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Chapter 31 Temperature Control Equipment Physiologic Temperature Control Numerous stud ies have shown that s ignif icant temperature changes routinely occur
in anesthetized patients (1 ). Inadverten t hypothermia is by far the most common
disturbance. Without specif ic in terventions , up to 90% of pat ients entering the
postanesthesia care un it (PACU) may be hypothermic (2). An except ion may be
patients undergo ing magnetic resonance imaging (MRI) in whom the absorpt ion of
radio f requency radiation may part ia lly offse t heat loss.
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In the unanes thetized patient, vasoconstric t ion main tains a temperature gradien t
between the core and periphery of 2°C to 4°C. Core body temperature is normal ly
maintained wi thin a narrow range of 37 ±0.2°C. When core body temperature goes
out of this range, physiologic mechanisms are ini tiated to reestablish the norm.
Anesthesia al ters the response threshold , al lowing the body to experience greater
variations in temperature before it attempts to reestab lish a 37°C core temperature .
Responses to al tered temperatures are less effect ive under anesthesia.
Hypothermia under anesthesia usually follows a characteris t ic pattern (1,3,4). Core
body tempera ture usually dec reases 0.5°C to 1.5°C during the f irs t hour (5,6 ,7) as
vasodi lata tion causes redistribut ion of body heat from the core to the periphery.
Warming peripheral t issues before induct ion of anes thesia (prewarming) dec reases
the central-to -peripheral tempera ture gradient, thereby minimizing the redistribut ion
of heat f rom the core to the periphery and reducing the ini t ia l dec rease in core
temperature (8,9 ,10,11,12,13,14,15,16,17,18,19). This redistr ibut ion cannot be
prevented by intraoperat ive skin surface warming (14).
Af ter the f irs t hour, core tempera ture typically decreases at a s lower rate as the
body's heat loss exceeds the metabolic heat product ion . This is followed by a
thermal plateau during which core tempera ture no longer s ignif icantly decreases. A t
this t ime, heat loss equ il ib rates wi th heat production and vasocons tric tion
constrains metabolic heat to the core compartment wh ile allowing peripheral t issues
to cont inue to cool . Pat ients wi th neuropathies have more severe hypothermia than
other patien ts, poss ib ly because the onset of vasoconstric t ion is delayed (20). A
plateau may never be reached when regiona l anesthesia blocks vasocons tric tion
(1).
In postanesthetic patients, vasoconstric t ion decreases rewarming rates. For this
reason, pat ients shou ld be warmed during surgery rather than a llowed to cool and
then be pos toperatively “rescued.” Warming may be accelerated by using certain
drugs (21,22) o r with a sympathetic block (23).
Etiologies of Heat Loss Most heat is lost v ia the skin surface. This loss is roughly proport ional to the sk in -
to-envi ronment temperature grad ient and the body surface area in contac t wi th a
lower temperature envi ronment. Ped iatric pa tients have a high surface area to body
mass rat io and thus tend to cool more qu ickly than adul ts bu t also rewarm more
quick ly (24).
Radiation Radiat ion is the major heat loss mechanism, accounting for 65% to 70% of the
body's heat loss (6). This is the loss of electromagnetic energy through inf ra red
rays f rom the warm body to colder objects in the room that do not contact the body.
Rad iant heat loss is a funct ion of the difference in tempera ture between the patient
and objects in the operat ing room (OR) and their heat emissiv ity (3 ). I t is
unaffected by ai r temperature , ai r movement, or the distance between the surfaces .
Convection The second major mechanism of heat loss is convection. This is the transfer of heat
to an a ir current. The magni tude of convective heat exchange is de termined by the
temperature gradient between the body and the ai r as we ll as the veloci ty of the ai r.
Surgical drapes prevent convective heat loss during surgery. Most of the heat los t
by this mechanism occurs when body surfaces are exposed prio r to surgica l
draping.
Conduction The thi rd heat loss mechanism is conduc tion. Heat is lost through di rect contact
between the patient and colder objects such as the opera ting table , l inens, surgical
instruments and skin preparat ion , irr iga tion, and in travenous (IV) f lu ids (25 ,26).
