

not done properly. Disposable adaptors are available but increase the cost.

Thermal burns have been reported with a mainstream CO 2 analyzer despite

use of mult iple layers of gauze, which kept the sensor from direct contact

with the skin (12). To prevent this, i t may be necessary to interpose a piece

of aluminum foil between two pieces of soft material to ref lect the radiant

energy.

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View Figure

Figure 22.3Mainstream infrared analyzer. A:Side

view. The light source and detector are housed in thesensor, which fits over the cuvette. The infrared light

shines through the windows of the cuvette and isdetected by the photosensor. B:Cross-sectionalview. Gases pass through the airway adaptor(cuvette). The infrared light that is transmittedthrough the windows is filtered and then detected bythe photodetector in the sensor.

Prolonged contact of the CO2 sensor assembly with the patient could cause

pressure injury.

D i v e r t i n g


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A diverting moni to r us es a pump to as pira te gas from th e sam plin g site th rough a

sampling tubing to the sensor that is located in the main unit . Keeping the sampling

tube as short as possible wil l decrease the delay t ime and result in more

satisfactory waveforms. These analyzers a re usually zeroed using room a ir and

calibrated using a gas of known composit ion. A mainstream monitor with divert ing

capabil i ty is shown in Figure 22.5. Gas is aspirated through a special cuvette and is

analyzed by the sensor.

To avoid water or part iculate contamination in the monitor, a number of devices

have been used. These include traps (Fig. 22.6) (which must be emptied

periodically), f ilters and hydrophobic membranes (which must be changed

periodically), and special tubing (which allows water to diffuse through its walls)

(13,14).

Water droplets and secret ions from the breathing system can enter the sampling

tube and increase resistance in the tubing, affect ing the accuracy. Some

instruments either increase the sampling f low or, to clear the contaminant from the

tube, reverse the f low (purge) when they sense a drop in pressure from a f low

restrict ion (15 ). I f this fai ls, the sampling port and/or the tube must be replaced.

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View Figure

Figure 22.4Mainstream infrared CO2analyzer with thesensor not completely covering the windows of the cuvette.This can result in falsely low CO2readings.

Accurac y decre as es wi th inc reasing res pira tory rate and lo nger sampling l ines (16 ).

Most diverting capnometers are accurate at those respiratory rates that are


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normally encountered in c l inical pract ice (20 to 40 breaths per minute). At higher

respiratory rates, accuracy is lower.

The sampling f low rate should be proport ional to the s ize of the patient. I t has been

recommended that a flow rate less than 150 mL/minute should not be used because

a low sampling f low may result in an elevated baseline, erroneously lo w peak

readings, and absence of an end-t idal plateau (Fig. 22.36), especially when the

respiratory rate is fast and t idal volume is small (17 ). A high f low rate wil l decrease

the delay and rise t imes but may cause fresh gas to be entrained into the sample

line with some breathing systems. This wil l result in incorrect end-t idal readings

and a capnogram with a decrease in CO 2 at the end of the expiratory plateau (Fig.

22.37).

View Figure

Figure 22.5Mainstream infrared CO2analyzer used as adiverting monitor. Gas is drawn through the cuvette by a

pump.

Devices

Fa c e Ma s k

A face mas k ha s a relatively large dead spac e rela ti ve to t ida l vol ume , mak ing i t

more dif f icult to obtain accurate end-t idal values. Figure 22.7shows a mask with a

sampling l ine for CO 2 . A sampling catheter can also be attached to the upper lip or

placed in the patient 's nares or the lumen of an oral or nasopharyngeal airway

under the mask (18 ). With a breathing system, the sampling tube is most often

attached t o a c omponent between the mask and the breathing system (Fig. 22.8), or

the

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The sampling site should be away from the fresh gas port. When a Mapleson

breathing system is used, continuous inf low of fresh gas that is close to the

sampling site can cause erroneous readings and an abnormal waveform (Fig.

22.37).

Tracheal tubes that incorporate a sampling lumen that extends to the middle or

patient end of the tube are available (Chapter 19). Tracheal tube connectors with

an attachment or hole for a sampling tube are available or can be created ( 21 ).

These may result in measurements that more closely approximate alveolar values,

especially in small pat ients and with breathing systems in which the fresh gas f low

can mix with exhaled gases (22 ,23,24,25,26).

View Figure

Figure 22.8Ports for gas sampling in breathing systemcomponents.

Su p r a g l o t t i c D e v i c e

With a supraglott ic airway device, a sampling tube can be inserted through the

connector (27,28). The preferred sampling site is the distal end of the shaft (29,30 ),

but in most patients, sampling at the connection to the breathing system wil l result

in sat isfactory readings (31,32 ,33 ,34,35 ).

A samp ling tu bing ma y be in serte d into a nasal airway (36,37 ,38).

Ox y g e n S u p p l eme n t a t i o n Dev i c e s

A re lat ivel y ne w dev ice , th e Ox yA rm, al lo ws simulta ne ous administ ra t ion of ox ygen

and carbon dioxide monitoring (39,40). I t consists of a headset that traverses

across the top of the head, oxygen supply and CO2sampling l ines attached to an

adjustable boom and a disposable arm dif fusor. I t can be used to administer oxygen


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and monitor CO2 in both nose and mouth breathers. A n asal cannula can be

modif ied to accept a sampling tubing

(39,40,41,42 ,43 ,44,45 ,46 ,47,48 ,49 ,50,51,52,53,54,55 ,56,57 ,58 ,59,60 ,61 ,62 ,63 ,64 ,6

5,66 ,67). They are available in seve ral configurat ions (Figs. 22.9, 22.10). Mouth

breathing, airway obstruct ion, and oxygen delivery through the ipsilateral nasal

cannula can affect accuracy (68 ). Caution should be observed in adapting a nasal

cannula; a piece may become dislodged and present a choking hazard (69 ).

A plas t ic ox yg en mask ma y be fi t ted wi th a samp l in g po rt (39,70 ) (Fig. 22.7).

Al te rnati vel y, th e sampl ing tub e ma y be conn ec ted to th e ma sk outl et (71), inserted

through a vent hole (42 ,57,72,73,74,75,76) or a sl it in the mask (77 ), or sl ipped

under the mask and attached near the nostri ls ( 37 ,78 ,79,80 ).

J e t Ve n t i l a t i o n

During jet venti lat ion, an injector incorporat ing a sampling lumen (81,82 ,83 ,84) or a

sampling tube placed in the airway (85,86) may be used. The venti latory frequency

may need to be lowered to measure the end-tidal CO 2 ( 83,87 ,88 ).

O t h e r

The end of a sampling l ine can be placed in front of or inside the patient 's nostri l

(89,90) or a nasopharyngeal airway (91). I f the patient is a mouth breather, the

sample l ine can be placed in front of the mouth or in the nasopharynx (89 ,92 ) orhypopharynx (93,94). A catheter can be placed in the trachea after extubation for

CO2 monitoring (95 ). A bite block can be modif ied to accommodate a sampling l ine

(96). A sampling l ine can be placed over a tracheostomy stoma (97).

Optimal placement should be determined by the CO 2 waveform. Mucosal irritation,

catheter blockage, and mechanical interference sometimes cause problems.

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View Figure

Figure 22.10This device is used for CO2sampling innonintubated patients who are either exhaling by mouth ornose.

Disadvantages

Problems with the sampling system (leaks, s ampling tube obstruct ion, or

failure of the aspirator pump) can occur. Particulate matter, blood,

secretions, or water can obstruct the tubing (102). The sampling l ine can be

connected to the wrong place (10 3,10 4). I f there is a leak i n the sample l ine,air wil l be added to the sample. This wil l di lute the sample and reduce the

values of end-t idal CO2 and anesthetic agents (10 5,10 6,107) (Fig. 22.29).

The sampling tube can kink, but this can be prevented by using an elbow

connector near the attachment to the patient end (10 8).

The aspirated gases must be either routed to the scavenging system or

returned to the breathing system. I f scavenging is employed, the fresh gas

flow may need to be increased to compensate for the gas removed or

negative pressure wil l be created in the breathing system (109 ).

In some divert ing monitors, room air used in the calibrat ion process is added

to the gas ex it ing the monitor. I f this air is returned to the breathing system,

it wil l c reate problems during closed c ircuit anesthesia.

Some delay t ime is unavoidable.

A supply of cal ib ra ti on gas mus t be av ailab le.

A numb er of disposable i tems (ad apto rs and cathete rs ) must be used.

There may be deformation of the waveform and erroneously low CO2

readings from the fresh gas dilut ion (Fig. 22.37).


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Compared with mainstream monitoring, sidestream measurements produce

more variable dif ferences between arterial and end-t idal CO 2 levels (110 ).

Technology

There are a number of dif ferent technologies available to measure respiratory and

anesthetic gases.

