Chapter 24 Pulse Oximetry P.776

Introduction Pulse oximetry, sometimes called the fi fth vi tal sign , is a noninvasive method of

measuring hemoglobin saturation (SpO2) by using a light s ignal transmitted through

t issue. A low SpO2 can prov ide warning of hypoxemia before other s igns such as

cyanosis or a change in heart rate are observed.

Unt il the 1980s , non invasive ox imeters, known as ear oximeters , were large,

expensive, and cumbersome. They required arte rial ization by heat or chemical

trea tment, and the ir u ti l i ty was l imi ted by dif f icul ties in different ia ting l igh t

absorbance of arterial blood f rom that of venous blood and tissues.

Technical advances, inc luding l igh t-emitt ing d iodes (LEDs), min ia turized

photodetectors, and microprocessors, allowed the creat ion of a new generat ion of

ox imeters, which were smal ler, less expensive, and easier to use. These

differentia te the absorp tion of l ight by the pulsa ti le arterial component f rom the

stat ic components, so they are cal led pulse oximeters .

A pulse oximeter may be a stand-alone device or incorpora ted into another device

such as a mult iparameter moni toring system. A relatively new development is a

combined pulse oximetry and transcutaneous carbon dioxide tension ear sensor

(1,2 ,3,4).

The American Soc ie ty of Anesthesiologis ts (ASA) and American Associat ion of

Nurse Anesthetis ts have made assessment of oxygenation a s tandard for

intraopera tive and postoperative moni toring. In 2005, an audible alarm for the pulse

ox imeter was added to the ASA monitoring s tandard (5). In ternat ional s tandards for

safe practice endorsed by the World Federation of Societ ies o f Anes thesiologists

highly recommend cont inuous use of a quanti tative monitor of oxygenation such as

pulse oximetry (6). In some states, the use of pulse oximetry is mandatory. A study

of c losed claims of anesthet ic -rela ted malpract ice cases determined tha t a

combination of pulse oximetry and capnography could have prevented 93% of

avoidable mishaps (7). One study determined tha t pulse ox imetry prov ided the f irs t

warning of an inc ident in 27% of si tua tions (8). The number of unant ic ipated

intensive care uni t admissions dec reased afte r the introduct ion of the pulse

ox imetry (9).

Operating Principles The pulse oximeter estimates SpO2 f rom the dif ferent ial absorpt ion of red and

infrared ligh t in t issue (10,11,12,13,14,15,16). The two wave lengths al low

differentia tion of reduced hemoglobin and oxyhemoglob in . Reduced hemoglobin

absorbs more l ight in the red band than oxyhemoglobin (Fig. 24 .1). Oxyhemoglobin

absorbs more l ight in the inf rared band. The pulse oximeter computes the ratio

between these two s ignals and relates this rat io to the arte rial oxygen saturat ion,

us ing an empirical a lgori thm.

Pulse oximeters discriminate between arterial b lood and other components by

determin ing the change in transmitted l ight caused by the f low of a rterial blood. The

ox imeter pulses the red and inf ra red LEDs ON and OFF several hundred times per

second. The rapid sampl ing rate allows recogni tion of the peak and trough of each

pulse wave. At the trough, the l igh t is transmitted through a vascular bed that

contains mainly capil lary and venous blood as wel l as intervening t issue. At the

peak, i t sh ines through all this plus arterial blood. A photodiode collec ts the

transmitted light and converts i t into elec trical signals . The emitted s ignals are then

amplif ied, processed, and displayed on the moni tor. Oximeters have a phase wi th

both LEDs OFF to allow detect ion of and compensat ion for extraneous l ight. Light

readings during the OFF period are subtracted from the next sequence.

Fract ional oxygen satura tion (% HbO2 ) is the ra tio of oxyhemoglobin to the sum of

al l hemoglobin spec ies present, whether avai lable for revers ible binding to oxygen

or not (17). Func tiona l oxygen saturat ion (SaO2 ) is def ined as the ratio of

oxyhemoglobin to al l functional hemoglobins . These must be determined by using

an

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in v itro oximeter. For pat ients wi th low dyshemoglobin levels , the d if fe rence

between f ract ional and functional sa turation is very smal l . However, when

dyshemoglobin levels a re elevated, the two values can vary great ly, and pulse

ox imeter readings may not agree wi th e ither the true f rac tional or func tiona l

sa turat ion values (18).

View Figure

Figure 24.1 Absorbance of light as a function of wavelength. The extinction coefficient is a measure of the tendency of a substance to absorb light. At the red wavelengths (650 to 750 nm), reduced hemoglobin absorbs more light than does oxyhemoglobin. In the infrared region (900 to 1000 nm), the reverse is true.

Transmission Pulse Oximetry The mos t common type of pulse oximeter is the transmission ox imeter. With th is

technology, a l ight beam is transmitted through a vascular bed and is detected on

the opposi te side of that bed.

Reflectance Pulse Oximetry Ref lectance oximetry rel ies on l igh t that is ref lected (backscattered) to determine

oxygen saturat ion. The probe has both an LED and a photodiode (Fig. 24.16).

Transmission pulse oximetry probes are not accurate when used in the manner of

ref lec tance oximetry (19). Ref lec tion orig inates f rom nonhomogene ity in the opt ical

path, tha t is , at the in terfaces between materials with different reflec tive indices.

The t issue mus t be wel l perfused to obtain a strong signal . Heating the

measurement s ite and app ly ing pressure may be helpful (20,21).

There are a number of l imi tat ions of ref lectance ox imetry. The probe design must

el iminate l igh t that is passed di rec tly to the probe or is sca ttered in the outer

surface of the sk in . The signa ls are weaker than those found in transmission

ox imetry, so the photodiode area needs to be as la rge as possible. I f the probe is

located over an arte ry or a vein , the read ing may be art ifactua lly low (22,23).

Vasoconstric t ion can cause overestimation of the oxygen saturation (23).

Physiology

Eff ic ient oxygen transport rel ies on the abil ity of hemoglobin to revers ib ly load and

unload oxygen. The relat ionship between oxygen tens ion and oxygen binding is

seen in the oxyhemoglob in dissociat ion curve (Fig. 24 .2), which plots the

hemoglobin oxygen saturation against the oxygen tension. The sigmoid shape of

the curve is essential for physiologic transport. As oxygen is taken up in the lungs,

the blood is nearly ful ly sa turated over a large range of tensions. During passage

through the

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systemic capi llaries, a la rge amount of oxygen is released with a relat ively smal l

drop in tens ion. This al lows oxygen to be released a t suf f ic ien tly high

concentrations to provide an adequate gradient for di f fusion into the cel ls .

View Figure

Figure 24.2 The oxyhemoglobin dissociation curve. Hemoglobin saturation is plotted as a function of oxygen tension.

The shape of the oxyhemoglobin d issociat ion curve l imi ts the degree of

desatura tion that can be tolerated. Between 90% and 100% saturat ion , the part ial

pressure of oxygen in arterial b lood (PaO2 ) wi l l be 60 torr or above. Below 90%

saturat ion, the curve becomes steeper, and smal l drops in satura tion correspond to

large drops in oxygen part ial pressure. If a problem develops, there may not be

much warning before the oxygen level becomes dangerous ly low. Normal satura tion

wi l l decrease as alti tude above sea level inc reases (24).

Equipment

Probes

The probe (sensor, transducer) is the part tha t comes in contact wi th the patient. It

contains one or more LEDs (photodiodes) that emit l ight at specif ic wavelengths

and a photodetector (photocell , t ransducer). The LEDs provide monochromatic

l ight. This means that they emit a constant wavelength th roughout their l i fe , so they

never need recal ib rat ion. LEDs cause relat ively l i t t le heating and are so

inexpensive that they may be used in a d isposable probe. The light, partial ly

absorbed and modula ted as i t passes through the t issue, is converted into an

electronic signal by the photodetector.

Figures 24.3 to 24 .11 show several types of p robes. Probes may be reusab le or

disposable. They have the same accuracy (25 ,26,27,28). A disposab le probe is

usual ly attached by using adhesive. Reusable probes either cl ip on or a re attached

by using adhesive or Velcro. Disposable probes may be easier to use, but reusable

probes are more economical as long as personne l are careful not to damage the

reusable probe (26,29,30,31). Self -adhesive (band, wrap) probes are less

susceptible to motion art ifact and are less likely to come off if the pat ient moves

than those that c lip on. However, they are usua lly not as wel l shielded f rom ambient

l ight as c l ip-on probes. A ttaching reusable probes by using an adhes ive or Velco

wrap may improve their s tab il i ty. P robes lined wi th soft material may be associa ted

wi th fewer motion art ifac ts (32).

