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Chapter 29
Temperature MonitoringIndications for Temperature MonitoringNumerous s tudies have shown that s ignif icant temperature changes routinely occur
in anesthetized patients ( 1 ,2 ,3 ) . The monitoring guidelines of the American Society
of Anesthesiologists s t ate that Every patient receiving anesthesia shall have
temperature monitored when cl inical ly signif icant changes in body temperature are
intended, anticipated or suspected. The Standards for Nurse Anesthesia Practice
state that one must monitor body temperature continuously on all pediatric patients
receiving general anesthesia and, when indicated, on all other patients ( 3A ).
Temperature monitoring should be performed whenever large volumes of cold blood
and/or intravenous f luids are administered, when the patient is del iberately cooled
and/or warmed, for pediatr ic surgery of substantial durat ion, and in hypothermic or
pyrexial patients or those with a suspected or known temperature regulatory
problem such as malignant hyperthermia. Major surgical procedures, especial ly
those involving body cavit ies, should be considered a strong indicat ion for
temperature monitoring.
Standards A U.S . s ta nd ard fo r el ec tro nic the rmo mete rs , not inc ludin g inf ra re d the rmo me ters ,
was published in 2000 ( 4 ) . Celsius display thermometers must be graduated in
intervals of not greater than 0.1C, while thermometers using a Fahrenheit display
must be graduated in intervals of not greater than 0.2F. The maximum error for an
individual reading is shown in Table 29.1 .
There is also a U.S. s tandard for infrared thermometers ( 5 ) . This specif icat ion does
not prescribe a method for determining cl inical accuracy.
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TABLE 29.1 Maximum Error for Electronic Thermometers
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Temperature Maximum Error
Celsius Scale
Less than 35.8C 0.3F
35.8C to less than 37.0C 0.2F
37.0C to 39.0C 0.1F
39.0C to 41.0C 0.2F
Greater than 41.0C 0.3F
Fahrenheit Scale
Less than 96.4C 0.5F
96.4C to 98.0C 0.3F
98.0C to 102.0C 0.2F
102.0C to 106.0C 0.3F
Greater than 106.0C 0.5F
(From ASTM 15111200, 2000).
Technologies
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A varie ty of tec hn ologi es are av ailab le to me asure tem pe ra tu re. No ne is su i tab l e fo r
al l s i tuat ions. Many devices simply display the temperature. These are less than
optimal, because a high or low temperature may go unnoticed for some t ime. Mostmodern devices have alarms that can be set i f the temperature exceeds high or low
limits . Temperature-monitoring capabil i ty is available on most physiologic monitors
used in the operating room and perioperative areas. Frequently, they have the
abil i ty to measure temperature at two different s i tes ( Fig. 29.1 ) . These devices
usually have the abil i ty to trend the temperature and to transfer temperature
information to an electronic record.
There are a number of s tand-alone devices, both mechanical and chemical . Those
stand-alone devices that are battery-powered should have a means to indicate
when battery power is low. Trend i ndicators are available on s ome of these
instruments.
T h e r m i s t o r
A therm istor is comp osed of a me tal ( i .e . , ma ng an es e, nickel , cob alt , i ro n , or zinc )
oxide sintered into a wire or fused into a rod or bead ( 6 ,7 ) . There must be a source
of current and a means to measure that current. Resistance of the metal oxide
increases as the temperature decreases and vice versa so the resistance can be
converted to a temperature. Advantages of thermistors include small s ize, rapid
response t ime, continuous readings, and sensi t ivi ty to s mall changes in
temperature. They are fair ly inexpensive. Probes c an be interchangeable and
disposable.
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View Figure
Figure 29.1 Monitor with capability for monitoringtemperature at two sites and alarms. (Courtesy of FisherPaykel Healthcare.)
T h e r m o c o u p l e
A therm oc oup le cons is ts of an elec tri ca l c i rc uit th a t ha s two diss imilar me tal s
welded together at their ends ( 6 ,8 ) . One of the two metal junctions remains at a
constant temperature. The other is exposed to the area being measured, producing
a voltage difference that is measured and converted to a temperature reading.
Adv an ta ge s of th erm ocoup les include accu ra c y, small s iz e, rap id res po ns e t ime ,
continuous readings, stability, and probe interchangeability. The materials are
inexpensive, so the probes can be made disposable.