In f r a r e d An a l y s i s

Infrared analysis is by fa r the most common technology in use today (3,11 1,11 2).

Technology

Infrared (IR) analyzers are based on the principle that gases with two or moredissimilar atoms in the molecule (nitrous oxide, CO 2 , and the halogenated agents)

have specif ic and unique infrared l ight absorpt ion spectra. Since the amount of

infrared l ight absorbed is proport ional to the c oncentrat ion of the absorbing

molecules, the concentrat ion can be determined by comparing the infrared l ight

absorbance in the sample with that of a known standard. The nonpolar molecules of

argon, nitrogen, helium, xenon, and oxygen do not absorb infrared l ight and cannot

be measured using this technology.

There are two general types of infrared technology available today.

B l a c k b o d y R a d ia t i o n T ec h n o l o g y

The most commonly used infrared technology ut i l izes a heated element called a

blackbody emitter as the source of infrared l ight (113). This produces a broad

infrared spectrum. The majority of the emitted radiat ion is redundant and must be

removed. Filters block radiation that is outside the desired range. This method

cannot remove the radiat ion that fal ls between discrete absorbing l ines because of

the continuous emission nature of the blackbody. The optical detectors must be

calibrated to recognize only infrared radiat ion that is modulated at a certain

frequency by using a spinning chopper wheel.

The analyzer selects the appropriate infrared wavelength, using an individual f i l ter

or a f i l ter wheel to maximize absorption by the selected gas at its peak wavelength

and to minimize absorption by other gases and vapors that could interfere with

measurement of the desired component. Some older infrared units are equipped

with a dial or switch to select the anesthetic agent being measured, while others

require a dif ferent f i l ter and scale to measure each agent. Most units in use today

can recognize the agents that can be monitored with this technology. After the


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sensor detects the transmitted infrared energy, an electrical signal is produced and

amplif ied, and the concentrat ion is displayed.

Monitors that identify and quantify halogenated agents use a separate chamber to

measure absorption at several wavelengths. Typically, these are single-channel,

four-wavelength infrared f i l ter photometers. There is a f i l ter for each anesthetic

agent and one to provide a

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baseline for comparison. Each f i l ter transmits a specif ic wavelength of i nfrared

light, and each gas absorbs dif ferently in the selected wavelength bands (114).

View Figure

Figure 22.11Sidestream optical infrared analyzer. A beamof infrared light is at one end, and a photodetection device is

at the other. The chopper wheel contains several filters,which are divided into sections that will allow passage of

only the frequencies most readily absorbed by the gases tobe measured. The filtered and pulsatile infrared light is

directed through both the sample chamber and a referencechamber with no absorption qualities. The amount of

infrared light absorbed at each frequency depends on thegas level in the sample chamber.

Most infrared instruments have an accuracy of 0.2% for CO 2 concentrat ions over

the range of 0% to 10% and 2.0% for nitrous oxide concentrat ions from 0% to

100%. For typical halogenated agents, the accuracy is 0.4% over a range of 0% to

5% (11 5). Most investigators believe that these monitors are suff icient ly accurate

for cl inical purposes (11 6,117,11 8), although they tend to underestimate the

inspired level and overestimate end-t idal values at h igh respiratory rates.

Diverting

Figure 22.11shows a div ert ing (side-stream) infrared analyzer. Infrared l ight is

continuously focused on a spinning (chopper) wheel. The wheel has holes with

f i l ters specially selected for the gases to be measured. The gas to be measured is

pumped continuously through a measuring chamber. The f i l tered and pulsed l ight is

passed through the sample chamber and also through a reference chamber with no


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absorption characterist ics. The l ight is then focused on an infrared photosensor.

The amount of l ight absorbed by the sample gas is proport ional to the part ial

pressures of gases whose infrared light absorption patterns correspond to the

wavelengths selected by the f i l ters on the chopper wheel. The changing l ight levels

on the photosensor produce changes in the electrical current that runs through it.

Rotat ing the wheel thousands of t imes per minu te provides hundreds of readings for

each respiratory cycle. For pract ical purposes, the waveform on the display is

continuous.

Monochromatic sidestream optical infrared analyzers use one wavelength to

measure potent inhalat ional agents and are unable to dist inguish between agents o r

to detect a mixture of agents (119 ). When such an analyzer is used, the clinician

must select which agent is to be monitored. I f an incorrect agent is selected,

incorrect values wil l be reported (12 0,121,12 2). Polychromatic infrared analyzers

use mult iple wavelengths to both identify and quantify the various agents (12 3).

This eliminates the need for the user to select the agent to be monitored and allows

a mixture of agents to be detected.

Most sidestream analyzers have a f ixed sampling f low rate, al though some permit

select ion of the f low rate. The measuring cell is calibrated to zero by using gas that

is free of the gases of interest (usually room air) and to a standard level by using a

calibrat ion gas mixture.

Nondiverting

With a nondiverting (mainstream) CO 2 monitor, the gas stream passes through a

chamber (cuvette) with two windows that are transparent to infrared light (Fig.

22.2). The cuvette is placed between the b reathing system and the patient. The

sensor, which houses both the l ight source and detector, f i ts ov er the cuvette. To

prevent water condensation, the sensor is heated sl ight ly above body temperature.

Infrared light shines through the window on one side of the adaptor, and the sensor

receives the light on the opposite side. After passing through the sample chamber,

the l ight goes through three ports in a rotat ing wheel, which contains (a) a sealed

cell with a known high CO 2 concentration,(b) a chamber vented to the sensor's

internal atmosphere, and (c) a sealed cell containing only nitrogen (Fig. 22.2B).

The radiat ion then passes through a f i l ter that sc reens the l ight to the correct

wavelength to isolate CO2information from interfering gases and onto a

photodetector. The signal is amplif ied and sent to the display module.


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Calibrat ion is performed by using two sealed cells in a plast ic unit that attaches to

the control unit (Fig. 22.2C). I t is shaped so that the sensor can clip over e ither

cell. The low c alibrat ion cell contains 100% nitrogen, while the high cell c ontains a

known part ial pressure of

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CO2 . Correct ions for nitrous oxide and/or oxygen must be en tered manually.

The sensor may become dislodged from the cuvette. I f i t is completely dislodged,

no waveform wil l be seen. I f i t is sl ight ly dislodged (Fig. 22.4), the readings may be

incorrect although the waveform appears normal (9,10 ). Condensed water,

secret ions or blood on the cuvette windows wil l interfere with l ight transmission and

cause erroneous readings (124).

M i c r o s t r eam T ec h n o l o g y

Microstream technology utilizes laser-based technology to generate infrared

emission that precisely matches the absorption spectrum of CO2(113). I t ut i l izes a

smaller sample cell and a low f low rate (50 mL/minute).

The emission source is a glass discharge lamp without an electrode that is coupled

with an infrared transmitting window. Electrons that are generated by a radio

frequency voltage excite nitrogen molecules. Carbon dioxide molecules are then

excited by coll ision with the excited nitrogen molecules. As the exc ited CO2

molecules drop back to their ground state they emit the signature wavelength of

CO2 .

The emission is split so that one part is di rected to the main optical detector via the

gas sample cell while the other part passes through a reference detector. This

channel is used as a continuous reference detector, compensating for changes in

infrared output.

The infrared source is electronically modulated so that measurements are made

every 25 msec. This provides a rapid response t ime. The amplitude of the signals

received by the detector depends on the amount of radiat ion absorbed from the gas

sample. The absorbed radiation is proportional to the CO2 c oncentrat ion.

The airway adaptor has three channels with narrow hydrophobic openings, each

facing a dif ferent direct ion. This permits the adaptor to be used in any orientat ion

and prevents the sample l ine from being occluded by water or secret ions. The

sample l ine has a hydrophobic f i l ter. A water trap is not necessary.


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Because of the low sample f low and s mall sample cell, this technology is useful for

measuring CO2 in very small pat ients, high respiratory rates, low-f low applicat ions,

and unintubated patients. Readings are not affected by high concentrat ions of

oxygen or anesthetic gases.

Advantages of Infrared Analysis

Mu l t i g a s Ca p a b i l i t y

Infrared analyzers are capable of measuring CO 2 , nitrous oxide, and all of the

commonly used potent volatile agents.

Vo l at i l e A g e n t D e t ec t i o n

Al though mo noc hromatic ana lyzers are unab le to ide nti fy anesth eti c ag en ts an d

mixtures of agents, most newer models provide agent detect ion and can detect and

quantify mixtures. Analyzers handle mixtures of agents in dif ferent ways. They may

give a display saying that there is a mixture of agents or may compensate for the

addit ional agent.

View Figure

Figure 22.12Microstream infrared analyzer. Smallhandheld device. (Picture courtesy of Oridion Medical.)

No Ne e d t o S c a v e n g e Ga s e s

Af ter measure ment, the ga ses can be return ed to the brea thing sys tem, if de si red.