Some probes are availab le in dif ferent s izes. If a p robe is too large for the patient,

some of the l ight ou tput from the LED can reach the photoce ll without pass ing

through tissue, and falsely high SpO2 readings wil l be produced (33,34). The

photocel l may not al ign wi th the probe, and readings wi l l not be possible.

Loss of reusable probes can be reduced by making i t diff icul t to separate the probe

f rom the cable (31). Attaching the probe to the oximeter case when not in use wil l

reduce damage and make it easy to f ind (35).

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View Figure

Figure 24.3 Disposable flexible probe. This can be placed on a variety of sites, including the finger, ear, cheek, tongue, toe, penis, hypothenar or thenar eminence, palm, foot, and wrist. (Picture courtesy of Masimo Corporation, Irvine, CA.)

To reduce contaminat ion, a glove, the f inger of a glove, or other covering may be

used either over the appl ica tion si te o r over the probe (36,37,38). Mi tts are

availab le to shield ambient l ight (F ig. 24.12).

Cable The probe is connected to the oximeter by an electrical cable. Cables from d if fe rent

manufac turers are not inte rchangeable.

Console Many d if fe rent consoles are avai lable (Figs. 24.13,24.14,24.15). Most oximeters

that are used in the operat ing room are part o f a phys io logic mon itor. Most s tand-

alone un its are l ine operated but wil l work on batte ries, making them useful during

transport. Some oximeters are hand-held (Figs. 24.14, 24.15).

A mic rocomputer mon itors and controls s ignal levels , perfo rms the calculat ions,

implements s ignal val id ity schemes, activates ala rms and messages, and monitors

i ts own c ircui try to warn of mal funct ions. A variety of messages may be provided to

info rm the operator of i ts

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functional s ta tus (13). The panel usual ly displays percent sa turat ion, pu lse rate,

and a larm l imits . Most uni ts have a bright display, al lowing them to be seen in a

wel l -l ighted room.

View Figure

Figure 24.4 Disposable flexible probe in place on a finger. (Reprinted by permission of Nellcor Puritan Bennett Inc., Pleasanton, CA.)

View Figure

Figure 24.5 Reusable probe. This is most commonly used on a finger or toe. In infants, this type of probe can be placed on part of the hand or foot. These probes offer good shielding from ambient light. (Picture courtesy of Masimo Corporation, Irvine, CA.)

View Figure

Figure 24.6 Reusable finger probe in place. (Reprinted by permission of Nellcor Puritan Bennett Inc., Pleasanton, CA.)

The disp layed values for SpO2 and pu lse rate are usual ly we ighted averages . Some

ox imeters allow the averaging period to be adjusted. A mode that averages over a

longer period of time may work better if there is much probe motion (39). Changes

in pulse rate or sa turat ion wil l be ref lec ted more rapidly if the averaging is done

over a shorter period of t ime.

Pulse ampli tude may be represented by a signa l indicator. Other uni ts use a

graphic that indica tes pu lse amplitude and may prov ide a plethysmographic

waveform.

Most instruments provide an audib le tone whose pi tch changes with the satura tion.

In this way, the opera tor can be made aware of changes in SpO2 without looking at

the oximeter. By using a variab le tone pu lse oximeter, anesthes ia providers

recognized an episode of oxygen desaturation more quickly than those using one

wi th a f ixed tone (40). There is usually a means to control the volume of the audible

s igna l.

Alarms are commonly prov ided for low and h igh pu lse rates and low and high

sa turat ion. Many units

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generate an alarm when the probe is not properly applied to the patient o r if for

some o ther reason the signal is inadequate. ASA s tandards fo r Basic Anesthetic

Moni toring require that the variable pi tch pulse tone and low thresho ld alarm be

audible (41).

View Figure

Figure 24.7 Disposable nasal probe in place. The clip from a disposable oxygen mask may be used to improve contact and to hold the probe in place. (Reprinted by permission of Nellcor Puritan Bennett, Inc., Pleasanton, CA.)

View Figure

Figure 24.8 Reusable probe designed for use on the ear. This may be used on other locations, including the cheek. (Picture courtesy of Masimo Corporation, Irvine, CA.)

View Figure

Figure 24.9 Reusable probe on the ear. (Reprinted by permission of Nellcor Puritan Bennett, Inc., Pleasanton, CA.)

Most pulse oximeters offer trend data. Inte rfaces for hard copy recording and data

management sys tems are usually available.

View Figure

Figure 24.10 Disposable wraparound probe on the foot. (Reprinted by permission of Nellcor Puritan Bennett, Inc., Pleasanton, CA.)

View Figure

Figure 24.11 Disposable wraparound probe on the toe. (Reprinted by permission of Nellcor Puritan Bennett, Inc., Pleasanton, CA.)

Oximeter Standards The internat ional and U.S. s tandards are quite s imilar (42,43). Among the

provisions are the following:

There must be a means to l imit the durat ion of continuous operation at

temperatures above 41C.

The accuracy mus t be stated over the range o f 70% to 100% SpO2. If the

manufac turer c la ims accuracy be low 65%, the accuracy mus t be stated over

the addi t ional range.

I f the manufacturer c laims accuracy during motion, this and the tes t methods

used to es tab lish i t must be disclosed in the ins tructions fo r use.

I f the manufacturer c laims accuracy during condi tions of low perfusion , th is

and the test methods used to establish it must be disc losed in the

instructions for use.

There must be an indicat ion when the SpO2 or pulse rate data is not curren t.

I f the pulse oximeter is p rov ided with any physiologic alarm, i t mus t be

provided wi th an a la rm system that moni tors for equipment faul ts , and there

must be an alarm for low SpO2 tha t is not less than 85% SpO2 in the

manufac turer-conf igured a la rm preset. An alarm for high SpO2 is optional .

An indication of s igna l inadequacy must be provided if the SpO2 or pulse rate

value displayed is potent ially incorrec t.

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View Figure

Figure 24.12 Special mitts are available to shield pulse oximeter probes from ambient light.

I f a variable pi tch audi tory s ignal is provided to ind ica te the pulse signa l, the

pi tch change shall follow the SpO2 read ing (i.e., as the SpO2 reading lowers,

the pi tch shal l also be lowered).

Use

Sites Finger The probe is most commonly attached over the f ingert ip (Figs. 24 .4, 24.6). The

failure rate is less, and accuracy is better when the probe is p laced on the f inger

than on the earlobe (44 ,45,46).

The f inger is re lat ively sensit ive to sympathet ic system vasoconstric tion (47,48). If

there is poor ci rcu la tion, a f inger b lock, digital pulp space infi l tration , or a

vasodi lato r may improve performance (48,49,50,51,52,53,54). Vigorously rubbing

the f ingert ip may temporarily improve c irculat ion to the area (50).

I f there is dark fingernai l polish or synthet ic f ingernails , the probe should be

oriented so tha t it transmits light f rom one side of the finger to the other (55). Some

c lear acryl ic na ils do not affect pulse oximeter readings (56).

A disadvantage of p lacing a probe on an extremity is that de tection of desaturat ion

and resaturat ion is s lower than when probes are placed more centrally

(48,57,58,59,60,61,62,63,64). Response time may be quicker when the probe is

placed on the thumb (62).

Motion art ifac ts are less frequent when the probe is placed on one of the larger

f ingers (32). The l it t le f inger may be useful i f the pat ien t is part icularly large (65).

The probe may be placed over a finger tha t has a burn (66).

The probe should not be on the index f inger during recovery. As a pat ient awakens,

the pat ient of ten wil l want to rub his o r her eye, usual ly wi th the index f inger. I f the

ox imeter probe is on that f inger, the cornea can be sc ratched.

In general , the arm opposi te f rom that on which the blood pressure cuff is applied

or in which an arterial ca theter has been inserted should be used. The pulse

ox imeter is sometimes in tegrated wi th the noninvasive blood pressure monitor so

that the pulse ox imeter wi l l not ala rm during the inf lat ion cycle if placed on the

same arm as the blood pressure cuff . Insert ion of a radia l artery catheter is

commonly fol lowed by a trans ient

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decrease in b lood f low and loss of an adequate s ignal for a pulse ox imeter if the

probe is on a f inger of that hand (67). However, performance is unaffected if pulse

ox imeter readings are made on the arm in which an arterial cannula is present (68).

Occasiona lly, poor function may occur wi th probe attachment to the same ex tremity

as the in travenous infusion, due to loca l hypothermia and vasoconstric t ion.

View Figure

Figure 24.13 The console may be a freestanding unit. (Reprinted by permission of Nellcor Puritan Bennett, Inc., Pleasanton, CA.)