The heat f low is proportional to the temperature difference between the two bod ies.
Thermal insulation between the surfaces wil l reduce heat transfer. Wetness
increases conduct ive heat loss (6). Relat ively l i tt le heat is lost to objects such as
the OR table pad, but heat lost when cold preparatory and irrigat ion solutions and
IV f lu ids are used can reduce body temperature s ign if icant ly .
Evaporation The fourth heat loss mechanism is evaporation. Evaporat ion losses occur f rom the
sk in, respiratory tract, open surgical wounds, pneumoperitoneum, or wet towels and
drapes that are in direct contac t with the pat ien t's body.
Other Factors A number of factors determine the severi ty of hypothermia. The longer the surgical
procedure, the greater the drop in tempera ture. The site of surgery is another
considerat ion , since large cavi ties are subject to cons iderable heat loss f rom
evaporat ion, whether open or laparoscopic techniques are used
(27,28,29,30,31,32,33,34,35,36). Administering large quanti t ies of coo l IV or
i rr igation f luids wi l l fu rther chil l the pat ient. Extremes of age, cachexia, female sex,
and low body mass are associa ted wi th inadvertent hypothermia.
Problems Associated with Hypothermia Hypothermia is a po tential cause of adverse patien t outcomes and may be
associated wi th l ife-threaten ing complications (37,38). Most compl icat ions are
ini tiated intraoperat ively, al though they are genera lly manifested
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or detected in the recovery period. The consequences of hypothermia wil l depend
on the s ize and condi tion of the patient. Smal l pa tients or those in weakened
condi tions wi l l be more suscept ible to the negative effects. Maintaining patient's
temperature decreases postoperat ive mortal ity and improves outcome
(39,40,41,42,43).
Metabolic Changes Adverse metabolic changes inc lude a lef tward shif t of the oxyhemoglobin
dissociation curve, accumulat ion of metabolic p roducts, and exacerbation of lac tic
ac idosis (44 ,45,46). Hemoglobin satura tion may be higher wi th warming (47).
Babies of mothers act ively warmed during cesarean del ivery have a higher
umbil ical vein pH (48).
Shivering and Thermal Discomfort Hypothermia is associa ted wi th postoperat ive shivering, which is often intense and
uncontro llable. I t causes patien t discomfort; inc reased metabol ic demand and
cardiorespiratory work; inc reased intraocular and intrac ran ia l pressures ; and
inte rferes with moni to ring, espec ia lly pulse ox imetry. Wound pain may be
aggravated by shivering. Many pat ients reca lled shivering and a feel ing o f in tense
co ld as the most d is tressing memory of their anesthet ic management, even af ter
rela tively short p rocedures. Some pat ients report the discomfort f rom shivering and
the cold sensat ion worse than the surgical pain (49,50). Skin temperature is of
equal importance wi th core temperature in determining thermal comfort (51).
Increased Recovery Time and Length of Stay Most studies have shown that intraoperat ive hypothermia causes s lower awakening
and longer t ime in the recovery room (even when temperature is not a d ischarge
criterion). Hypothermia may cause postoperat ive confusion (52). Higher
postanesthesia scores, earl ier extubation, and shorter PACU t imes are associated
wi th normothermia (53,54,55,56,57,58,59,60,61,62,63,64,65). Mild hypothermia
does not prolong recovery in pedia tric pa tients hav ing peripheral surgery (66).
Maintaining normothermia may shorten hosp ital ization (45).
Impaired Drug Tolerance Drug distr ibut ion is al tered, drug metabol ism is decreased, and the behavior of
anesthet ic drugs is al te red. This often results in higher blood concentrations and
prolonged dura tion of action (44,67,68,69).
Hypovolemia Hypothermia can lead to f luid shif ts from the vascular to the extracel lular space and
a re lat ive hypovolemia. For every degree cent igrade of hypothermia, 2.5% of the
intravascular volume may be lost (6). Cold-induced diuresis can occur, add ing to
the problem. As the pat ien t rewarms , vasodi la tat ion may occur and more fluids wi l l
need to be given to accommodate the loss. Pat ients with hypothermia have
s ignif icant ly greater f lu id and transfusion requirements (41 ,43,52).