Po r t a b i l i t y

The units (Fig. 22.12) are small, compact, and l ightweight. They may be

incorporated into an anesthesia machine or physiologic monitor and be used in

remote areas of the facil i ty.


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Qu i c k Re s p o n s e T im e

The response t ime is fast enough to measure both inspired and exhaled

concentrat ions. Response t imes for anesthetic agents and nitrous oxide are longer

than for CO2(115 ).

Sh o r t Wa rm - u p T im e

The warm-up t ime is short. The instruments do not need to be kept i n a standby

mode.

Co n v e n i e n c e

Al though ea rl y uni ts require d a comp licated cal ibra ti on wi th te st ga ses wi th ea ch

use, newer units only require periodic calibrat ion with a s tandardized gas mixture.

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L a c k o f I n t e r f e r e n c e f r om O t h e r G as e s

Argo n or low concen tra ti ons o f ni tr ic ox id e do not interf ere wi th vol at i le ag en t

monitoring by infrared analyzers (12 5,126). Infrared spectrometry is accurate in the

presence of 5% CO2 (12 7).

Det ec t i n g A n e s t h et i c B r e ak d o w nAgent-identifying inf ra red ana lyze rs ma y pro vide warn ing of des fl ura ne breakdown

that produces carbon monoxide by d isplaying wrong o r mixed agents (12 8).

Disadvantages

Ox y g e n an d N i t r o g e n n o t Me as u r e d

Oxygen and nitrogen cannot be measured by infrared technology.

Gas I n t e r f e r e n c e

While oxygen is not absorbed by infrared l ight, i t causes broadening of the CO 2 absorpt ion spectra, which results in lower CO2 readings (115). In a typical infrared

CO2 analyzer, 95% oxygen causes a 0.5% decline in measured CO 2 (101). Some

units have a user-actuated electronic offset for oxygen.

There is some overlap of the CO2 and nitrous oxide infrared absorption peaks so

that nitrous oxide can cause falsely high CO2 readings, with an increase of 0.1 to

1.4 torr per 10% nitrous oxide. Most infrared analyzers that measure both CO 2 and

nitrous oxide automatically co rrect for nitrous oxide's effect on the C O2 reading.

Some require the user to indicate when nitrous oxide is present.


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I f the analyzer is set to measure a v olat i le agent dif ferent from that present in the

gas mixture being analyzed, CO 2 and nitrous oxide as well as agent readings wil l

be incorrect (12 9). A mixture of agents can cause erroneous readings (13 0).

Desflurane may disturb the infrared CO2sensor so that it reads higher-than-

expected concentrat ions (13 1).

Helium in the gas mixture may cause the infrared analyzer to underestimate the

concentrat ion of CO2 (132).

In a c c u r a c y f r om O t h e r S u b s t a n c e s

Ethanol, methanol, isopropanol, diethyl ether, acetaldehyde, or acetone in sampled

gases can cause spuriously high volat i le agent readings

(122 ,133,134,135 ,136,13 7,138 ). Ether that is used to soak gauze that packed aprosthesis can cause a monitor to incorrect ly identify isof lurane (139).

Polychromatic analyzers are less affected, and some display a warning that the

interfering agent has been detected (11 8,123 ,140,141). Some analyzers de tect

halogenated propellants as anesthetic gases (142 ,143,14 4,145,146).

Methane, which can accumulate during low-f low anesthesia, causes inaccuracies

with monitors that use the 3.3 m wavelength range (14 7,148).

In t e r f er e n c e f r o m Wa t er V a p o r

Water vapor absorbs infrared l ight at many wavelengths and wil l cause increasedCO2 and volat i le agent readings (115 ). Monitors use special tubing, water traps,

f i l ters, and/or hydrophobic membranes to minimize this. Water that gets into the

monitor can cause expensive or irreversible damage (102).

S l o w R e s p o n s e T i m e

With rapid respiratory rates, the response time may be too slow to measure

inspired and end-t idal levels of volat i le agents accurately (14 9).

Rad i o F r e q u e n c y In t e r f e r e n c e

Handheld two-way radios in use near an infrared analyzer may cause CO 2 readings

to be increased (150 ).

D i f f i c u l t y A d d i n g New Vo l a t i l e A g e n t s

As new vola ti le ag en ts are ad ded to the anesth etic arm amenta rium , th es e mon ito rs

need to be revised to accept the new agents. This revision may require anything

from a software change to an expensive change to the analyzing bench. In some

cases, a correct ion factor can be used to convert one channel of a monitor to


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monitor another agent (151). In some cases, it wil l be necessary for the user to

manually select the agent being used (15 2,153 ).

Par amagne t ic O x y g e n Ana l y s i s

When introduced into a magnetic f ield, some substances locate themselves in the

strongest portion of the f ield (154 ,155). These substances are termed

para magne tic. Oxygen is the only paramagnetic gas that is important in anesthesia.

When a gas t hat contains oxygen is passed through a switched magnetic f ield, the

gas wil l expand and c ontract, causing a pressure wave that is proport ional to the

oxygen part ial pressure.

To obtain a high degree of accuracy, it is necessary to compare the pressure in the

gas sample with a reference signal that is obtained by using air o r oxygen. When

air is used as a reference gas, nitrogen may accumulate in the breathing system

during closed-circuit anesthesia if the reference gases are redirected to the

anesthesia ci rcuit . I f oxygen is used as the reference gas, the accumulat ion of

nitrogen is signif icantly reduced (156 ).

A paramagn etic ox yge n an al yze r is shown in Figure 22.13. Reference and sample

gases are pumped through the analyzer. The two gas paths are joined by a

dif ferential pressure or f low sensor. I f the sample and reference gases have

dif ferent part ial pressures of oxygen, the magnet wil l cause their pressures to

dif fer. This dif ference is detected by the transducer and converted into an electrical

signal that is displayed as oxygen part ial pressure or volumes percent.

The short r ise t ime allows both inspired and end-t idal oxygen levels to be measured

even at rapid respiratory rates. Many monitors combine infrared analysis of CO 2 ,

P.697

volat i le anesthetic agents, and nitrous oxide with paramagnetic oxygen analysis in

the same monitor using the same diverted gas (Fig. 22.1). This allows most gases

of interest to be monitored by a single monitor.


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View Figure

Figure 22.13Paramagnetic oxygen analyzer. A referencegas of known or no oxygen content and the gas whoseoxygen level is to be measured are pumped through theanalyzer and converge into a tube at the outlet. The two gas

paths are joined at their midpoints by a differential pressureor flow sensor. The magnet is switched on and off at a rapid

rate. Because the reference and sample gases have differentoxygen levels, the pressures in the paths will differ. The

pressure difference is detected by the sensor.

I f the sample gas from the analyzer is returned to the breathing system and air was

used as a reference gas, it wil l di lute the o ther gases and cause an increase in

nitrogen (10 1). This is especially a problem during closed-circuit anesthesia. I f

oxygen is used as the reference gas, the accumulat ion of nitrogen is s ignif icantly

reduced (156). Desflurane may disturb the paramagnetic oxygen sensor so that it

reads higher than expected (13 1). Failure of a paramagnetic oxygen analyzer has

been reported (157 ).

E le c t r o c h em i c a l O x y g e n An a ly s i s

An el ec tro che mi ca l ox yge n an alyze r consists of a sens or, wh ich is expo sed to the

gas being analyzed, and the analyzer box, which contains the electronic circuitry,

display, and alarms (Fig. 22.14). The sensor contains a cathode and an anode

surrounded by electrolyte. The gel is held in place by a membrane that is

nonpermeable to ions, proteins, and other such materials, yet is permeable to

oxygen. The membrane should not be touched, because dirt and grease reduce itsusable area. In most cases, the sensor is placed in the inspiratory l imb of the

breathing system.

Most of these analyzers respond slowly to changes in oxygen pressure, so they

cannot be used to measure end-t idal concentrations. Some newer sensors can

analyze oxygen quickly enough to measure inspired and exhaled concentrat ions.

Technology

Ga l v a n i c Ce l l ( Fu e l Ce l l , M i c r o f u e l Ce l l )


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Oxygen diffuses through the sensor membrane and electrolyte to the cathode,

where it is reduced, causing a current to f low (114,158 ,159 ,160). The rate at which

oxygen enters the cell and generates current is proport ional to the part ial pressure

of oxygen in the gas outside the membrane. For convenience, however, the display

scale is usually marked in percent oxygen. A gain control al lows the analyzer to be

calibrated with gas with a known part ial pressure of oxygen (usually air).

A gal van ic cell sensor is sho wn in Figure 22.15. I t consists of an anode and two

cathodes su rrounded by electrolyte. The cathode acts as the sensing electrode and

is not consumed. The hydroxyl ions formed there react with the lead anode, forming

lead oxide. The anode is g radually consumed.