P l at i n u m W i r e
The electr ical resistance of plat inum wire varies almost l inearly with temperature.
By employing an extremely small d iameter wire, rapid thermal equil ibrat ion is
possible. Resistance is measured in a manner similar to a thermistor. These
thermometers are accurate and giv e continuous readings. Probes can be made
interchangeable.
If a probe with a thermocouple, thermistor, or plat inum wire is to be used inside the
body, i t must have
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an outer sheath. The end of the probe designed to be inserted into the patient is
sealed and the electr ical connection made at the o ther end. The connection needs
to be kept d ry. If i t becomes wet, erroneous readings can result ( 9 ,10 ,11 ).
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View Figure
Figure 29.2 The flexible adhesive-backed strip of thisliquid crystal temperature monitor has a black background.To use, the covering over the adhesive is removed, and themonitor is placed on the skin.
L i q u i d C r y s t a l
Certain organic compounds in thermal transformation from a solid to a l iquid s tate
pass through an intermediate phase that exhibits anisotropic (optical ly act ive)
propert ies ( 12 , 13 ). The term liquid crystal is used to describe this s tate. When l ight
shines on such a material , crystals scat ter some of the l ight , producing ir idescent
colors. The l iquid crystals are encapsulated so that the colors form let ters and
numbers.
A l iqu id crys tal tem pe ra ture mo ni to r is shown in Figure 29.2 . I t consists of a f lexible
adhesive backing with plast ic-encased l iquid crystals on a black background that
prevents reflect ion of the transmitted l ight and enhances the color resolution. The
covering over the adhesive is removed, and the disc or s tr ip is placed on the skin.
The l iquid crystal thermometer is available in two forms: one displays the skin
temperature direct ly; the other has a buil t- in correct ion factor (offset) so that the
temperature displayed est imates core temperature ( 7 , 13 ).Liquid crystal thermometers are safe, convenient , noninvasive, easy to apply and
read, disposable, nonirr i tat ing, and inexpensive. They give fast , continuous
readings and involve no electronic circuitry. They can be applied before induction
of anesthesia and are easi ly transferred to the recovery area with the patient .
Disadvantages of l iquid crystal thermometers include the need for subject ive
observer interpretat ion and the inabil i ty to interface with a recording system. They
are less accurate than other devices. Extreme ambient temperature, humidity, and
air movement can cause inaccuracy ( 13 ). Other disadvantages include diff icult ies
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with adhesion secondary to skin secret ions, al lergic reactions to adhesive backing,
inaccuracy, and imprecision ( 14 , 15 ). They are capable of measuring temperature
only on the skin. If lef t in the sun for an extended period, an error indicat inghyperthermia can be produced. If the device is frozen, al l the numbers can be
visible at once ( 13 ). Infrared heating lamps may cause erroneously elevated
readings ( 12 ). There is a report of a l iquid crystal thermometer that gave a falsely
high temperature ( 16 ).
I n f r a r e d
An infra red th erm om ete r is an el ec tron ic in str umen t th at mea sure s a port ion of the
infrared radiat ion from surfaces within i ts f ield of v iew ( 17 , 18 ,19 ). Electromagnetic
radiation is emitted by an object in proportion to its temperature. Because theprobe is placed in the outer part of the ear canal and usually has a relat ively wide
view angle, i t detects infrared emissions from parts of both the ear canal wall and
tympanic membrane and computes an av erage or highest temperature. The
displayed value may be the actual temperature (unadjusted, calibration mode) or
with an offset to est imate temperature at another si te based on selected study
samples (si te equivalent mode) ( 5 ,7 ,17 ,20 ).
The instrument has an otoscopelike probe ( Fig. 29.3 ) . Disposable probe covers are
used for hygiene and to prevent cerumen buildup. The probe should be placed
carefully but f irmly as far as possible into the ear canal , aiming toward the
tympanic membrane. The probe window should be clean and s hiny, and a n ew
probe cover should be used each t ime.
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View Figure
Figure 29.3 The infrared thermometer's probe is insertedinto the external ear canal.
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Patients tolerate infrared thermometry well ( 18 ). Because i t does not contact any
surface, there should be no trauma associated with its use. Measurements are
rapid. This technology is often used in the postanesthesia care unit and other
cri t ical care areas.