View Figure

Figure 24.14 Small handheld, battery-operated pulse oximeters are often used, especially during patient transport. This unit is in its recharger. (Picture courtesy of Masimo Corporation, Irvine, CA.)

The posi tion of the arm may a ffect the reading. In most patien ts , the SpO2 fa lls

af te r the monitored arm is raised (69). I t may also fall when the arm is lowered

(70).

Toe The toe is an alternate s ite when the f inger is not available or the s igna l from the

f inger is unsatisfactory. Detection of desatura tion or resaturat ion wil l not be as

rapid as with more centrally placed probes (63). The de lay in detec tion of

hypoxemia may be up to 1 to 2 minutes (64,71). The toe may provide a more

reliable signa l in patients who have had an epidural block (72). An inc rease in

pulse ampli tude from the toe may be a s ign of a successful block (73).

View Figure

Figure 24.15 Combined pulse oximeter and carbon dioxide monitor. (Reprinted by permission of Nellcor Puritan Bennett, Inc., Pleasanton, CA.)

Nose The nose is usual ly a convenient loca tion. Nasal probes respond more rapidly to

changes in saturation than probes placed on ex tremities. The bridge (Fig. 24 .7), the

wings of the nostri ls , and the nasa l septum have been used (74,75,76). The nose

c lip f rom a disposab le oxygen face mask can be attached to the outer surface of a

f lex ib le probe to make it f i t snugly over the bridge (77).

Accuracy at this s i te is controversial . It has been recommended under condi tions

such as hypothermia, hypotension, and infusion of vasoconstric to r drugs. In

hypothermic pa tients, the nasal septum was a more re liable si te than the f inger

(74,76). However, some studies have found tha t nasal probes often give grossly

erroneous results and have a higher fai lu re ra te than other s ites under condi tions of

poor perfusion (78 ,79).

I f the patien t is placed in the Trendelenberg posi tion , venous congestion may occur

around the nose, causing the pulse oximeter to display art i f icial ly low saturations

(80,81).

Ear An ear probe (Figs. 24.8, 24 .9) may be held in place by a plastic semic i rcula r

device hung around the ear.

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Stabi l izing devices such as headbands or around-the-ear loops may be useful .

Using a c lip may improve the qual i ty of the s ignal (82). The earlobe should be

massaged for 30 to 45 seconds wi th a lcohol o r vasodi la tor or EMLA cream can be

applied for 30 minutes prior to probe app lica tion to increase perfusion (83).

An ear probe can be part icula rly useful when there is finger motion . Response t ime

is fas ter with an ear probe than wi th a f inger probe (48,58,62,64,84,85). Under

condi tions of poor perfus ion , some ear probes perform better than finger probes

(78). The ear is rela tively immune to vasoconstric t ive effec ts of the sympathetic

system (47). The ampli tude of ear plethysmography wi l l respond mainly to changes

in pulse pressure. Ear probes may g ive more erroneous readings than f inger p robes

in patien ts wi th tricuspid incompetence (86). A steep head down posit ion may resul t

in e rroneous readings (87). An ear oximeter may be combined wi th a

transcutaneous carbon dioxide sensor (4,88).

Tongue A tongue probe can be made by placing a mal leable aluminum strip behind the

probe to allow i t to bend around the tongue (89,90). A disposable probe wrapped

around the t ip of the tongue in the sagi ttal p lane may also be used (91).

Ref lectance pulse oximetry has been used on the superior surface of the tongue.

The mouth should be c losed.

Glossal pulse ox imetry has been shown to be accurate (91). This s ite may be

especial ly useful in pat ients who have burns over a large percentage of thei r body

surface (90,91). Desaturat ion and resatura tion is detected by a probe at the tongue

quicker than one on the f inger or toe (63).

A l ingual probe is more resistant to s igna l in terfe rence from electrosurgery than

probes p laced on peripheral s ites but may be dif f icult to maintain in place during

emergence (89,90). Tongue quivering may mimic tachycardia. Other problems are

venous conges tion f rom a head down posi t ion and excess oral secre tions . The

probe mus t be posit ioned af te r tracheal intubation or insert ion of a supraglottic

ai rway. I t can be easi ly dislodged.

Cheek A probe with a meta l s trip backing can be used to hold a disposable probe around

the cheek or l ips (92,93). A cl ip -on probe wi th a cover over the part of the probe on

the buccal surface can also be used (94,95,96,97). Th is method of use is not

recommended by the manufacturer (98,99). Probes special ly designed for this s ite

are commercial ly avai lab le .

Buccal pulse oximetry is more accurate than f inger pu lse oximetry (93,100). Probes

at this location detect increases and decreases in saturat ion more quickly than

f inger or toe probes (63). Buccal oximetry has been found to be ef fective during

hypothermia, decreased cardiac output, increased sys temic vascular res istance,

and o ther low pulse pressure states. This site is useful in pa tients who have burns

(101). Disadvantages inc lude dif f icult placement, poor acceptance by awake

patients, and art i fac ts during ai rway maneuvers .

Esophagus This probe uses ref lectance oximetry. The esophagus, a core organ, is bette r

perfused than the extremit ies during states of poor peripheral perfus ion and may

therefore provide a more consistent, reliable source for pulse oximetry wi th

hemodynamic instabi l ity (102,103,104,105,106,107,108). It ref lec ts changes in

arte rial sa turat ion more qu ickly than peripheral sites such as the f inger. Th is s i te

may be useful for pat ients who have ex tensive burns, on whom conventional p robes

wou ld be dif f icul t to place (109,110).

Accurate placement of the esophagea l probe requires prac tice by the user (107).

Ach ievement of a re liable s ignal may be a problem (104).

Forehead A f lat ref lectance pulse oximeter sensor can be used on the forehead (82,111,112)

(Figs. 24.16 , 24.17). I t should be placed just above the eyebrow so that i t is

centered s light ly lateral of the i ris (113). The sensor s ite shou ld be c leaned wi th

alcohol before app lying the sensor to help secure the adhesive. Pressure on the

probe f rom a headband or pressure dressing may improve the s ignal (21,82).

This si te is usually easily accessible. The fo rehead is less affec ted by

vasoconstric tion from cold or poor

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perfusion than the ear or f inger (114,115). Changes in sa turat ion can be detected

more rapidly at the fo rehead than at the finger (48,116). However, pool ing of

venous blood due to compromised return to the heart may cause low saturation

readings in sup ine patients (81). It should not be used if the patien t is in the

Trende lenburg posi t ion. There are usually few motion art i fac ts when the forehead is

used (117,118).

View Figure

Figure 24.16 Reflectance pulse oximeter probe. The light source and sensor are situated next to each other. (Picture courtesy of Masimo Corporation, Irvine, CA.)

View Figure

Figure 24.17 Reflectance pulse oximeter on the forehead. Note the headband securing it in place. (Picture courtesy of Masimo Corporation, Irvine, CA.)

Other Pharyngeal pulse oximetry by using a pulse oximeter attached to a laryngeal mask

may be useful in patien ts wi th poor peripheral perfusion (119,120).

Flexible probes (Fig . 24.3) may work th rough the palm, foo t, penis, ankle, lower

ca lf , or even the arm in infants (27 ,121,122) (Fig. 24.10).

Pulse oximetry may be used to moni tor fetal oxygenation during labor by attaching

a ref lectance pulse oximetry probe to the presenting part (20,23,123,124). A

disadvantage is that the probe has to be placed bl indly and may be positioned over

a subcutaneous vein or artery, which wi l l affect the reliab il i ty of the readings (23).

Fixation Proper probe placement is c rucial fo r good performance. A malposi tioned probe can

resul t in false-posi tive and false-negative alarms. Probes can be total ly o r partially

dislodged without this be ing noticed.

Adhesive probes may s tay on better than c lip-on probes. It may be benef icial to

tape probes in place when they wi l l be inaccessible during surgery, but it is

importan t to avoid compression of the f inger or other part . Wrapp ing the l imb l ightly

wi th gauze may help to f ix the probe in posit ion . Another method is to s lip the cut

f inger of a glove over the probe (125). The probe should be protected from bright

l ight (F ig. 24.12).

Stabilizing the Signal The search that the pulse oximeter goes through when a probe is ini tia l ly appl ied

(or dis lodged) includes sequential tr ia ls of various intensit ies of l ight in an effort to

f ind a signal s trong enough to transmit th rough the t issue but not so s trong that the

detec tion system is saturated (12). Once a pulse is found, there is usua lly a delay

of a few more seconds whi le SpO2 values for several pulses are averaged.

Appearance of a satis fac tory waveform is an indicat ion that the readings are

reliable. Comparison of the pulse rate shown by the oximeter and that by an

electrocardiograph moni tor is a lso an ind ication that saturation readings are

reliable. A disc repancy between the rates is f requently an indication of probe

malposit ion or malfunction . A discrepancy can also occur during certain

dysrhythmias.