Cathode: O2+ 2H2O + 4e-4OH

-

Anode: 4OH-+ 2Pb 2PbO + 2H 2O + 4e

-

P.698

View Figure

Figure 22.14Electrochemical oxygen analyzer. The sensoris connected by a cable to the analyzer box, which contains

the meter, alarms, and controls. A thermistor compensatesfor changes in oxygen diffusion caused by temperature. An

amplifier is present in the polarographic analyzer. Thosemonitors with manual calibration require adjustment of a

gain control until the correct reading is obtained for astandard oxygen concentration. Those with automatic

calibration simply require a button to be pressed in thepresence of a gas of standard concentration (usually air).

This puts the monitor into calibration mode, and it returns tonormal readings automatically when calibration is complete.

Since there are two cathodes, two voltages are generated. These are compared,

and if a certain amount of dif ference is present, the operator is prompted to check

the cell. Because the current is strong enough to operate the meter, a s eparate

power source is not required to operate the analyzer. A power source (either

battery or mains current) is required to power the alarms.


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The chemical reaction is temperature dependent. In order to compensate for

temperature differences, a temperature-dependent resistor (thermistor) may be

connected in parallel with the sensor.

View Figure

Figure 22.15Galvanic cell sensor. The membrane is

permeable to gases but not to liquids. At the cathode,oxygen molecules are reduced to hydroxide ions. At theanode, hydroxide ions give up electrons. An electron flow

between the anode and cathode is generated, which isdirectly proportional to the partial pressure of oxygen in thesample gas.

P.699

View Figure

Figure 22.16The life of a galvanic (fuel cell)electrochemical oxygen analyzer can be prolonged by

leaving it exposed to room air when not in use.

The sensor comes packaged in a sealed container that does not contain oxygen. I ts

l i fe span begins when the package is opened. I ts useful l i fe is cited in percent


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hours, which is the product of hours of exposure and oxygen percentage. I f i t is

exposed to a high oxygen concentrat ion, its l i fe expec tancy wil l be decreased.

Sensor l i fe can be p rolonged by removing it from the breathing system and

exposing it to air when not in use (Fig. 22.16). Galvanic sensors require no

membrane or electrolyte replacement. The whole s ensor cartr idge must be replaced

when it becomes exhausted (Fig. 22.17).

Po l ar o g r a p h i c E l e c t r o d e

A pol arog raphic (C lark elec tro de) sensor is shown in Figure 22.18. I t consists of an

anode, a cathode, an electrolyte, and a gas-permeable membrane. There is a power

source (battery or alternating current [AC] l ine) for inducing a potential between the

anode and the cathode.Oxygen molecules dif fuse through the membrane and electrolyte. When a polarizing

voltage is applied to the cathode, electrons combine with the oxygen molecules and

reduce them to hydroxide ions. A current that is proport ional to the oxygen part ial

pressure in the sample f lows between the anode and cathode.

Polarographic sensors may be either preassembled disposable cartr idges or units

that can be disassembled and reused by changing the membrane and/or electrolyte.

Use

Calibration

Calibrat ion should be performed daily before use and at least every 8 hours after

that. Some instruments remind the user when calibrat ion is needed and wil l not give

a reading unti l calibrat ion is performed (Fig. 22.19). The calibrat ion can be checked

by exposing the sensor to room air and verifying that i t indicates approximately

21% oxygen.

Checking the Alarms

The sensor should be put i n room air and the low oxygen alarm set above 21%. The

visual signal should f lash, and the audible alarm should sound. I f the unit has a

high oxygen alarm, the sett ing for that should be moved below 21%. Both visual

and audible signals should be act ivated. I f the v isual signal fai ls or the audible

signal is weak, the batteries should be replaced and the alarms rechecked. I f this

fails to remedy the problem, the unit should not be used.


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View Figure

Figure 22.17Galvanic cell sensor. The entire sensor mustbe replaced when it becomes exhausted.

P.700

View Figure

Figure 22.18Polarographic sensor. Oxygen diffusesthrough the membrane and electrolyte to the cathode. When

a polarizing voltage is applied to the cathode, the oxygenmolecules are reduced to hydroxide ions. The current flow

between cathode and anode will be proportional to thepartial pressure of oxygen. (Redrawn fromBageant RA. Oxygen analyzers. Respir Care 1976;21:415.)

Placement in the Breathing System

Sites for placing the sensor in breathing systems a re discussed in Chapter 9 . The

sensor service l i fe of some galvanic cell analyzers is reduced by exposure to CO2 ,

so locating the sensor on the inspiratory side of the system may be preferable. The


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sensor should be upright or t i l ted s light ly to prevent moisture from accumulating on

the membrane. The junction between the cable and the sensor should not be under

strain.

Setting Alarms Limits

The low oxygen alarm should be set a l i t t le below the minimum and the high oxygen

level alarm a l i t t le above the maximum acceptable concentrat ions. There should be

places on the anesthesia record for recording the alarm set points and oxygen

percentage.

Advantages

Eas y t o U s e

Electrochemical oxygen analyzers are dependable, accurate, and user-fr iendly.

Galvanic analyzers may be more reliable than polarographic analyzers (16 1).

L o w C o s t

These instruments cost less than other means of oxygen analysis.

Comp a c t

Compared with other technologies for measuring oxygen, the electrochemical

analyzer takes up l i t t le space.

No E f f ec t f r o m A r g o n

Argo n does no t af fec t galvan ic cel l mon itori ng (126 ).

Disadvantages

Ma i n t e n a n c e

While maintenance on newer models has been simplif ied, some instruments need

frequent membrane and electrolyte changes. Polarographic monitors require more

maintenance than the galvanic cell monitors (161 ).


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View Figure

Figure 22.19The display on the anesthesia machineprovides a reminder that the oxygen analyzer needs to becalibrated.

P.701

Ca l i b r a t i o n

These instruments need to be calibrated before use each day and at least every 8

hours.

Us e r E n a b l i n g

Instruments that are not an in tegral part of anesthesia machines need to be turned

on by the user.

S l o w R e s p o n s e T i m e

Most of these analyzers cannot be used to measure end-t idal oxygen.

P i ez oe l ec t r i c An a ly s i s

TechnologyThe piezoelectric method uses vibrat ing crystals that are coated with a layer of l ipid

to measure volat i le anesthetic agents ( 158,162,163 ,164 ) (Fig. 22.20). When

exposed to a volat i le anesthetic agent, the vapor is adsorbed into the l ipid. The

result ing change in the mass of the l ipid alters the vibrat ion frequency. By using an

electronic system consist ing of two oscil lat ing circuits, one of which has an

uncoated (reference) crystal and the other a coated (detector) crystal, an electric

signal that is proport ional to the v apor concentration is generated. Piezoelectric


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analyzers are divert ing devices. Some piezoelectric-based units have a separate

nondispersive infrared sensor to dif ferentiate inspirat ion and expirat ion.

Advantages

A c c u r a c y

Investigat ions show an accu racy of better than 0.1% (162 ,163). Water vapor and

nitrous oxide affect the reading, but the worst-case interference is less than 0.1%.

The analyzer does not give art ifactual results in the presence of aerosol propellants

that are used to administer bronchodilators ( 142 ).

F as t R e s p o n s e T im e

These analyzers can measure inspired and expired levels of halogenated agents.

View Figure

Figure 22.20Piezoelectric analyzer. A:One vibratingcrystal is coated with lipid, and the other is uncoated. By

comparing the vibration frequencies of the crystals, thelevel of anesthetic agent in the gas being analyzed can be

measured. B:Piezoelectric crystals. (Courtesy ofBiochemical International, Inc.)

P.702


29/129

View Figure

Figure 22.21Colorometric carbon dioxide detectors. Acolor code around the outside provides a reference. A:Adultsize. The device is supplied with caps that must be removed

before use. (Reprinted by permission of Nellcor PuritanBennett, Inc., Pleasanton, CA.) B:Adult and pediatricversions. The paper strip must be removed to activate the

device.

No Ne e d f o r S c a v e n g i n g

Because the agents are not altered, the gas removed can be returned to the

breathing system.

Sh o r t Wa rm - u p T im e

The warm-up period is shorter than with an infrared analyzer.

Comp a c t

These units are small.

Disadvantages

On l y O n e Ga s Me as u r e d

This analyzer cannot measure oxygen, CO 2 , nitrogen, or nitrous oxide.

No A g e n t D i s c r im i n at i o n

This device cannot discriminate between agents. The user must tel l the monitor

which agent is being measured. I f the wrong agent is selected, the reading can be

in error by as much as 118% (162 ).

In a c c u r a c y w i t h Wa t er V a p o r

Water wil l cause errors with the piezoelectric monitor. In one case, the l ines to the

pump were reversed so that water vapor was removed after its passage rather than

before (16 5). This caused erroneously high readings.