There are a number of problems with infrared thermometry. Poor penetration,
improper aiming, and obstructions such as c urvatures of the ear canal can result in
signif icantly lower temperatures. Measurements are intermit tent . This device is
general ly not useful for monitoring in the operating room.
M e r c u r y - g l a s s
These thermometers are not useful for intraoperat ive anesthesia, as they have a
long equil ibrat ion period (usually 3 to 5 minutes) and cannot be read without beingremoved from the monitoring site. In the future, they will be banned to reduce
mercury contamination in the environment.
If a new thermometer is used on each patient , there is a waste-disposal problem. If
the thermometer is reused, i t must be cleaned and steri l ized. If a thermometer is
broken, the mercury vaporizes and can present a health hazard.
Thermal Compartments
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Al th oug h arbi tr ary, i t is some wh at us eful to div ide th e bod y into two th erm a l
compartments: the core that includes the deep, vi tal internal organs and a shell of
peripheral t issue that serves as insulat ion for the core.
C o r e
Core temperature is uniform and high compared with the rest of the body. I t
normally varies between 35.7C and 37.8C ( 21 ,22 ,23 ). When signif icant changes
in body heat are expected, core temperatures should be monitored.
Sites differ in how well they reflect core temperature. The difference may depend
on the rate of temperature change. A si te that reflects core temperature accurately
when temperature change is s low may fai l to reflect rapid changes.
P e r i p h e r yNormally, thermoregulatory vasoconstriction maintains a temperature gradient
between the core and periphery (shell) of 2C to 4C. Regional temperature
variat ions exist i n the periphery. The correlat ion between temperatures measured
at different body si tes depends on several factors, including the stabil i ty of the
body temperature and whether or not there have been recent cold or warm
challenges. Skin and axil lary temperatures are usually considered shell
temperatures.
Monitoring SitesBody temperature can be monitored at a number of sites. The temperature can vary
considerably in different parts of the body at any t ime. Factors influencing
temperature at any given si te include the t issue 's heat production, the temperature
and rate of blood flow through the area, the amount of insulation from the
environment, and external influences on the site ( 24 ).
The best site for temperature monitoring depends on the purpose of the
measurement, durat ion of the surgical procedure, the surgical s i te , the anesthesia
technique, and av ailable equipment. Considerat ions should include accuracy,
speed, convenience, access, safety, pat ient acceptabil i ty, and cost-effect iveness.
Abs ol ute ac curac y is us uall y no t nec essa ry , but mea surem ents mu st be c lo se
enough to detect temperature changes that may influence treatment decisions. It
may be helpful to monitor two si tes. The difference between core and a second si te
can provide indirect information on blood f low (slow change = poor blood f low) and
is helpful in guarding against an overshoot during warming or cooling.
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Studies comparing temperature measurement among different sites are often
confusing. Many try to relate peripheral s i tes to c ore temperature and use different
si tes as the standard for core temperature.
P u l m o n a r y A r t e r y
Pulmonary artery temperature can be measured in patients who have a Swan-Ganz
catheter with a thermistor in place. It is thought by many to be the best method of
measuring core body temperature ( 25 ,26 ).
Pulmonary artery temperature generally correlates well with intrathecal and jugular
bulb temperatures, even with rapid cooling and rewarming ( 27 ,28 ). Poor correlat ion
with brain temperature was found during profound hypothermia ( 29 ).
Pulmonary artery readings are not rel iable during thoracotomy or cardiopulmonarybypass when there is no f low through the heart and lungs and may be direct ly
affected by the cardioplegia used during cooling.
E s o p h a g u s
Esophageal temperature measurement can be accomplished by using a simple
probe, an esophageal stethoscope with thermistor ( Fig. 29.4 ), or a gastric tube with
the temperature sensor some distance from the end of the tube ( Fig. 29.5 ).
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View Figure
Figure 29.4 Esophageal stethoscope and temperature probe.
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Temperatures in the esophagus may vary up to 4C, depending on the temperature
probe location within the esophagus ( 7 ). The esophageal temperature should be
measured with the sensor located in the lower third or fourth of the esophagus ( 30 ). At thi s de pth, th e es opha gus l ies be tween th e heart an d the des cen ding ao rt a.