Reusing Disposable Probes Because d isposable probes are costly , many inst itut ions reuse them

(126,127,128,129,130,131,132,133,134). Although concerns about this have been

raised (135,136), several s tudies show tha t the fai lure rate of reprocessed probes

is equal to or less than that of new probes, and the accuracy is not affected

(132,133,134,137).

Testing Dev ices that can be used to test pulse oximeters are available (138). Some al low

the tes ter to se t the p lethysmograph ic waveform at dif ferent ampl itudes and to test

the accuracy of the heart rate as we ll as the SpO2.

Applications

Monitoring Oxygenation Anesthetizing Areas Oxygen desatura tion can occur a t anytime during anesthesia , regardless of the ski l l

and experience of the anesthes ia provider. Desatura tion greater than 10%

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occurs in up to 53% of anes thetized patien ts (139,140,141,142). Pediatric pat ients

are especially at r isk (121,143,144,145,146,147). Mos t severe desaturations occur

during induct ion or emergence. During maintenance, desaturations are milder but

more frequent (148). Studies have shown that a reduc tion in the number of

hypoxemic events occurred when pulse oximetry was used (144,149). The inc idence

of myocardial ischemia was also decreased (150).

Pulse oximetry may help to detect inadvertent bronchial intubation (8,151,152,153).

This method is not always reliable, part icula rly if an elevated inspired oxygen

concentration is being used (12 ,154). The absence of desaturat ion does not rule

out bronchial intubation (155). Methods to detect b ronchial intubat ion are d iscussed

in Chapter 19 .

Oximetry is useful in managing one-lung anes thesia to help assess the

ef fectiveness of measures taken to increase the oxygen sa turat ion (156) Chapter

20).

Oximetry is useful for patients undergoing regional and moni tored care anesthesia

(157,158). Of ten, the signs of hypox ia are confused with restlessness from an

inadequate block. Ins tead of supplying oxygen and assist ing resp irat ion , add it ional

sedation is provided, which compounds the problem. With oximetry, the pat ien t's

oxygenation status can be assessed and measures taken to improve SpO2.

Pulse oximetry may be useful to conf i rm correct tracheal tube placement when a

functional carbon dioxide moni tor is not avai lab le (159). I f oxygen saturation rises

af te r in tubation , correct tube placement is l ikely. However, a pu lse oximeter should

not be rel ied on fo r th is purpose because preoxygenation may delay the onset of

desatura tion beyond the time when esophageal intubation would be considered

l ikely.

Other problems that can cause a drop in oxygen saturat ion include fat embolism,

amniot ic f luid embolism, pulmonary edema, breathing system disconnec tions and

leaks , aspirat ion, tracheal tube obstruction, hypoxic gas mix ture, oxygen del ivery

failure, hypoventilat ion, anaphylaxis , bronchospasm, pneumothorax, malignant

hyperthermia, and pulmonary embol ism (8,160,161,162,163,164,165,166,167,168).

Causes of hypox ia related to equ ipment are discussed in Chapter 14 .

Postanesthesia Care Unit The recovery room is another location where desatura tion is common

(141,169,170,171,172,173,174,175,176,177). Routine oxygen administration to

recovering patients may not be necessary when patients are monitored with pulse

ox imetry (178). Before leaving the recovery room, a tr ial of breathing room a ir wh ile

moni toring oxygen saturat ion may provide an indicat ion of the need to cont inue

oxygen beyond the pos tanesthesia care un it (PACU) or to re ta in the patient in the

unit for a longer time (179).

Transport Unrecognized oxygen desaturation may occur while the patien t is being transported

between the opera ting room and the PACU and between that unit and other areas

(180,181,182,183,184,185,186,187,188,189). Pulse oximetry is included on mos t

transport moni tors , and portable pulse oximeters are available.

Other Intrahospital Areas Patients frequently experience hypoxic episodes in the pos topera tive period after

leaving the PACU (190,191,192,193,194). Pulse oximetry can detect these episodes

and a id in deciding when oxygen therapy should be discontinued. Telemetric pulse

ox imetry monitoring may be a cost-effect ive method of maximizing qual i ty of care

when used to monitor patien ts on a genera l care f loor (195).

Pulse oximetry is useful fo r moni to ring pat ients in the intens ive care unit (196). It

may be he lpful during weaning from art if ic ial venti lat ion (197,198).

Pulse oximetry has been used during cardiopulmonary resusci tation (199,200,201).

Because of art ifacts and lag t imes, i t is more useful in primary respiratory arres t

than in card iac arrest. I t is useful in assessing oxygenation during newborn

resusci tation (203).

Another area where pulse oximetry has proved useful is the emergency department

(204,205,206).

Ref lectance pulse oximeters can be useful for assessing fetal s ta tus during labor

and delivery by applying a forehead probe (207).

Pulse oximetry is useful in identify ing wh ich patien ts wi th tonic-c lonic seizures are

at r isk of hypoxic cerebral brain damage (208).

Out-of-hospital Use Pulse oximetry is useful in the prehospi tal setting, inc luding when transporting

patients by hel icopter or ambulance (209,210,211,212,213,214,215,216).

Controlling Oxygen Administration Pulse oximetry allows the lowest safe oxygen flow and concentration compatible

wi th safe levels of a rterial oxygenation to be administered. Keeping the oxygen

concentration and f low low wi l l help to dec rease the risk of a f ire (Chapter 32).

Monitoring Peripheral Circulation Pulse oximetry can detect arm posit ions that compromise c i rculat ion (217). The

pulse oximeter tha t is attached to a toe can help to warn of decreased perfusion at

the foo t in patients in the l ithotomy posit ion (218). However, i t cannot rel iably

detec t inadequate perfusion (219,220).

Moni toring oxygen saturat ion during shoulder arthroscopy has been recommended

as a test for

P.787

brach ial artery compress ion (221). However, an adequate pulse s ignal may be

present wi th brachial plexus compression (222).

Pat ients with l imb fractures may have compromised c i rcula tion distal to the

f rac ture. Pulse oximetry may serve as a usefu l guide to b lood flow to that area

(223,224,225). However, i t may not be helpful in warn ing that a compartment

syndrome is developing, because diminution of the arterial pulse dista l to the

compartment is a late s ign (226).

Pat ients who undergo mediastinoscopy are at r isk fo r brachiocephal ic artery and

aort ic arch compression between the mediast inoscope and the sternum. Arte rial

compression may be detec ted by measuring pulse wave ampli tude on a pulse

ox imeter (227,228).

Pulse oximetry may be used to evaluate the effect of a sympathetic block as

indicated by an increase in peripheral blood f low (229,230). It may be useful during

and after angiography to detect inadequate blood f low (231).

Pulse oximetry may be used to determine the best s i te of amputation or a rterial

bypass surgery (232). I t has been used to moni to r reimplanted or revascularized

digi ts (233,234,235).

Pulse oximetry can be used to measure pa lmar col la teral c irculat ion

(236,237,238,239,240,241,242,243,244). However, i ts usefulness for this has been

disputed (245,246). There is a report of radial artery occlusion that was detected by

pulse oximetry (247). It has proved usefu l in evaluat ing a pain ful hand af ter

creat ion of an arte riovenous f is tula (248). A s imilar test of the collateral ci rculat ion

may be performed on the foot by using pulse oximetry (237).

Determining Systolic Blood Pressure A pulse oximeter can be used to determine the sys tol ic blood pressure

(249,250,251,252,253,254,255,256). The blood pressure cuff is applied to the same

arm as the pulse oximeter. The cuff is inf lated slowly, and the pressure at the point

at wh ich the waveform is lost is no ted. It also can be determined by inf lat ing the

cuff we ll past the systolic pressure and looking for the onset of a s ignal as the cuff

is def lated . One study found that the bes t agreement wi th Korotkoff sounds and

noninvasive blood pressure equipment occurred when the average of blood

pressures est imated at the disappearance and reappearance of the waveform was

taken as the systolic pressure (257). In pediatric pat ien ts , blood pressure

determined by th is method was found to be more accurate than tha t determined by

an automatic noninvasive blood pressure monitor (258).

Pulse oximetry has been used for patients with pulseless d iseases of the

ex tremit ies to monitor sa turation and sys to lic blood pressure (259).

Locating Arteries When the axil lary artery cannot be palpated, i t may be located by plac ing a pu lse

ox imeter on a f inger on that s ide and pressing in the axil la unti l the pulse wave

disappears (260,261,262). Pu lse oximetry has also been used to locate the femoral

and dorsalis pedis arteries by using a pulse oximeter app lied to a toe

(231,263,264).