Chem i c a l Car b o n D i o x i d e De t ec t i o n


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A chemi ca l (colori me tri c) de tec to r (Fig. 22.21) consists of a pH-sensit ive indicator

enclosed in a housing (3,166 ,16 7,16 8,169 ). When the indicator is exposed to

carbonic acid that is formed as a product of the reaction between CO 2 and water it

becomes more acidic and changes color. During inspirat ion, the color returns to its

rest ing state unless it is used with a breathing system that allows rebreathing.

The inlet and outlet ports are 15 mm, so the device can be placed between patient

and the breathing system or resuscitat ion bag. With the Mapleson F sys tem

(Chapter 8), it may be placed between the expiratory limb and the bag ( 17 0).

Pediatric versions are available (Fig. 22.21B).

Technology

H y g r o s c o p i c

The hygroscopic CO2 detector contains hygroscopic f i l ter paper that is impregnated

with a colorless base and an indicator that changes color as a function of pH. The

filter paper is visible through a clear window. The color chart on the dome was

designed to be read under f luorescent l ight. An auxil iary color c hart that is included

in each package should be consulted if o ther l ight ing is encountered. A purple o r

mauve (A) color indicates a low CO2 (2%) (171 ,172). The mean minimum concentrat ion of

CO2 needed to produce a color change is 0.54%, with a range f rom 0.25% to 0.60%

(169 ).

The hygroscopic CO2 detector's useful l i fe may last from a few minutes to several

hours, depending on the humidity of the gas being monitored (173). Reducing the

relat ive humidity of exhaled gases by using an HME to trap moisture before it

reaches the device prolongs the detector's useful l i fe.

H y d r o p h o b i c

A hydro pho bi c in dicator in a colo rimetri c dev ice shows a color cha nge f ro m bl ue to

green to yellow when exposed to CO2 ( 173 ,174). Liquid water may cause the device

to not function properly. I f the device is allowed to dry, i t wil l recover its act ivity. I t

has a faster response t ime, performs better at high respiratory frequencies, and is

less affected by humidity than the hygroscopic model (17 3,174 ).

Use


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A chemi ca l CO2detector is useful for confirming successful tracheal intubation

when a capnometer is not available. I t is useful for intubations that are performed

out of the hospital, in the emergency department, or on the wards

(175 ,176,177,178 ). It can be used to determine the position of the Combitube ( 179 )

(Chapter 21). I t can be used during an intubation in a hyperbaric chamber (180). A

manual resuscitator may have a built- in colorimetric CO2detector (181 ,182).

Because it is disposable, i t may be especially useful to confirm tracheal intubation

in patients with respiratory diseases such as severe acute respiratory syndrome

(SARS) (183 ).

Advantages

The device is easy to use.

I ts performance is not affected by nitrous oxide or anesthetic vapors.

I ts small size, portabil i ty, and lack of need for a power source allow it to be

used in locations where use of a CO 2monitor is not possible.

The cost is low compared with other methods of CO2 analysis.

Studies show the device to be accurate in diagnosing esophageal intubation

(166 ,167,16 8,171 ,176 ,17 7,178 ,182,18 4,185 ,186 ,187 ,188,189,190 ,191,192,19

3).

The device can serve to evaluate resuscitat ion or as a prognostic indicator of

successful short-term resuscitation after the tracheal tube has been correct ly

posit ioned (186,18 8,194 ).

It offers minimal resistance to flow.

I t is always ready for use, does not require cleaning, and minimizes the risk

of transmission of infect ion.

Carbon monoxide does not interfere with the c hemical CO2 detectors (12 7).

Disadvantages

I t may take several breaths before conclusions can be drawn about the

tracheal tube location to avoid errors caused by false-posit ive results, as

discussed below. I t is usually recommended to wait six breaths before

making a determination.

False-negative results may be seen with very low t idal volumes and low end-

t idal CO2 concentrat ions, such as i n cases of compromised lung perfusion


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(190 ,192). During cardiopulmonary resuscitat ion, a posit ive test indicates

that the tracheal tube is in the airway, but a negative result (suggesting

esophageal placement) requires an alternate method of confirming tracheal

tube posit ion. I f there is l i t t le or no ci rculat ion to the lung, CO2 wil l not be

available for the detector to verify correct tracheal tube placement. Failure to

inf late the tracheal tube cuff may cause equivocal color change (167). Other

methods to determine tracheal tube posit ion are discussed in Chapter 19.

Drugs inst i l led in the trachea or gastric contents can cause irreversible

damage to the device (195 ,196).

False-posit ive results can occur if there is CO2 in the stomach (from ingested

carbonated beverages or antacids or mask v enti lat ion) (197,198,19 9,200 ).

The display may init ial ly turn color and only slowly revert to its original color.

Diff iculty in dist inguishing color changes has been reported (197 ). I t may be

dif f icult to determine whether a subtle color change is d ue to the patient 's

low end-tidal CO2or a misplaced tracheal tube.

There is no alarm or CO2 waveform.

This device may not be c ost effect ive for routine use when compared with

use of a capnometer (201 ). I ts cost-effect iveness may be greater with a s mall

number of applicat ions (20 2).

Airf lo w obs truc ti on f rom a manuf ac tu ring defec t ha s been rep orte d (20 3).

This device is semiquantitat ive and cannot give accurate measurement of

CO2 . For this reason, its applicat ion is l imited to tracheal tube posit ion

verif icat ion.

Re f r a c t ome t r y

In an o ptical interference refractometer ( interferometer), one port ion of a split l ight

beam passes through a chamber into which the sample gas has been aspirated,

while the other portion passes through an identical chamber containing air

(204 ,205,206,207 ). Because vapor slows the velocity of l ight, the port ion passing

through the vapor chamber is delayed. The beams are then recombined to form an

interference pattern that consists of dark and l ight bands. The posit ion of these

bands, observed through an eyepiece against a scale superimposed on the pattern,

yields the vapor concentration. In

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order to use this device, one must know the refract ivity number of the gas being

analyzed. Refract ivity is a nonlinear function of the part ial pressure of the gas.

This instrument is used primarily for v aporizer calibrat ion. I t cannot be used to

measure vapor levels of halogenated agents in a typical anesthetic gas mixture of

oxygen and nitrous oxide because of its sensit ivity to nitrous oxide (208).

Gas Measurement

Ox y g e n

The standards for basic anesthesia monitoring of the American Society of

Anesthes iolog is ts (ASA) and Ame ric an As soc iati on of Nurse Anesth etists (AA NA )

state that the concentrat ion of oxygen in the patient breathing system shall be

measured by an oxygen analyzer with a low oxygen concentration alarm in use. The

use of more than one device to monitor oxygen is desirable.

Standard Requirements

International and U.S. standards on respiratory gas monitors analyzers was

published in 2004 and 2005 (209,210 ). The following requirements are in those

standards.

Oxygen readings shall be within 2.5% of the actual level. This accuracy

shall be maintained for at least 6 hours of continuous use.

The high and low oxygen level alarms must be at least medium priority. A

high-priority alarm is required for an inspired oxygen concentrat ion below

18%. Alarm priorit izat ion is discussed in Chapter 26.

I t shall not be possible to set the low oxygen alarm limit below 18%.

An ox ygen an alyze r wi th an ala rm th at can be set below 18% is dangerous (21 1).

TechnologyOxygen levels may be measured by using electrochemical or paramagnetic

technology. In most cases, electrochemical analysis provides only mean

concentrat ions. Paramagnetic technology has a suff icient ly rapid response t i me to

measure both inspired and end-t idal levels. I t may be desirable to measure the

inspired oxygen with non-intubated, spontaneously breathing patients. This is

possible with a diverting device such as a paramagnetic analyzer but not with an


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electrochemical monitor. End-t idal oxygen can be measured during jet v enti lat ion

(212 ).

Applications of Oxygen Analysis

Det ec t i n g H y p o x i c o r H y p e r o x i c M i x t u r e s

The f irst l ine of defense against hypoxemia is to avoid a hypoxic inspired gas

mixture. An oxygen monitor provides an earlier warning of inadequate oxygen than

pulse oximetry. In a s tudy of 2000 crit ical incidents, 1% were f irst detected b y the

oxygen monitor (213 ). Hypoxia is discussed in Chapter 14.

Oxygen analysis can also help prevent problems result ing from hyperoxygenation,

such as patient movement during surgery, awareness, damage to the lungs and

eyes, and f ires. Fires are discussed in detail in Chapter 32.

Det ec t i n g D i s c o n n e c t i o n s a n d L e ak s

Disconnection of the tubing to an oxygen mask may be detected by using a

divert ing oxygen analyzer (214).