Placing the sensor in this posi t ion wil l minimize (but not c ompletely el iminate) the
effect of respired gases ( 31 ). When the sensor is part of an esophageal
stethoscope, the ideal depth of placement is 12 to 16 cm dis tal to the point of
maximum heart sounds ( 32 ). If the probe is placed higher in the esophagus, the
reading will be lower ( 30 ,32 ,33 ,34 ). If the probe is placed in the stomach, it may
record temperatures higher than core, reflecting liver metabolism. In addition, the
response time to temperature changes is slow with the probe in the stomach.
The probe is most accu rately placed by using an electrocardiographic lead buil t into
the probe ( 35 ,36 ). The posi t ive lead is at tached to the probe, and the negative lead
is attached to the right shoulder. A biphasic P wave indicates that the probe tip is
at the midatr ial level .
In adults , the ideal posi t ion is approximately 38 to 42 cm below the central inc isors
(32 ) or at least 24 cm below the larynx ( 30 ). For nasal insert ion, the fol lowing
formulas can be used ( 37 ):
L (cm) = 0.228 (standing height) - 0.194
orL (cm) = 0 .479 (si t t ing height) - 4.44
where L is the length from the opening to the nares.
View Figure
Figure 29.5 Gastric tube with temperature probe. This alsofunctions as an esophageal stethoscope.
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In children, the ideal dis tance in centimeters below the corniculate cart i lages is
approximated by the fol lowing formula ( 38 ):
10 + (2 age in years)/3 cm
A temp era ture prob e can be ins ert ed into the es oph ag us thro ugh the drain tub e of a
ProSeal laryngeal mask. The optimal location for the temperature probe is 15 to 20
cm distal to the drain tube ( 39 ).
Esophageal temperature is considered core temperature by many investigators.
Temperatures measured in this location have shown good agreement with
pulmonary artery temperature ( 28 , 40 , 41 ,42 ,43 ,44 , 45 ). During rapid warming or
cooling, esophageal temperature shows less lag time than that measured at most
other si tes ( 46 ), although some studies found that bladder temperature showed a
closer approximation to pulmonary artery temperature than the esophageal
temperature ( 47 ,48 ). Brain temperature may be adequately reflected by esophageal
temperature during mild, but not profound, hypothermia ( 29 ).
Patients having act ive or passive airway humidificat ion have sl ightly h igher
esophageal temperatures ( 31 ,34 ).
Contraindicat ions to use of an esophageal probe include procedures on the face,oral cavity, nose, airway, or esophagus and patients who have esophageal
disorders. I t is poorly tolerated by awake patients . Correct placement may be
diff icult , and probes may become displaced. Esophageal temperatures are
unrel iable during thoracic surgery. Continuous gastr ic suctioning wil l cause a
decrease in esophageal temperature ( 49 ). When an esophageal probe is used with
the patient in the si t t ing or prone posi t ion, oral secret ions can track down to the
connection between the probe and monitor cable. This ca n lead to incorrect
readings ( 9 ) .
N a s o p h a r y n x
The temperature of the nasopharynx is measured with a sensor that is in contact
with the posterior nasopharyngeal wall posterior to the soft palate. This location
should place i t c lose to the hypothalamus.
Al th oug h some s tudie s sho w a goo d c orr elat ion of nasop haryn gea l te mp era tu re wi th
core temperature ( 29 , 46 , 50 ,51 ,52 ), other s tudies have found the correlat ion less
sat isfactory ( 53 ,54 ,55 ).
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Electronic rectal thermometers may have two measurement modes: dwell or
monitor. In the dwell mode, the temperature is displayed continuously. The dwell
mode requires a minimum of 2 to 3 minutes to reach a stable temperature. Thepredictive mode estimates the temperature on the basis of the curve of temperature
rise. This mode requires only 30 seconds. A comparison of the measurements of
the two modes yielded similar values during steady-state condit ions and mild
temperature f lux.
Disposable probe covers can be used to avoid c ross contamination. Probes should
be inserted to at least 8 cm in adults and 3 cm in children ( 65 ). The depth of
insert ion can be marked on the probe. I t should be checked for placement after the
patient is moved. The probe should be securely taped to the patient 's buttocks, and
the lead wire should be secured to a garment or sheet to avoid probe displacement.