Avoiding Hyperoxemia In p remature neonates, administration of oxygen may be associated with

development of retinopathy and other pathologic condit ions. Pulse oximetry can aid

in t i trat ing insp ired oxygen by detecting hyperoxemia (265,266,267,268,269). I t is

recommended that the high SpO2 alarm be set at 95% or lower fo r th is purpose

(269,270,271).

Monitoring Vascular Volume and Sympathetic Tone I f the pulse oximeter beg ins skipping beats or performing intermit ten tly, the cause

could be hypovolemia (272). A corre lat ion between pu lse waveform ampli tude

variation during pos itive-pressure venti la tion and hypovolemia has been reported

(272,273,274,275,276). I f brief interruption of vent ilation causes the waveform to

return to normal or more cons tant funct ion, a tr ial of f lu id therapy may be

warranted.

One of the mos t useful and commonly overlooked plethysmographic features is

waveform ampl itude (277). Ampl i tude changes can be concealed by the auto-gain

function found on most pulse oximeters . When the auto-gain is tu rned OFF, certain

observat ions can be made. The plethysmograph s igna l ampl i tude is di rectly

proport ional to the vascular dis tensibil i ty over a wide range of card iac output.

During anes thesia, the unga ined pulse oximeter s ignal may be used to de termine

the extent of attenuat ion of the sympathetic response to st imuli .

Another important fea ture of the waveform is the dic ro tic notch (277). The notch

tends to descend toward the basel ine during increas ing vasod ilation and gets

higher with vasoconstric t ion .

Other Uses Other s i tuat ions where oximetry may be usefu l inc lude high-frequency jet

venti lation and determining the effect iveness of therapeutic bronchoscopy. I t can be

combined with measurement of mixed venous oxyhemoglobin saturat ion to estimate

oxygen consumption (278,279).

Pulse oximetry has been used to gauge pulmonary b lood flow in infants and

ch ildren with cyanot ic congen ital heart lesions (280,281).

P.788

Pulse oximetry can give warning of f luid extravasation (282).

Advantages

Accuracy Pulse oximetry is accurate , and accuracy does not change with time. Numerous

studies have shown that the dif fe rence between sa turat ion determined by pulse

ox imetry and arterial blood gas analys is is c linical ly ins ignif icant above an SpO2 of

70%

(13,14,26,28,46,57,60,121,156,267,283,284,285,286,287,288,289,290,291,292,293,

294,295,296,297,298,299,300). Mos t manufac turers claim that errors are less than

3% at saturat ions above 70% (243). This accuracy shou ld be suff icient for mos t

c linical purposes , except possibly neonatal hyperoxia. Changes in accuracy are

negligible over temperatures encountered in c linical use (301).

Pulse oximetry is accurate in pat ients wi th dysrhythmias, p rov ided tha t the SpO2 is

s table and the plethysmogram is noise-free and has reasonable ampli tude (302).

The SpO2 may be correct even if the pulse rate is not.

Independence from Gases and Vapors Pulse oximetry readings are not affected by anes thetic gases or vapors.

Fast Response Time Pulse oximetry has a fas t response time, especia lly compared wi th transcutaneous

measurements (156).

Noninvasive Pulse oximetry is non invasive , which al lows it to be used as a routine moni tor. I t is

readi ly accepted by awake patients, so it can be applied before induct ion of

anesthesia. The bleeding, arterial insuffic iency, emboliza tion, and in fection

sometimes seen after a rterial punc ture are avoided. Temporary elevat ion of the

PaO2 induced by pain and apprehension is avoided.

Continuous Measurements Saturation , pulse rate , and blood f low are continuously moni to red. Develop ing

trends can be detec ted and remedial act ion taken before severe hypoxia ensues.

Separate Respiratory and Circulatory Variables Continuously moni toring the quality of the peripheral pulse may be helpful in

determin ing whether a hypotensive patient has good cardiac output. If blood

pressure is low and pulse signal s trength is high, the pat ient is probably

vasodi lated but perfusing adequately. If , however, both blood pressure and pu lse

strength are low, perfusion may be inadequate.

Perfusion is indica ted by the pulse s ignal s trength, and oxygenation is indicated by

sa turat ion. Unl ike trans -cu taneous monitoring, the values disp layed do not require

inte rpretat ion . Most oximeters wi l l signal if the flow is not adequate to prov ide a

saturat ion value. Th is is helpful in determining a truly low sa turation value as

opposed to one caused by low f low.

Convenience The probe is simple and fast to apply. Si te prepara tion is minimal . Arte rial ization

of the sk in is not usually not necessary, except when the earlobe is the moni toring

s i te. No cal ibra tion or changing of electrolyte or membrane is required . A variety of

different p robes are available for d if fe rent si te app lica tions .

Fast Start Time There is minimal delay in ob taining the oxygen saturation . Readout typically begins

wi th in a few beats after applica tion of the probe. This is a distinct advantage over

transcutaneous monitoring , which requires a prolonged warm-up time.

Tone Modulation Changes in pulse tone wi th varying saturat ion a llow the user to be continuously

updated on SpO2 wi thout tak ing his or her eyes off the pat ient. Tone modulation

al lows a much quicker recogn ition of hypoxic episodes than does a f ixed tone (40).

Most anes thesia providers can detect the direction (but not the magni tude) of a

change in satura tion by l is tening to the change in pi tch of a pulse ox imeter tone

(303).

User-friendliness Most instruments are user-f riendly. Min imal training is required to learn to operate

the instrument.

Light Weight and Compactness The conso le can be made lightweight and compact. This fac i li ta tes use during

transport. Hand-held pulse oximeters are avai lable . Oxygen saturation monitoring is

availab le in nearly al l physiologic moni tors .

P.789

Probe Variety The wide variety of probe configurat ions confers broad cl in ical applicabil i ty to all

types of patients , inc luding preterm infan ts. The abil ity to use various vascular

beds offers advantages f rom the standpoin t of access during surgery and avoids

disturbing the surgica l field.

No Heating Required Heating the sk in is no t required . The probe can usual ly be lef t in place for extended

periods wi thout r isk of thermal inju ry.

Battery Operated Most stand-alone uni ts and those incorporated in to transport mon itors can be

operated on batteries .

Economy The use of pulse oximetry can save money by l imiting oxygen administrat ion to

s i tuat ions where i t is really needed and by decreas ing the number of blood gas

analyses (178,194,304,305,306). I t may be cost-effect ive to moni to r certa in pat ients

at high risk for transfer to the intens ive care un it (307). The use of monitors with

superior art ifac t f i l tering abi li ty may result in cost savings (308,309,310).

Limitations and Disadvantages

Failure to Determine the Oxygen Saturation There is a small but defini te incidence of failu re wi th pu lse oximetry

(140,311,312,313,314,315). Factors that are reported to contribute to higher fai lure

rates inc lude ASA phys ical s tatus 3, 4, or 5 patients; young and elderly pat ien ts ;

orthoped ic, vascular, and cardiac surgery; electrosurgery use; hypothermia ;

hypotension ; hypertension; durat ion of in traoperative procedure ; chronic renal

failure; low hematoc ri t; and motion (140,312,313,314,316,317,318,319). The actual

failure rate varies with the monitor (320).

A pulse oximeter may zero out, meaning that i t displays 00 for the SpO2 and pulse

rates values when i t fai ls to produce a measurement or it might display ______ fo r

the values (321). Some pu lse oximeters blank the display or g ive a message such

as Low Qual ity Signal or Inadequate Signal . Others freeze the disp lay.

Poor Function with Poor Perfusion Pulse oximeters require adequate pulsat ions to dis tingu ish l ight absorbed f rom

arte rial blood f rom venous b lood and tissue l ight (322). Readings may be unreliable

or unavailable if there is loss or diminution of the peripheral pulse (proximal blood

pressure cuff inf lation, external pressure, improper posi tioning, hypotension,

hypothermia, Raynaud's phenomenon, cardiopu lmonary bypass, low cardiac output,

hypovolemia, peripheral vascular disease, a Valsalva maneuver such as seen in

laboring pat ients or in those wi th infusion of vasoactive drugs

(78,140,251,287,319,322,323,324,325,326,327,328,329,330,331,332,333).

Methods to improve the s ignal inc lude applying vasodilating cream, performing

sympathet ic and dig ital nerve blocks, administe ring intra-arteria l vasodi la tors, and

warming cool extremities (49,51,52,83,334,335,336,337). The use of a probe on a

better perfused si te such as the cheek, tongue, nasal septum, or esophagus may be

helpful . Improved signa l techno logy by newer pulse oximeters can improve

performance during low-perfusion condit ions (320,330,338,339,340,341).