An ox ygen monitor can dete ct disconnections in the bre ath in g sys te m

(215 ,216,217). However, oxygen monitoring cannot be depended on for this

purpose (218,219 ). Whether or not the oxygen l evel fal ls at the point being

monitored depends on several factors, including the type of breathing system in

use, posit ion of the sensor, si te of disconnection, alarm set points, i f the patient is

breathing spontaneously or v enti lat ion is controlled, and the type of venti lator in

use. I f oxygen is the driving gas and there is no physical barrier between the

driving oxygen from the venti lator and the breathing system gas, a disconnection at

the common gas outlet wil l result in a r ising percentage of oxygen (220 ).

Disconnections are discussed i n Chapter 1 4.

With a si destream analyzer, a decrease in inspired and expired oxygen may result

from a leak in the sampling system (221).

De t ec t i n g H y p o v e n t i l at i o n

Normally, the dif ference between inspired and expired oxygen is 4% to 5%. A

dif ference of more than 5% after a s teady state has been reached is a s ensit ive

indicator of hypoventi lat ion (222,223,224). Hypoventi lat ion is discussed in detail in

Chapter 14.

O t h e r

End-tidal oxygen has been used to measure the adequacy of preoxygenation

(225 ,226,227,228 ,229).


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Knowing the expired oxygen concentrat ion allows an est imate of the patient 's

oxygen consumption and c an aid in the diagnosing malignant hyperthermia. Oxygen

consumption can be estimated from the difference between the inspired and

exhaled oxygen concentrat ions (230,231 ).

The concentrat ion of nitrous oxide c an be est imated from the concentrat ion of

oxygen.

End-t idal oxygen has been used to detect air embolism. When a signif icant amount

of air enters the

P.705

vascular bed, there is an increase in end-t idal oxygen and a decrease in the

dif ference between inspiratory and end-t idal oxygen c oncentrat ions (232 ).

Car b o n D io x i d e An a ly s i s

ASA gu ide lines for basic anesth eti c moni to ring state that wh en a trac he al tube or

supraglott ic airway device is inserted, its c orrect posit ioning must be verif ied by

identifying CO2 in the expired gas. Continual end-t idal CO 2 analysis shall be

performed unti l the device is removed or the patient is transferred to a

postoperative care location. In 2005, an audible alarm was added to the monitoring

standard (233).

A court case has he ld that a reasonab ly prud en t hea lth care fac ili ty would supp ly a

CO2 monitor to a patient undergoing general anesthesia (234). Some states have

mandated the use of CO 2monitors (158).

Carbon dioxide analysis provides a means for assessing metabolism, circulat ion,

and venti lat ion and can de tect many equipment- and patient-related problems that

other monitors either fai l to detect or detect so slowly that patient safety may be

compromised. A c losed claims analysis found that capnography plus pulse oximetry

could potential ly prevent 93% of avoidable anesthetic mishaps (235 ). In one study,

10% of intraoperative problems were init ial ly diagnosed by CO2 monitoring (236). In

another study, end-t idal CO2 was useful in confirming 58% of already suspected

anesthesia-related crit ical incidents and was the initial detector of 27% ( 23 7). In yet

another study, it was est imated that a capnometer used on its o wn would have

detected 55% of crit ical incidents if they had been allowed to evolve and 43% would

have been detected before any potential organ damage (238). Carbon dioxide

monitoring detects acute complete airway obstruction and extubation more rapidly

than pulse oximetry or vital s ign monitoring (239 ). In major trauma vict ims, using


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capnography to guide prehospital venti lat ion resulted in less hypoventilat ion on

hospital admission (24 0,24 1).

The respiratory cycle ( i.e., inspirat ion vs. expirat ion) is defined in terms of CO2

measurement, so end-tidal values for other gases depend on CO 2 measurement.

Terminology

Capnometry is the measurement of CO2in a gas mixture, and a capnometer is the

device that performs the measurement and displays the readings in numerical form.

Capnographyis the recording of CO 2 concentrat ion versus t ime, while a

capnograph is the machine that generates the waveform. The capnogram is the

actual waveform (24 2). I t may be possible to connect a capnometer to another

patient monitor and/or recorder to generate a waveform. Waveforms are availableon all modern physiologic monitors.

The Capnometer

S t a n d ar d s Re q u i r em e n t s

An internati on al and a U.S. standa rd on capn omete rs hav e been pub li shed

(209 ,210). They contain the following specif icat ions.

The CO2 reading shall be within 12% of the actual value or 4 mm Hg (0.53

kPa), whichever is greater, over the full range of the capnometer.

The manufacturer must disclose any interference caused by ethanol,

acetone, methane, helium, tetrafluoroethane, and dichlorodifluoromethane as

well as commonly used halogenated anesthetic agents.

The capnometer must have a high CO 2 alarm for both inspired and exhaled

CO2 .

An al arm for low exha led CO2 is required.

T e c h n o l o g y

Methods to measure CO2 levels i nclude infrared and chemical colorimetric analysis.

A wide vari ety of di sp la y forma ts are av ailable on CO2 monitors. The CO2 level may

be reported as either part ial pressure or volumes percent and may be displayed

continuously or as the peak (normally end-t idal) value. Other parameters such as

respiratory rate and I:E ratio may be displayed.

Portable, battery-operated CO2 monitoring devices are available

(243 ,244,245,246 ,247,24 8,249 ,250 ) (Figure 22.12). These are useful in emergency


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medicine and patient transport (Fig. 22.12). At least one has been reported to not

give correct values during rebreathing (25 1). MRI-compatible infrared CO2monitors

are available.

Many capnometers are included in multipurpose physiologic monitors with other

parameters such as blood pressure, pulse oximetry, and analysis of other gases.

The CO2 waveform may be one of several on a display.

A tm o s p h e r i c P r e s s u r e E f f ec t s

Atmo sph eri c pressure can in f luence CO2 readings (111 ,112,252,253,254 ,255 ).

Some instruments incorporate a barometer to compensate for c hanges i n

atmospheric pressure. Others require the user to enter the atmospheric pressure

manually. St i l l others do not correct for a tmospheric pressure. The c apnometerstandards (20 9,210 ) require that the manufacturer disclose the quantitat ive effects

of barometric pressure on c apnometer performance in the i nstruct ions for use.

Sidestream Analyzers

When a sides tream infrared reports results in v olumes pe rcent, the atmospheric

pressure at measurement time must be known to correctly compute the CO 2 value.

P.706

TABLE 22.1 Capnography and Capnometry with Altered Carbon Dioxide Productiona

Waveform on

Capnograph

End-tidal

Carbon

Dioxide

Inspiratory

Carbon

Dioxide

End-tidal to

Arterial

Gradient

Absorption of CO2fromperitoneal cavity

Normal 0 Normal

Injection of sodiumbicarbonate

Normal 0 Normal

Pain, anxiety, shivering Normal 0 Normal


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Increased muscle tone (as frommuscle relaxant reversal)

Normal 0 Normal

Convulsions Normal 0 Normal

Hyperthermia Normal 0 Normal

Hypothermia Normal ! 0 Normal

Increased depth of anesthesia(in relation to surgical

stimulus)

Normal ! 0 Normal

Use of muscle relaxants May seecurarecleft

! 0 Normal

Increased transport of CO2tothe lungs (restoration of

peripheral circulation after ithas been impaired, e.g., afterrelease of a tourniquet)

Normal 0 Normal

a

Normal end-tidal CO2is 38 torr (5%). Inspired CO2is normally 0. The arterial to end-tidal gradient is normally less than 5 torr.

For example:

FetCO2= part ial pressure (atmospheric pressure - water vapor pressure) 100

At 760 mm Hg atmospheri c pres sure and a CO2 level of 38 mm Hg,

FetCO2= 38(760 - 47) 100 = 5%

If the atmospheric pressure is reduced to 500 mm Hg,

FetCO2= 38(500 - 47) 100 = 8%


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If a correction for atmospheric pressure is not made, the capnometer will read

erroneously high volumes percent at increased alt i tude. Other options a re to

calibrate at alt i tude with a gas that has a known CO2concentrat ion or to set the

device to read part ial p ressure.

Ma i n s t r e am I n f r a r e d A n a l y z e r s

Mainstream infrared instruments are calibrated from sealed gas cells of known

part ial pressure. These instruments wil l report measurements in units of part ial

pressure correct ly (252). I f such an analyzer reports results in volumes percent, the

atmospheric pressure at measurement t ime must be known.

Clinical Significance of Capnometry

Carbon dioxide is produced in the body t issues, conveyed by the circulatory system

to the lungs, excreted by the lungs, and removed by the breathing system.

Therefore, changes in respired CO2may ref lect alterat ions in metabolism,

circulat ion, respirat ion, or the breathing system. Tables 22.1 to 22.4 l ist some

sources of changes in CO2 levels.