Rectal temperature is influenced by heat-producing flora, the temperature of the
blood returning from the legs, and insulat ion by feces. Rectal temperature is
usually somewhat higher than that measured at more c entral s i tes during s teady-
state condit ions ( 51 , 66 ,67 ,68 ,69 , 70 , 71 ). The rectum is not a vascular area, and the
lag time may be prolonged with shifting temperature ( 26 ,52 , 55 , 72 ,73 ).
The rectum is usually accessible and monitoring relat ively noninvasive. Rectal
temperature is not influenced by ambient temperature. I t is general ly disl iked by
patients as uncomfortable, by hospital personnel as cumbersome, and by b oth asaesthetical ly object ionable. Probes are p rone to extrusion during recovery from
anesthesia. Other disadvantages are the relat ive inaccessibi l i ty during surgery and
the r isk of bacterial contamination. Contraindicat ions include gynecologic and
urologic procedures. Bowel perforat ion is a r isk ( 74 ). A pararectal abscess and
pneumoperitoneum have been reported with its use ( 75 ,76 ).
Ty m p an i c M e m b r an e
Two different technologies ut i l ize the ear. One uses a probe that contacts the
tympanic membrane, and the other measures infrared radiation.
Contact
The anatomical posi t ion of the tympanic membrane is deep within the skull and
separated from the internal carotid artery by only the narrow air-f i l led c left of the
middle ear and a thin shell of bone, making i t an at tract ive si te for temperature
measurements. The tympanic membrane and the hypothalamus share a common
blood supply, so measuring temperature at this s i te may reflect thermal information
at the primary si te of thermoregulat ion ( 17 ).
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Temperature can be measured by inserting a thermistor or thermocouple probe into
the external auditory canal un ti l i t contacts the tympanic membrane. Because of the
danger of perforating the membrane, clinicians tend not to place the probe farenough into the canal . If i t does not touch the membrane, the readings wil l not be
accurate. In the awake patient , i t should be inserted unti l the patient feels the
thermocouple touch the tympanic membrane; appropriate placement is confirmed
when the patient easi ly detects a gentle rubbing of the at tached wire ( 77 ). The
lower anterior quarter of the membrane should be used ( 78 ). After the probe is
inserted, the aural canal should be occluded with cotton wool and cov ered
externally to prevent air movement from cooling the probe ( 79 ,80 ).
Contact tympanic membrane probes are shown in Figure 29.7 . The sensor is
enclosed in soft foam. I t usually has a widened segment, piece of foam, or a
feather or barb to hold i t in place after insert ion. After insert ion, the reading should
stabil ize quickly. If i t does not , the probe should be s lowly advanced unti l the
reading stabil izes.
Numerous studies have shown a good correlation between tympanic membrane
temperature and temperature measured in the esophagus, pulmonary artery, or
urinary bladder ( 31 , 71 ,78 ,79 , 80 , 81 ,82 ,83 ,84 ,85 ). Temperature may be less
accurate during rapid changes ( 29 , 40 ,79 ).
Head posit ion can cause as ymmetric c hanges in the tympanic temperatures ( 86 ).Upon assuming a lateral position, the temperature on the lower side increases
while that on the upper side decreases.
Adv an ta ge s of ty mp an ic mem bran e te mp erat ure mo ni tori ng inc lud e c le anline ss an d
convenience. I t is tolerated by c onscious patients , making i t useful for
postoperative monitoring. The si te is readily accessible during most surgical
procedures.
Complications have been reported. Oozing from the ear, trauma to the external
auditory canal with subsequent external ot i t is , and perforat ion of the membrane
have been reported ( 81 ,87 ,88 ). These complicat ions occurred before the advent of
f lexible cotton-t ipped probes that are less traumatic ( 7 ). Recommended methods to
avoid trauma include otoscopic inspection of the canal and drum before insertion,
stopping insert ion as soon as resistance is fel t , and placement in awake patients to
assess discomfort . Care should be taken that the
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probe is not pushed into the canal when the head is moved.
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View Figure
Figure 29.7 Tympanic membrane temperature probes.
Contraindicat ions include any ear abnormali ty that would prevent correct
placement, a skull f racture that passes through the osseous meatus, and
perforation of the tympanic membrane.