Difficulty in Detecting High Oxygen Partial Pressures At PaO2 values above 90 mm Hg, small changes in saturat ion are assoc iated wi th

rela tively large changes in PaO2. Thus, i t has l imited abil ity to dist inguish high but

safe levels of a rterial oxygen from excessively elevated levels (342).

Delayed Hypoxic Event Detection While the pulse oximeter response t ime is general ly fas t, there may be a significant

delay between a change in alveolar oxygen tension and a change in the oximeter

reading. It is possible for arte rial oxygen to reach dangerous levels before the

pulse oximeter ala rm is ac tivated (343). Sett ing the low SpO2 alarm threshold

higher wil l decrease the delay.

Delayed response can be re la ted to probe locat ion (62,85). Desaturation is

detec ted earl ier when the probe is p laced more centrally. Lag time wil l be increased

wi th poor perfusion (251,344,345). Venous obstruc tion, periphera l vasoconstric tion,

co ld, and motion art ifacts wi l l cause increases in the t ime to detect hypoxemia

(32,325,327).

The algori thms that are used to prevent false alarms may increase the delay in

detec ting hypoxic events (346). A pulse ox imeter may respond to a noisy or weak

s igna l by s imply holding on to an old value (321). Increasing the time over which

the pulse s ignals are averaged also inc reases the delay t ime.

Erratic Performance with Dysrhythmias I rregular heart rhythms can cause the pulse oximeter to perform errat ical ly (347).

During aortic balloon pulsat ion, the augmentation of diasto lic pressure exceeds

P.790

that of sys tol ic pressure. This leads to a double- or triple-peaked arterial pressure

waveform that confuses the pulse oximeter, so it may not prov ide a reading

(348,349). Pulse oximetry works in pa tients who have had an aortomyoplas ty (105).

Inaccuracy Different Hemoglobins Most pulse oximeters are designed to detect only two species of hemogobin:

reduced and oxygenated. Whole blood of ten contains other moiet ies such as

carboxyhemoglobin, sulfhemoglobin, and methemoglobin. Th is dis turbs the

absorbance rat io of the wavelengths used to de termine oxygen saturation (350).

Methemoglobin Normally less than 1% of the to tal hemoglob in , methemoglobin (metHb) is an

ox ida tion produc t of hemoglobin that forms a reversible complex wi th oxygen and

impairs the unloading of oxygen to t issues (351,352). Methemoglobinemia can be

congen ital (353) or acquired . Drugs causing methemoglobinemia inc lude

ni trobenzene (354), benzocaine (355,356), pri locaine (357,358,359), and dapsone

(352,360,361). Methemoglobin absorbs l ight equal ly a t the red and inf ra red

wavelengths that are used by most pu lse oximeters. When compared wi th func tional

sa turat ion, most pulse oximeters give falsely low read ings fo r saturations above

85% and falsely high values for saturations below 85% (351,362,363,364,365,366).

The disc repancy between SpO2 and functional sa turation inc reases as the level of

metHb increases and functional hemoglobin satura tion dec reases (351). With

trea tment of the methemoglobinemia, the SpO2 readings become more accurate

(357,358,361,365).

I f there are conf l ic ting resul ts between the pulse oximeter and arte rial blood gas

analys is, methemoglobinemia should be suspec ted, and the diagnosis should be

confi rmed by mult iwavelength co-oximetry. The s tandard blood gas analys is is no t

capable of de tecting and measuring metHb (367).

A new pulse ox imeter capab le of measuring metHb as we ll as carboxyhemoglobin is

now avai lable (Fig . 24.18).

Carboxyhemoglobin Carboxyhemoglobin (HbCO, COHb), formed when hemoglobin is exposed to carbon

monoxide (CO), has an absorpt ion spectrum s imila r to that of oxyhemoglob in , so

most pulse oximeters wil l over-read SpO2 by the percentage of carboxyhemoglobin

present (18,368,369,370,371,372,373,374,375). In one study, the pulse oximeter

reading did not go below 96% with carboxyhemoglob in levels as high as 44% (376).

In v itro CO-oximetry can measure the percentages o f other moiet ies by using more

than two wavelengths .

View Figure

Figure 24.18 Pulse oximetercarbon monoxide monitor.

An increase in HbCO may occur during laser surgery in the ai rway, bu t the levels

are not high enough to keep pu lse oximetry from rel iably est imating saturat ion

(377). Carbon monoxide production in assoc ia tion wi th dry carbon dioxide

absorbent is discussed in Chapter 9.

Pulse oximeters that different ia te between oxyhemoglobin and carboxyhemoglobin

and that can measure carboxyhemoglobin are now avai lable (Fig. 24.18).

Fetal Hemoglobin Most studies show that fetal hemoglobin (Hb F) does not appear to affect the

accuracy of pulse oximetry to a c linically important degree (378,379,380,381,383),

al though very high levels may cause i t to read sl ight ly low (382).

Hemoglobin S The use of pulse oximetry in the pat ient wi th s ick le cell disease is controvers ial .

Severa l investigators have concluded that pulse oximetry is inaccurate in these

patients, wh ich makes i t unreliable for de tec ting serious hypoxemia (383,384,385).

Other studies have found i t to be suff icient ly accurate to be useful

(386,387,388,389,390).

Sulfhemoglobin Sulfhemoglob inemia may be caused by drugs such as metoclopramide, phenacetin,

dapsone,

P.791

and sulfonamides (391). Sulfhemoglobin causes the pulse oximeter to d isplay

arti factual ly low oxygen sa turation.

Other Hemoglobinopathies Hemoglobin Koln is associated with art ifac tually low oxygen satura tion as measured

by the pulse oximeter (392,393).

Hemoglobin Hammersmith and Hemoglob in M (Mi lwaukee) affec t pulse oximeter

reading so much that ox imetry is not usefu l (394,395,396).

Hemoglobin-H disease wil l cause the pulse ox imeter to indicate a higher saturat ion

than is actual ly present (397).

A pat ient wi th hemoglobin Constant Springs and alpha-thalassaemia 2 in which

pulse oximetry readings were cons isten tly low has been reported (398).

Heinz Body hemolyt ic anemia causes the pulse oximeter to read low (399).

Bilirubinemia Severe hyperbil i rubinemia can cause an artifactual elevation of metHb and

carboxyhemoglobin when using in v itro oximetry but does not affec t pulse ox imetry

readings (283,378,400,401,402,403).

Low Saturations Pulse oximetry becomes less accurate at low oxygen saturat ions

(10,26,57,58,61,84,284,285,288,291,293,294,297,298,306,404,405,406,407,408,40

9,410,411,412,413,414). This inaccuracy is greater in patien ts wi th dark sk in (415).

I t should be used wi th caution in pat ien ts wi th cyanotic heart disease

(410,411,416). Measuring PaO2 or SaO2 at low saturations is recommended fo r

importan t c linical decis ions.

Malpositioned Probe Oximeters wi th probes that are not applied we ll vary greatly in their behavior,

depending on both the actual SpO2 and the manufac turer and model of the oximeter

(70,417,418,419,420,421,422). If the probe is not properly posi t ioned, i t may al low

the l ight from the emitter to the detector to only graze the t issue instead of passing

through i t. Th is penumbra effec t reduces the s ignal-to -no ise rat io and may result in

spurious SpO2 values in the low 90s in normal pa tients. I f the pat ient is hypoxic ,

the oximeter may overestimate the true value (418,423). In one case, the probe was

comple tely unattached but cont inued to prov ide apparent ly accurate readings.

Closer examinat ion of the waveform revealed an unusual pattern (424).

To avoid the problems of probe posi tion, the posi tion should be checked frequently

and inaccessible loca tions avoided. The use of too large or too small a probe may

resul t in inaccurate readings (33,34,300). Long f ingernai ls can cause inaccura te

posit ioning (425).

Venous Pulsations Pulse oximeter design assumes that the pulsa ti le components of l igh t absorbance

are due to arterial b lood. Prominent pulsations of venous b lood may lead to

underest imation of the SpO2 (70,426,427,428,429,429A). Pulse rate determination

may be correct. The error may be worse when probes are used on the head (86,87)

but less when the probe is placed on the f inger. In pat ien ts wi th low sys temic

vascular res istance, the pulse ox imeter may under-read the saturat ion , possibly

because the ox imeter is sensing pulsat i le venous f low (430).

High ai rway pressures during art if icial venti lat ion may cause phasic venous

conges tion, which may be in terpreted by the oximeter as a pulse wave (431). In

some cases , it may be necessary to turn the vent ilator OFF to obta in a correct

reading.