TABLE 22.2 Capnographic and Capnometric Alterations as a Result of Circulatory

Changes

Waveform on

Capnograph

End-tidal

CarbonDioxide

I nspiratory

CarbonDioxide

End-ti dal to

ArterialGradient

Decreased transport of CO2to the lungs (impaired

peripheral circulation)

Normal ! 0 Normal

Decreased transport of CO2

through the lungs (pulmonaryembolus, either air or

thrombus; surgicalmanipulations)

Normal ! 0 Elevated

Increased patient dead space Normal ! 0 Elevated

P.707


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TABLE 22.3 Capnometry and Capnography with Respiratory Problems

Waveform on

Capnograph

End-tidal

Carbon

Dioxide

I nspiratory

Carbon

Dioxide

End-tidal to

Alveolar

Gradient

Disconnection Absent 0

Apneic patient,stopped ventilator

Absent 0

Hyperventilation Normal ! 0 Normal

Hypoventilation,mild to moderate

Normal 0 Normal

Upper airwayobstruction

Abnormala 0 Elevated

Rebreathing, e.g.,(under drapes)

Baselineelevated

Normal

Esophagealintubation

Absent 0

a

See Figure 18.34.

Me t a b o l i sm

Monitoring CO2 el imination gives an indicat ion of the patient 's metabolic rate (256 ).

An inc reas e or decrea se in end-t idal CO2 is a reliable indicator of metabolism only

in mechanically venti lated s ubjects. For spontaneously breathing patients, PetCO2

may not increase with increased metabolism because of compensatory

hyperventi lat ion by the patient (257 ,258 ).


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Table 22.1l ists some metabolic causes of increased or decreased CO 2 excret ion.

These include increased temperature, shivering, convulsions, excessive

catecholamine production or administration (259 ), blood or bicarbonate

administrat ion (260), release of an arterial clamp or tourniquet

(261 ,262,263,264 ,265), and parenteral hyperalimentat ion (266). Carbon dioxide

production falls with decreased temperature and increased muscle relaxation.

Increased exhaled CO2 can result from CO 2used to inf late the peritoneal cavity

during laparoscopy (267,268,269,270 ,27 1,27 2), the pleural cavi ty during thorascopy

(273 ,274), a joint during arthroscopy (275), or to increase visualizat ion for

endoscopic vein harvest (276).

TABLE 22.4 Capnographic and Capnometric Alterations with Equipment

Problem Waveform on

Capnograph

End-tidal

Carbon

Dioxide

Inspiratory

Carbon

Dioxide

End-tidal to

Ar terial Gradient

Increased apparatus deadspace

BaselineElevated

Normal

Rebreathing with circle

system: faulty or exhaustedabsorbent, bypassedabsorber (may be masked by

high fresh gas flow)

Baseline

ElevatedSeeFigure

18.35

Normal

Rebreathing with Maplesonsystem (inadequate fresh gasflow, misassembly, problemwith inner tube of Bainsystem)

BaselineElevatedSeeFigure18.35

Decreased

Rebreathing due tomalfunctioningnonrebreathing valve

BaselineElevatedSee

Figure18.35

Decreased


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Obstruction to expiration inthe breathing system

SeeFigure

18.34

0 Decreased

Blockage of sampling line Absent 0 0

Leakage in sampling line SeeFigure18.39

! 0 Increased

Low sampling rate withdiverting device

SeeFigure18.41

! Increased

Too high a sampling rate

with diverting device

See

Figure18.42

! 0 Increased

Inadequate seal aroundtracheal tube

SeeFigure18.44

! 0 Increased

P.708

Malignant hyperthermia is a hypermetabolic state with a massive increase in CO2

production. The increase occurs early, before the rise in temperature. Early

detect ion of this syndrome is one of the most important reasons for routinely

monitoring CO2 ( 236 ,277). Capnometry can be used to monitor the effectiveness of

treatment.

C i r c u l a t i o n

Table 22.2l ists some of the circulatory changes that affect exhaled CO2 . A

decrease in end-t idal CO2 is seen with a decrease in cardiac output if venti lat ion


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remains constant (278 ,279 ,280,281 ,28 2). End-t idal CO2 increases with increased

cardiac output (283).

In addit ion to reduced cardiac output, reduced blood f low to the l ungs can result

from surgical manipulat ions of the heart or thoracic vessels (284 ), a dissecting

aort ic aneurysm compressing a pulmonary artery (285), wedging of a pulmonary

artery catheter, and pulmonary embolism (thrombus, tumor, gas, fat, marrow, or

amniot ic f luid) (286,287,28 8,28 9,29 0,29 1,29 2,293,29 4,29 5). I f the embolized gas is

CO2 , the end-t idal CO2 wil l i nit ial ly increase and then decrease

(296 ,297,298,299 ,300,30 1). Although not as sensit ive as the Doppler for detect ing

air embolism, CO2 monitoring is less subject ive, is unaffected by electrosurgery

apparatus, and can be used in major ear, nose, and throat (ENT) cases for which

the Doppler method is not applicable. Capnometry may not be suff icient ly sensit ive

to detect fat and marrow microemboli (302).

During resuscitat ion, exhaled CO2 is a better guide to the effect iveness of

resuscitat ion measures than the electrocardiogram (ECG), pulse, or blood pressure

(303 ,304,305,306 ,307,30 8,309 ,310 ,311,312). The capnometer is not susceptible to

the mechanical artifacts that are associated with chest compression, and chest

compressions do no t have to be interrupted to assess c irculat ion. The colorimetric

CO2 detect ion device has also been shown to be an effect ive monitor during

resuscitat ion. However, i f high-dose epinephrine or bicarbonate is used, end-t idal

CO2 is not a good resuscitat ion indicator (313,314,31 5,316,31 7,31 8).

End-t idal CO2 levels may be of use in predict ing the outcome of resuscitat ion

(186 ,304,307,310 ,319,32 0,321 ,322 ,323,324 ,325 ,326 ,327,328 ,329 ,330,331,33 2) and

the resolut ion of a p ulmonary embolus (333).

Re s p i r a t i o n

Carbon dioxide monitoring gives information about the rate, frequency, and depth of

respirat ion. I t can be used to evaluate the patient 's abil i ty to breathe spontaneously

as well as the effect of bronchodilator or nitr ic oxide treatment or altered venti lat ion

parameters. I t al lows control of v enti lat ion with fewer blood gas determinations.

End-t idal analysis is noninvasive, available on a breath-by-breath basis, and not

affected by hyperventi lat ion that is induced by drawing an arterial blood sample.

Table 22.3l ists some respiratory causes of increased and decreased end-t idal CO2 .

A capno me te r can wa rn of esophageal intuba tio n, ap nea , extu bation, discon ne ction,

venti lator malfunction, a change in compliance or resistance, ai rway obstruct ion,

poor mask f it , or a leaking tracheal tube cuff.


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A dependable me ans to dete rmin e wh en a trachea l tube ha s been corr ec t ly

posit ioned in the tracheobronchial tree obviously is of great v alue. Esophageal

intubation has been a leading cause of death and cerebral damage in the past. A

discussion of ways to detect inadvertent esophageal placement is found in Chapter

19. Carbon dioxide monitoring is usuall y considered the most reliable method.

Carbon dioxide measurement to detect esophageal placement has some drawbacks

and l imitat ions, so its use as the only means of c orrect tube placement should be

strongly discouraged. Absence of circulat ion, severe bronchospasm, equipment

malfunction, and applicat ion of cricoid pressure occluding the tracheal tube t ip can

result in fai lure to detect CO2 (33 4,33 5,33 6,337 ,338,339 ,34 0,34 1,342,34 3,34 4). The

analyzer may be in a calibrat ion mode when the tube is p laced.

With esophageal intubation, small waveforms may be transiently seen as a result of

CO2 that has entered the stomach during mask ventilation or from carbonated

beverages or medications (19 7,199 ,345,34 6,347 ,348). This could give the

impression that the tube is co rrect ly placed in the trachea. However, rapidly

diminishing concentrat ions and abnormal waveforms wil l usually dif ferentiate

esophageal from tracheal intubation (345,34 9,350).

A cas e has bee n rep ort ed where a normal capno gram was pre sen t desp ite an

esophageal intubation (351). There was a cuffed oropharyngeal airway in place,

and the patient was breathing spontaneously. Carbon dioxide from the trachea was

thought to have gotten under the cuffed airway and forced down the esophagus,

where it was aspirated from the tracheal tube. Inflating the tracheal tube cuff

interrupted the waveform.

While esophageal intubation wil l l ikely be detected by using end-t idal CO2 , there is

no guarantee that the tube is in the trachea. Carbon dioxide can be se nsed from a

tracheal tube that is posit ioned above the vocal cords (352).

A diverting CO2 monitor can be used to monitor respiratory rate and exhaled CO2 in

unintubated patients who are breathing s pontaneously

(50,51,52,53 ,54 ,55,67 ,72 ,78,89 ,90 ,353,354,355 ,356,357 ,358,35 9). Apnea, airway

obstruct ion, or disconnection of the oxygen source may be detected. I f v enti lat ion

of the breathing space under the surgical drapes is inadequate, rebreathing wil l

occur and may be detected by a rising inspired CO2 level (52 ,354 ,360).