View Figure
Figure 29.8 Disposable probe for measuring skintemperature.
InfraredInfrared ear thermometry (infrared emission detection thermometry, infrared
tympanic membrane thermometry, infrared tympanic thermometry) is performed by
insert ing an otoscopelike probe into the external ear canal ( 19 ). The t ip is usually
covered by a disposable cover. The device detects the amount of infrared heat
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emitted. The wide angle is too large to measure only the tympanic membrane, so it
also reads the temperature of the ear canal .
Since these thermometers are known to give temperatures lower than coretemperatures, some manufacturers provide offsets to estimate the temperature at
other si tes ( 89 ). These offsets vary for different brands ( 17 ,26 ,90 , 91 ).
A tug on th e ea r to s tra ig hte n th e cana l wh ile ta k ing a reading wa s foun d by som e
investigators to improve correlation with core temperatures ( 92 ,93 , 94 , 95 ,96 ). Other
studies did not f ind that an ear tug is helpful ( 20 ,69 ). The device should be inserted
with a gentle back and forth motion, and f irm but gentle pressure should be applied
to seal the canal from ambient air ( 94 ).
Many studies have compared temperatures obtained by infrared ear thermometers
to temperatures obtained f rom other si tes
(19 ,25 ,41 , 42 , 44 ,52 ,58 , 66 , 69 ,70 ,83 ,90 ,91 , 93 ,97 ,98 , 99 , 100 ,101 ,102 ,103 ,104 ,105 ,106
, 107 , 108 ,109 ,110 ,111 ,112 ,113 ,11 4 ,11 5 ,116 , 117 ,118 ,119 ,120 , 121 ,122 ,12 3 ). The
correlat ions vary from excellent to poor. There is considerable va riat ion among
different instruments and with different users of the same instrument
(20 ,109 , 114 ,120 ). A difference of 1.6C between the two ears has been reported
(114 ). Readings are affected by the ambient temperature ( 12 4 ,125 ). If the face is
cooled, there will be a decrease in the reading ( 126 ). Acute ot i t is media wil l not
influence the reading unless there is suppurat ion, in whichP.866
case the reading wil l be sl ightly increased ( 98 ,10 2 ,117 ,12 7 ).
Adv an ta ge s of in frare d te mp era tu re mo ni tori ng in c lud e spe ed an d red uc ed po ten tia l
for cross contamination. I t is noninvasive, requires no clothing removal , and is well
tolerated by conscious patients . The si te is usually readily accessible. The ease
with which the measurements can be obtained has made these the thermometers of
choice in many pre- and postoperat ive units . Because the readings are intermit tent
and subject to such variability, this technology is not recommended for
intraoperative use.
S k i n
Skin temperature can be measured by using a l iquid crystal device or f lat d isc or t ip
of a lead containing a thermocouple or thermistor ( 6 ) ( Fig. 29.8 ). Some skin probes
have a special backing to at tach to the body and material to insulate the sensor
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from ambient condit ions. An opaque dressing and/or tape over the sensor may
decrease the effect of environmental factors.
Skin temperature is most commonly measured at the forehead, because this sitehas a fai r ly good blood f low, and there is not much underlying fat . The back, chest ,
anterior abdominal wall, fingers, toes, and the antecubital space inside the elbow
have also been used.
The correlat ion between skin temperature and core temperature is controversial .
Skin surface temperature is typically 2C to 4C less than core temperature. Some
manufacturers build in an offset to compensate for this difference ( 128 ). In some
studies, skin temperature has been found to correlate well with core temperature
(129 , 13 0 ). However, most invest igat ions have found that skin temperature does not
accurately reflect core temperature, and there may not be a relat ionship between a
change in skin and co re temperature ( 29 ,31 , 61 ,78 , 10 8 , 13 1 ,132 , 13 3 ).
Monitoring skin temperature carries few risks. The si te is e asi ly accessible. The
main disadvantage is that skin temperature is a poor est imate of core body
temperature and may give e rroneous information ( 16 ). Skin temperature readings
are affected by ambient temperature, skin surface warming devices, intraoperat ive
changes in cardiac output , and regional vasoconstr ict ion ( 134 ,135 , 13 6 , 13 7 ). The
usefulness of skin temperature monitoring as a screening device for malignant
hyperthermia is l imited, as cutaneous vasoconstr ict ion may occur with thissyndrome ( 13 8 ).