Mixing Probes SpO2 measurements may not be accura te if one manufacturer's p robe is used with

a dif ferent manufacturer's instrument (432,433).

Severe Anemia The pulse oximeter may overes timate SpO2, especially at low saturations, in

patients with severe anemia (434,435,436,437). However, i t is accurate for non-

hypoxic SaO2 values in these patien ts (438).

Skin Pigmentation Although some earl ier s tudies have shown that pulse oximeter readings were

s light ly high in pat ients wi th dark sk in (286,439,440), newer studies have shown

that pigmenta tion does not make a s ign if icant dif fe rence in pulse oximeter accuracy

(26,441,442). Spurious readings were reported in a pat ient wi th oculocutaneous

albinism who was taking a herbal remedy (443).

Dyes Certain dyes inc luding methylene blue, indocyanine green, lymphazurin ( isosulfan

blue), indigo carmine, ni trobenzene, and patent blue when injec ted intravenously,

intra-arteria lly, into the lymphatics , intradermal ly, or into the uterine cavi ty can

resul t in decreases in SpO2 without actual decreased saturation

(354,360,444,445,446,447,448,449,450,451,452,453,454). In v i tro oximetry may

also be affected by dyes (360,445,455,456,457). Usual ly, the in te rference lasts only

a few minutes but may persist much longer, even hours, when lymphatics are

injected (451,452).

The reac tion of the pulse oximeter to exogenous dyes has been used as a means of

confi rming intravascular catheter placement. The dye is injected into the catheter,

and the pulse oximeter is observed (458). The pu lse oximeter may be useful to

es timate card iac output by the dye di lut ion method (459).

P.792

Fingerprin ting ink wil l cause a low satura tion reading (460). Henna, a stain used by

some Middle Eastern women on the fingers and toes , can cause a low satura tion

reading (461). Chi ld ren who have been f inger paint ing wi th b lue pa in ts may exhibit

low SpO2 readings (462).

Optical Interference Stray l ight or l ight f l ickering a t frequencies s imilar to the frequencies of the LEDs,

including sunligh t, f luorescent l ights , operat ing room lights , infrared heat ing lamps,

infrared radiant warmers, l ight sources for various scopes, xenon lamps , bi l i rubin

l ights , phototherapy, or surgical imaging instruments, can enter the photodetector

and resu lt in inaccura te or e rratic readings

(14,332,463,464,465,466,467,468,469,470,471,472,473,474). Having probes on two

adjacent f ingers can cause an abnormal trace (475).

One clue that optica l in terference is occurring is inconsis tency between the pulse

rate on the pulse oximeter and tha t on other mon itors (350). A lthough excessive

ambient l ight usua lly prevents the oximeter f rom track ing the pulse , it can resul t in

apparent ly normal but inaccurate measurements in some ins tances (470).

Oximeters vary s ignif icantly in their susceptibil i ty to opt ical inte rference (326,466).

Some manufacturers try to minimize the effect of s tray l igh t by tak ing in termittent

readings when both of the LEDs in the probe are turned OFF and then subtrac ting

these background readings from measurements taken by the photodetector when

ei ther LED is turned ON. Sensi tiv i ty to l igh t may be increased with reduced pu lse

ampli tude.

There are a number of ways to minimize the e ffects of op tical interfe rence. These

include selection of the correct p robe for the patient and use, applying the probe so

that the detector is across f rom the LEDs , making certain the probe remains

properly posi t ioned, and shielding the probe f rom l ight and o ther nearby probes

(Fig . 24.12). Extraneous l ight can be eliminated by covering the probe wi th an

opaque material such as a surg ical towel , gauze, f inger cot, blanket, alcohol wipe

packet, or o ther foil shield (59,127,476). This may also help to stabil ize the probe.

All l igh t may not be adequately shielded by a s imple covering (463,470).

Nail Polish and Coverings Some shades o f brown, black, blue, and green (but not red or purple ) nai l po lish

may cause s ignif icant ly lower satura tion readings (477,478,479). Synthet ic nai ls

may interfere wi th pulse oximetry readings . The presence of onychomycosis, a

ye llowish gray color caused by fungus, can cause fa lsely low SpO2 read ings (480).

Dirt under the na il can also cause diff icul ty in obta in ing rel iable readings (481).

Al though there is one report of dried blood on a f inger that caused erroneous low

sa turat ion read ings (482), other authors have found tha t dried b lood does not affec t

pulse oximeter accuracy (483,484). Pat ients rece iv ing docetaxe l may have

discolored f ingernai ls that cause lower saturation readings (485).

In most cases, this problem can be overcome (wi thout removing the pol ish or the

synthetic na il ) by turn ing the probe 90 degrees so that i t transmits l ight from one

s ide of the f inger to the other s ide (55,481).

Electrical Interference Electrica l in terference from an electrosurgical unit can cause the oximeter to give

an incorrect pulse count (usual ly by counting extra beats) or to falsely register a

decrease in oxygen satura tion (486). This problem may be inc reased in pat ients

wi th weak pulse s ignals (13). Manufacturers have made s ign if icant p rogress in

reduc ing their instruments ' sensi tiv i ty to electrica l in terference (13,207,243,326).

Some monitors d isplay a notice when s ignif icant in terference is p resent. Some

f reeze the SpO2 display during such interference, which may give a false sense of

security. Addi tional s teps to minimize e lectrical interference inc lude locating the

electrosurgery grounding plate as close to, and the ox imeter probe as far f rom, the

surgical field as possible ; routing the cable from the probe to the oximeter away

f rom the electrosurgery apparatus; keeping the pulse oximeter probe and console

as far as possible from the surgical s i te and the electrosurgery grounding plate and

table; and operating the uni t in a rapid response mode. The electrosurgical

apparatus and pulse ox imeter should not be p lugged into the same power c ircui t

(486).

Motion Artifacts Motion o f the probe can cause an arti fact that the pulse ox imeter is unable to

differentia te f rom normal a rterial pulsat ions. Motion artifact creates both fa lse-

posit ive (false alarm) and fa lse-negative (missed hypoxemia) errors (32,487).

Changing alarm thresholds to reduce one of these errors wil l of ten inc rease the

incidence of the other type of e rror (488).

Motion is usual ly no t a problem during genera l anesthesia, but i f the patien t is

sh ivering, has a condit ion such as Park inson's disease, o r is mov ing about o r being

transported, motion art ifac ts can be s ignif icant (489,490). Evoked potential

moni tors and nerve stimulators can produce motion artifacts if the pulse oximeter

probe is on the same extremity (491,492,493,494,495). Motion arti facts have been

caused by patients tapping their fingers whi le under regional anesthesia (496).

The oximeter's abi l i ty to deal with motion art ifact depends on the correlat ion with

the onset of the motion and the s tart of monitoring. If the motion precedes the

onset of moni toring, there is a greater decrement in performance (497,498).

In the 1990s, pulse oximeter manufac turers began to make design improvements,

and the newer genera tion

P.793

instruments have improved abi l ity to f i l ter motion art ifacts

(320,321,338,341,488,497,498,499,500,501,502,503,504,505). In addi tion to

reduc ing alarms, the use of these instruments has resul ted in less need for arte rial

blood gas measurements and fas ter weaning f rom high concentra tions of oxygen

(309). They are also associated wi th a shorter loss of s ignal when placed d istal to a

blood pressure cuff or tourniquet (506).

Lengthening the averaging t ime wi l l increase the l ikel ihood that enough true pulses

wi l l be detected to reject motion artifacts (15,507) but may delay detection of

hypoxemia. Most pulse oximeters al low the user to select one of several t ime-

averaging modes.

Motion art ifac ts can usual ly be recognized by false or e rrat ic pu lse rate d isplays or

dis torted plethysmographic waveforms. Inc reased pulse ampli tude indicates

movement but not necessari ly arti factual SpO2 readings (487,508).

Motion art ifac ts may be decreased by applying the probe to a less active si te.

Flexible probes that a re taped in p lace are less suscept ible to motion art i facts than

are c lip-on probes (13,15). Larger f ingers may be less susceptible to motion art ifac t

(32).

Pressure on the Probe Pressure on the probe may resul t in inaccurate SpO2 readings wi thout affect ing

pulse rate determinat ion (429).

Hyperemia I f a l imb becomes hyperemic af te r blood f low is interrup ted, the oxygen satura tion

shown by the pulse oximetry may be arti f ic ia lly low (509). A pulse ox imeter placed

near the s ite of blood transfusion may show transien t decreases in oxygen

sa turat ion wi th rapid blood infusion (510).

Probe Damage A damaged pulse oximeter probe can cause the oxygen saturat ion to be higher than

the actual value (511,512). The use of a cleaning agent that is not recommended by

the manufacturer on a reusable probe can resul t in damage to the probe, prevent ing

i ts reuse (513).