Capnometry has been used to help determine the posit ion of a double-lumen tube

(361 ,362). Methods to determine proper double-lumen tube placement are

discussed in Chapter 20. Correct placement can be checked b y examining the


45/129


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Carbon dioxide analysis can be used to de tect a disconnected oxygen tubing to a

mask over the face during local or regional anesthesia (355). I f the oxygen source

becomes detached, there will be a rise in CO2 because of rebreathing.

Rarely a phantom CO2wave may be noted despite a disconnection. One case was

reported when the gas sample l ine from the CO 2 monitor was connected to the

breathing system just upstream of the expiratory unidirect ional valve (392 ). During

inspirat ion, the inspiratory unidirect ional valve opened, allowing fresh gas plus the

gas (containing CO2) from the monitor to pass to the patient, where i t was detected

during inspirat ion. This problem does not occur if the gas is either directed to the

scavenging system or returned to the breathing system downstream of the

expiratory valve. In another case, square wave capnographic tracings were

observed after a patient was disconnected from a venti lator that had not been

turned OFF (39 3). The gas analyzer aspirated CO2 from the expiratory tubing,

generating a series of diminishing tracings o n the capnograph.

View Figure

Figure 22.22A spirogram that plots CO2against volumewill illustrate the inspiratory valve leak by a decreaseddownslope on the inspiratory side of the loop better than acapnogram. (Redrawn fromBreen PH, Jacobsen P. Carbon dioxide spirogram [but not

capnogram] detects leaking inspiratory valve in a circlecircuit. Anesth Analg 1997;85:13721376

[Fulltext Link][CrossRef]

[Medline Link].)

O t h e r U s e s

A diverting capno me te r can be us ed to lo cal ize the si te of leaks in CO2 insuff lat ion

equipment (394), diagnose a tracheoesophageal or bronchoesophageal fistula

(395 ,396), guide blind intubation (397,39 8,39 9,40 0,40 1,402,403,40 4,405 ),

determine when the t ip of an exchange catheter or f iberscope is in the trachea


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(406 ,407), or confirm that the needle or catheter is posit ioned in the trachea during

a

P.710

cricothyrotomy or percutaneous dilatational tracheostomy (Chapter 2 1)

(408 ,409,410).

Carbon dioxide analysis may be used to assess the posit ion of an enteric tube

(411 ,412,413,414 ,415,41 6,417 ,418 ,419,420 ,421 ,422 ,423). If the tube is placed into

the trachea, CO2 wil l be detected at the free end. I f CO 2is not detected, the tube is

l ikely in the esophagus. I f an enteric tube passes into the trachea in an intubated

patient, the capnograph wil l show a downsloping alveolar plateau (42 4).

Correlation between Arterial and End-tidal Carbon Dioxide

Levels

Numerous s tudies have shown that the correlat ion between arterial and end-t idal

CO2 tensions in children and adults without cardiorespiratory dysfunction is good

enough to warrant routine monitoring (4,110,271 ,425 ,432 ). End-t idal CO2 is usually

lower than PaCO2 by 2 to 5 torr (433 ). The gradient may be less or even negative if

the functional residual capacity is reduced, as in pregnant or obese patients

(428 ,434,435) and is reduced with rebreathing (436 ). Tables 22.1through 22.4show some condit ions with altered end-t idal to arterial gradients.

Predict ion of PaCO2 from end-t idal CO2 alone is unreliable in some patients and

may be potential ly deleterious in some patient subgroups. A study of neurosurgical

patients found that end-t idal CO2 did not accurately ref lect changes in the arterial

CO2 tension (437 ), although in healthy patients during elect ive neurosurgical

procedures, the PaCO2-PetCO2 dif ference remains stable over t ime (430 ).

Transcutaneous CO2 monitoring has been found to be more accurate in evaluating

CO2 levels during one-lung venti lat ion (438 ,439 ), in obese patients (440), during

neurosurgical procedures in adults (44 1), and in older children (442 ,443).

The relat ionship between arterial and end-t idal CO 2 tension may be constant or

vary, sometimes in dif ferent direct ions, both within and between patients (44 4).

Al though the re usually is a l inear relati onshi p between end-t idal an d art eri al CO2,

the gradient may be unexpectedly large or even negative

(445 ,446,447,448 ,449,45 0). End-t idal CO2 cannot replace the measurement of

PaCO2 in the intensive care unit or emergency room, although it is useful for

trending or screening (43 0,450,451,452,45 3).


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P r o b l em s w i t h S am p l i n g

Accurate me asurement of end -t id al CO2is especially dif f icult with high venti latory

frequencies (16 ). In small pat ients, sampling at the patient end of the tracheal tube

results in a closer approximation to arterial CO2 than sampling at the breathing

system end (22 ,23 ,24 ,25,26). While placing the gas s ampling l ine on the machine

side of an HME may avoid contamination and water logging of the sample, this may

result in erroneous values and poor waveforms (45 4,455,45 6).

One source of sampling error is a leak at the interface between the patient and the

equipment. Poor mask fit, using an uncuffed tracheal tube or a tube with a defective

cuff, or a loose connection or leak in the sampling catheter may cause erroneously

low end-t idal CO2 readings (106). The correlat ion between arterial and end-t idalCO2 tensions is better during venti lat ion with a supraglott ic device than a face

mask (457 ). The correlation can be improved by sampling at the patient end of the

supraglott ic device (30).

With unintubated, spontaneously b reathing pa tients, p oor correlat ion between end-

t idal and arterial CO2 is associated with part ial airway obstruct ion, high respiratory

rates, low t idal v olumes, oxygen delivery through the ipsilateral nasal cannula, and

mouth breathing (68,93 ,458). Results may be improved by isolating insufflated

oxygen from exhaled gases, observing the waveform for normal configuration, and

decreasing the oxygen flow rate (53,459 ).

When a sidestream capnometer is used with a Mapleson system, exhaled gas may

be diluted by fresh gas during the latter port ion of expirat ion if the expiratory f low

rate is less than the sampling f low rate of the capnometer. This wil l cause the end-

t idal CO2reading to be lowered even if the alveolar phase of the capnogram is f lat

or has a small posit ive slope. The amount that the end-t idal CO 2is lowered wil l

depend on several f actors, including whether spontaneous or controlled venti lat ion

is used, the type of venti lator and breathing circuit , the fresh gas f l ow, the sampling

rate, and the expiratory flow rate (22,460 ,46 1). Maneuvers to obtain a PetCO2

reading that is closer to the PaCO2with Mapleson systems include using lower

fresh gas f lows, extending the t ime of expi ratory f low, adding dead space between

the breathing system and gas sampling point, and using a ci rcuit that automatically

interrupts the fresh gas f low after i nspirat ion or prevents mixing of exhaled and

fresh gases (460,46 2). A quick method of checking to see whether the PetCO 2 i s

art ifactually low is to temporari ly disconnect the fresh gas supply (463). I f there is


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an abrupt r ise in PetCO2 when the dilut ing effect of the fresh gas is removed, then

the f irst breaths that fol low wil l give a better measure of the true PetCO2 .

During high-frequency venti lat ion, PetCO2 is a poor index of PaCO 2 (88). In order

to measure the end-t idal CO2, the high-frequency venti lat ion should be interrupted

to impose a few slow breaths (83 ,87,88 ,464 ,465 ,466 ).

D i s t u r b a n c e s i n t h e V e n t i l at i o n : P e r f u s i o n Rat i o

When there is venti lat ion-perfusion mismatching, the relat ionship between end-t idal

and arterial tensions of CO 2 is disturbed. Clinical condit ions that can alter the

volume and/or distr ibut ion of pulmonary blood f low i nclude pulmonary embolism,

pulmonary artery s tenosis or

P.711

occlusion, reduced cardiac output, pulmonary hypotension, hypovolemia, and

certain heart lesions (433 ,467 ,468,469 ,470 ,471 ,472 ).

The end-t idal to arterial CO 2gradient increases as venous admixture (r ight to left

shunt) occurs. This can be caused b y atelectasis, bronchial intubation, or ce rtain

heart condit ions. The effect is less dramatic than that caused by an increase in

dead space, but when the venous admixture is l arge (as in cyanotic congenital

heart disease), i ts contribut ion can be considerable (425 ,46 9,473 ,474).

Changes in body posit ion, such as the lateral or prone posit ion, may cause an

increase in the Pa/PetCO2 gradient (475 ,476 ).

Patients with pulmonary disease have an uneven d istr ibut ion of v enti lat ion and, to a

lesser extent, blood f low. This leads to an increased gradient (433 ,477,478 ,479).

Since positive end-expiratory pressure (PEEP) may decrease the gradient

(4

Documents

Chapter 22. Gas Monitoring