Skin temperature may be used to evaluate the quali ty of a regional block. A rise in
skin temperature is an indicat ion that the block is successful . Another use is in
microsurgery. An increase in skin temperature may indicate that blood flow to that
area has increased.
A x i l l a
To measure axillary temperature, a mercury-glass thermometer or a probe with a
thermocouple or thermistor is posi t ioned over the axil lary artery and the arm
adducted ( 31 ). Significant differences have been found between mercury and
electronic axil lary temperatures ( 68 , 111 ). Temperature should not be measured on
the same side where a blood pressure cuff is on the upper arm. Equil ibration may
take as long as 10 to 15 minutes ( 72 ,13 8 ).
Al th oug h a few s tu die s fou nd sat is fa c to ry corr elati on wi th mo re centra l s i te s
(133 , 13 9 ,140 , 141 , 14 2 ), most s tudies show poor correlat ion
(20 ,25 ,29 , 45 , 51 ,58 ,68 , 69 , 98 ,11 5 ,116 , 12 9 , 14 3 ,144 ,14 5 ,14 6 ,147 , 14 8 , 14 9 ).
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Axil la ry temp eratu re me asureme nts are eas y to tak e and are no t obj ec tiona b le to
the patient or nursing staff . The si te is usually accessible. However, i t is not
considered accurate or rel iable in adults . I t i s used most frequently on infants andchildren because the patient 's small s ize and greater surface vasculature make a
relatively uniform temperature ( 17 , 31 ). Readings are influenced by contact with the
probe, skin perfusion, exposure to the environment, skin warming devices, and
proximity of the probe to the axil lary artery ( 20 , 26 ,56 , 14 8 ).
M o u t h
Sublingual temperature is measured by placing a p robe in one of the pockets on
either s ide of the frenulum of the tongue ( 15 1 ). The patient 's mouth should be
closed and enough t ime al lowed for the reading to be accurate. This area containssmall muscular arteries that respond to masticat ion or hot or co ld l iquids by
expanding or contract ing.
Correlat ion with temperatures measured at more c entral s i tes varies s omewhat but
in general is fair ly sat isfactory ( 20 ,58 , 77 ,113 , 11 5 ,15 1 ,152 , 15 3 ).
Sublingual temperatures are well tolerated by patients . Readings are not affected
by the presence or absence of teeth, in tubation, administrat ion of oxygen, a
nasogastr ic tube on continuous suction, or the temperature of inspired gases
(145 , 15 0 ,154 ). Sublingual temperature may be inaccurate if the patient is a mouth
breather or has tachypnea ( 15 5 ). Warm and cold ambient temperatures have a
small effect on oral temperatures ( 12 4 ,125 ).
Tr a c h e a
Temperatures in the trachea can be measured by using a tracheal tube with the
temperature sensor in the c uff . Studies differ in how well temperatures measured at
this s i te correlate with those at other s i tes ( 156 ,157 ,158 ,15 9 ).
P.867
In g u i n a l A r e a
The inguinal area has been used to measure temperature ( 45 ,72 ,160 ). The leg is
f irst abducted and the femoral pulse determined. The sensor is placed just lateral
to the femoral artery, and the leg is adducted to c reate a seal . A s ignif icant
drawback to this s i te is the t ime required to reach equil ibrium.
H y p o p h a r y n x
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A therm oc oup le can be a tt ac hed to the cuf f of a supra glott ic dev ice to meas ure
temperature in the hypopharynx ( 161 ,162 ). Temperatures measured in this location
have correlated well with those at other s i tes .
Hazards of Thermometry
D am a g e t o t h e M o n i t o r i n g S i t e
Tympanic membrane and rectal perforation and trauma to the nose, external
auditory canal, rectum, and esophagus have been reported
(74 ,75 ,76 , 81 , 87 ,88 ,16 3 ,164 ). Probes can have sharp edges that could p resent a
hazard to the patient ( 16 5 ).