False Alarms A high percentage of pulse oximetry alarms are spurious or triv ial

(489,503,505,514,515,516,517). Art ifac t-induced ala rms occur in two ways. When

an artifact is mis taken fo r a pulse, i t can corrup t the measurement and resul t in an

alarm. When an artifact obscures the pulse, it can resul t in a loss -of -pulse alarm.

False alarms are mos t commonly caused by motion artifact bu t are also associa ted

wi th poor s ignal qua li ty , probe displacement, external p ressure, and interfe rence.

False alarms are a more s ignif icant problem outs ide the opera ting room because

patients are commonly moving, they are of ten poorly perfused, and there are many

sources of elec tronic and opt ical interference.

False alarms do not represent a di rec t danger to the patient but may encourage the

care provider to take inappropriate ac tions such as disabling the alarm, setting the

l imi ts to inappropria te values, or lowering the ala rm volume. Misinterpretat ion of

alarms can resul t in failu re to treat hypoxemia or unnecessary treatment.

Some false alarms can be avoided by s imple measures such as putt ing the probe

on a different extremity than the automated blood pressure cuff and in a locat ion

where i t is un likely to be affected by ex terna l pressure.

Newer pulse oximeters tha t are designed to reduce motion-related art i fac ts can

s ignif icant ly reduce the inc idence of false alarms

(320,497,498,499,503,518,519,520,521,522,523,524,525,526). However, some of

these have shown less rel iabi l i ty in ident ifying hypoxic episodes and bradycardia

than older models (346).

Delaying the time between detecting low SpO2 and alarm activa tion, us ing a longer

averaging t ime, and sett ing the low SpO2 alarm l imit lower can reduce the number

of false alarms (311,517,527,528,529) but may increase the lag t ime before

detec ting hypoxemia. With some pulse oximeters , tu rning OFF the low pu lse rate

alarm prevents a la rming when the blood pressure cuff is inf lated (13).

Synchronizing the pulse oximeter wi th the electrocardiogram (ECG) moni tor can

lessen art ifac ts (32,39). However, the oximeter may synchronize with ECG art ifac ts

generated by motion or shivering , resul ting in e rroneous readings (13).

Furthermore, with this system, the pulse rate displayed by the oximeter wil l

necessari ly be equa l to the pulse rate shown by the ECG monitor, so equal ity of the

pulse rates cannot be used as an indicat ion tha t the d isplayed sa turation data are

valid.

Failure to Detect Impaired Circulation The presence of a pulse oximeter signa l and a normal reading does not necessari ly

imply that tissue perfus ion is adequate. Some pulse oximeters show pulses despi te

inadequate tissue perfusion (251,327) or even when no pulse is present

(200,530,531,532). Ambient l ight may produce a fa lse s ignal (471).

Pulse oximetry is not rel iable in diagnosing impaired perfusion wi th increased

intracompartmental pressures (533,534).

Discrepancies between Readings from Different

Monitors A discrepancy in readings between different b rands of ox imeters on the same

patient a t the same time is no t uncommon (17,270,535,536,537,538). There is a lso

variation in

P.794

the time tha t it takes various moni tors to detect resaturation (539).

Failure to Detect Hypoventilation Hypoventi lation and hypercarbia may occur wi thout a dec rease in hemoglobin

oxygen saturat ion, espec ial ly if the pat ient is receiv ing supplemental oxygen

(540,541,542). Pulse oximetry cannot be relied on to detect leaks, disconnect ions

(543), or esophagea l in tubation (544). Methods to detect esophageal intubat ion are

discussed in Chapter 19.

Problems with Sound Recognition There is cons iderable variat ion in the volume and aud io spectrum of available pulse

ox imeters between models and even wi th in mode ls (545). Some c linic ians have

trouble detecting changes in the pi tch o f the sound emitted by pulse oximeters as

the saturation changes (303,546).

There is wide variat ion in the pitch f requency of di fferent pulse oximeters (547).

Thus, in locat ions where d if ferent pulse ox imeters are encountered, the potential

for confusion exists.

Lack of User Knowledge Pulse oximetry is of ten used by personnel whose knowledge of it is l imi ted.

Physicians, nurses, and others who use the instrument of ten do not know the bas ic

principles and make serious errors in interpreting readings (548,549,550,551).

Interference with Other Monitors Electromagnetic in terfe rence from the pu lse oximeter power supp ly may cause

arti facts and fa lse readings on certa in thoracic impedance moni tors (552).

I f a pulse oximeter probe is placed in front of certain plasma display touch screens ,

a normal-appearing waveform and 100% saturation is displayed (553).

Patient Complications

Corneal Abrasions Patients recovering from general anesthes ia f requently rub their eyes. If there is a

pulse oximeter on the index f inger, a cornea l abrasion may resul t (65,554,555). A

f inger other than the index f inger may be a more appropriate loca tion fo r the probe

during recovery (556).

Pressure and Ischemic Injuries Inju ries ranging from pers istent numbness to ischemic injury at the s ite on which a

probe was placed have been reported

(557,558,559,560,561,562,563,564,565,566,567,568). Loss of the s ignal may occur

(569). These risks are increased by pro longed probe applica tion, compromised

perfusion of the extremity, and tight appl icat ion of the probe. Frequent examination

of the s i te and moving the probe to different s ites wi l l reduce the l ikel ihood of

injury. Pat ients wi th large f ingers should not have a c i rcumferent ial probe placed on

the f inger. I f the pulse oximeter reading appears to be weak, the s i te should be

checked for inc reased pressure .

Burns Inju ries ranging from reddened areas to third-degree burns under pulse oximeter

probes have been reported

(562,570,571,572,573,574,575,576,577,578,579,580,581). Considering the mil l ions

of long-term applica tions , the inc idence of these burns is quite low (43).

Burns can result f rom incompatib il i ty between the probe f rom one manufacturer with

the pulse oximeter of another (575,578). A number o f pulse oximeter probes have

connec tors that f it d if fe ren t pulse ox imeters, bu t the probes are not compatible. The

use of a damaged probe can resul t in a burn (570,577). A pu lse oximeter probe may

provide an al ternate pa thway for electrosurg ical currents (582).

Burns that are associated with hypothermia have been reported (579). Burns have

been reported when a pu lse oximeter was used during photodynamic therapy

(583,584).

To avoid these injuries, f requent inspect ion of the probe s i te and s ite ro tat ion are

recommended (585). When a probe is placed on a finger or toe, the l igh t source

should be placed on the nai l rather than on the pulp (576). A glove can be placed

on the finger to protect i t f rom thermal inju ry wi thout affect ing the accuracy of the

instrument (36). If the pu lse oximeter display freezes, the cause should be

investigated. Only the probes recommended by the ox imeter manufacturer should

be used (578).

Burns that are associated with pulse oximetry during magnetic resonance imaging

(MRI) as a resul t of induced skin current beneath looped cables ac ting as antennae

have been reported (586,587,588). The MRI envi ronment is discussed in Chapter

30. During MRI, the danger of burns can be reduced by the fol lowing measures:

All potential conduc tors should be checked before use to ensure that there is

no f rayed insulat ion, exposed wires, or other hazards.

All unnecessary conduct ive materials such as unused surface coils should be

removed f rom the MRI system bore before pat ient moni toring is ini t iated.

P.795

The probe should be placed as far f rom the imaging s i te as possible.

Cables , leads , or wires f rom monitoring devices should be posit ioned so that

no loops are fo rmed. A braid should be made of the slack port ion of wires .

I f possible , no potential conduc tors should touch the patient a t more than

one location.

A thick layer of thermal insu lat ion should be p laced between any wires or

cables and the patien t's sk in.

Monitoring devices that do not appear to be operat ing properly should be

removed f rom the pat ient.

Electric Shock An electrica l shock related to dia thermy has been reported (589). In this case,

there were bare wires in the pu lse oximeter probe.

Carbon Monoxide Monitoring Carbon monoxide can accumulate in the breathing system from various sources,

including the react ion between anes thetic agents and desiccated absorbent. This is

discussed in detai l in Chapter 9. It is diff icul t to de termine if CO is present in the

inspired gases when the pat ient is anesthetized.

In 2005, a combinat ion pulse CO-oximeter capable o f measuring

carboxyhemoglobin (SpCO) became avai lable (Fig. 24 .18). The same sensor is

used for measuring both SpCO and SpO2 . It ala rms wi th CO concentra tions

between 5% and 50%. The instrument uti l izes an e ight-wave length sensor to

dis t inguish between oxygenated blood, deoxygenated blood, and blood containing

CO. I t can a lso measure metHb.

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