B u r n sBurns can occur at the measurement si te i f the probe acts as a ground for the
electrosurgical apparatus ( 166 , 16 7 , 16 8 ). No insulat ion can c ompletely block radio
frequency currents , and if there is no other sat isfactory return path, the current can
burn through insulat ion ( 16 9 ). Using a battery-operated device is no guarantee of
electr ical safety, because the chassis may be g rounded through a metal support .
Temperature probes should be examined before use to detect damage to the
insulat ion. Esophageal burns may be avoided by insert ing the probe via a small
tracheal tube ( 170 ). The probe may be pulled back into the tube during periods of
maximal electr ical act ivi ty.
In c o r r e c t In f o r m a t io n
A faulty prob e can cau se an inco rr ec t tempe ra tu re to be displ ay ed
(171 , 17 2 ,173 , 174 ). Secret ions or f luids in the connection between the probe and
reading instrument can result in falsely elevated readings ( 9 , 10 ). If the battery in a
battery-powered device is depleted, the unit may s top functioning or give incorrect
information ( 6 ). Using jacks that fit the receptacle but have internal electronics
incompatible with the monitor can give false readings.P r o b e C o n t a m i n a t io n
Reusable temperature probes may be a s ource of bacterial or v iral pathogens even
if protect ive covers are used ( 6 ,175 ).
F a u l t y Pr o b e s
Af ter a p ro be is re mov ed f ro m i ts pa ckagin g, i t shoul d be ins pe cted . A part o f th e
sheath on a p robe could break off and be aspirated.
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For the fol lowing quest ions, answer
i f A, B, and C are correct i f A and C a re correct i f B and D a re correct i f D is correct i f A, B, C, and D are correct .
1. Advantages of thermistors include
A. Sma ll s ize
B. Interchangeable and disposable probes
C. Continuous readings
D. Rapid response t ime
View Answer 2. With the platinum wire thermometer
A. Cu rr ent f lo ws in prop ort ion to th e tem pe ra ture d if fe re nce
B. Rapid thermal equil ibrat ion is possible because of the small diameter of the wire
C. Two wires of different metals are welded together at their ends
D. Resistance of the wire varies with temperature
View Answer 3. Advantages of the liquid crystal thermometers include
A. Fa s t cont in uous re ad ing sB. Infrared lamps do not interfere with their readings
C. They can be applied prior to induction
D. Accuracy even at the extremes of ambient temperature
View Answer 4. Advantages of infrared temperature monitors include
A. Th ey do no t con tac t th e ea rd rum di re c tl y
B. Tolerated well by patients
C. Rapid measurement
D. Continuous measurement
View Answer 5. The accuracy of axillary temperature determinations is
influenced by
A. Prox imi ty to the ax ill ary artery
B. Perfusion of the sk in
C. Skin contact with the probe
D. Patient age
View Answer 6. Factors influencing the temperature reading in the naso-
pharynx include
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A. Te mp eratu re of th e inspi red ga ses
B. Leak in the tracheal tube cuff
C. Gastr ic suctionD. Temperature in the hypothalamus
View Answer 7. For correct use of the esophageal temperature monitor,
A. Th e sen sor shoul d be lo cated in the lo we r third to lowe r fou rth of the esop hag us
B. Stomach placement of the probe will provide temperatures lower than core
C. The probe should be placed 12 to 16 cm dis tal to the point of maximum heart
sounds
D. Temperature of the respired gases is not a factor in the lower third of the
esophagus
View Answer 8. Factors that can cause esophageal temperatures to be
unreliable include
A. Airwa y hum idificat ion
B. Thoracic surgery
C. Continuous gastr ic suction
D. Patient in the si t t ing posi t ion
View Answer 9. When using the tympanic membrane to measure
temperature,
A. Th e pro be do es no t hav e to ac tu al ly contac t the me mb ra neB. Perforat ion of the membrane can occur
C. Readings wil l be stable if the probe is in c lose proximity to the membrane
D. The probe should have a means to hold i t in place
View Answer 10. Factors that will cause incorrect readings from an
infrared ear thermometer include
A. Ce ru men
B. Ambient air temperature
C. Oti t is media with suppuration
D. Oti t is media without suppuration
View Answer 11. Factors affecting the temperature measured in the
rectum include
A. Cysto scopy
B. Peri toneal lavage
C. Rectal contents
D. Colon pathology
View Answer