Peripheral vasoconstric tion can make i t more dif f icult to insert peripheral venous
ca theters. Active local warming fac il i tates IV ca theter insert ion (70).
Cardiovascular System Effects Hypothermia enhances sympathetic act iv ity. Catecholamine concentra tions may rise
(71). Periphera l vasoconstric tion, which reflec ts the body's effort to conserve heat,
can resu lt in increased b lood pressure and cardiac work load and
electrocardiographic changes (65,72,73). Risks inc lude cardiac dysrhythmias, de-
creased contracti l i ty, myocardial ischemia and infa rct ion, and cardiac arrest
(74,75,76). Hypothermia can resul t in inc reased adverse hemodynamic events and
increased requirements for vasoac tive drugs (41,77). Normothermia is assoc ia ted
wi th a reduct ion in the incidence of pos toperative morbid cardiac events in patien ts
wi th known risk factors for coronary artery d isease (46,65,74,76,78). However, the
ef fects are modest in relat ively young, genera lly heal thy patients (55).
Rap idly rewarming patients with profound hypothermia can result in shock due to
redistribut ion of blood to the periphery as tissues vasodi late.
Effects on Coagulation Hypothermia inhibits platele t funct ion and act ivat ion o f the coagu la tion cascade
(79). It may be associated with inc reased blood loss and higher transfusion
requirements (41,53,58,80,81,82,83,84,85,86,87).
Reduced Resistance to Infection Even mild hypothermia may delay hea ling and predispose patients to wound and
other infect ions (45,88,89,90,91,92). Warming may prevent postoperat ive wound
infect ion (93). Maintain ing normothermia may attenuate protein breakdown after
surgery (94).
Interference with Monitoring Thermoregu latory vasoconstric t ion decreases cutaneous blood f low and may
interfere wi th pulse oximetry and o ther fo rms of mon itoring (95,96).
Increased Costs Hypothermic pat ients have prolonged stays in the intensive care uni t and heal th
care fac i li ty (41,87). Heart surgery pa tients have shorte r durations of venti lato ry
support wi th normothermia . A reduction in cos ts may resul t f rom act ive warming
(53,97).
Other Cold agglut inins may be found in associa tion wi th infection . Vascular obs truct ion
and even gangrene may resul t (98). Cool ing may cause a decrease in urine output
(99). The agreement of central and peripheral venous
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pressures deteriorates at lower temperatures (100). Inc reased pa in and anxiety
may be associated wi th hypothermia (101).
Warming Devices
Standards Two U.S. standards on warming dev ices were publ ished in 2002: one for c i rcula ting
l iquid and forced-air pa tient temperature management devices (102) and one for
f lu id warmers (103).
The fol lowing are in the standard for c i rculat ing l iquid and fo rced a ir devices :
• For forced-air devices, the maximum contact surface temperature shal l not
exceed 48°C, and the average contac t surface temperature shall not exceed
46°C during normal cond it ions.
• For c irculating l iquid dev ices, the contact surface temperature shal l not
exceed 43°C, and the average contac t surface temperature shall not exceed
42°C during normal cond it ions.
The f lu id warming standard requires that the device does not heat the f luid above
44°C under normal condi tions .
Warming Methods I t is generally accepted that no s ingle technique alone is superior in combating
hypothermia. The bes t resul ts are l ikely to be achieved by combining methods. The
costs , risks, and benef i ts of warming should be spec if ical ly considered fo r each
patient, factoring in preexisting medical condi tions and the surg ical procedure.
Forced-air Warming Devices Forced-air warming devices (convective warming devices , warm air blowers) entrain
ambient ai r through a microbial f i l ter. The air is warmed using an e lectric heater
thermostatica lly control led , and then blown through a hose that is connec ted to an
inf latable pat ient cover (Fig. 31 .1). Some devices moni tor the tempera ture setting
wi th in the warming un it . Some newer uni ts monitor the temperature at the end of
the ai r delivery hose (104). Most offer a se lec tion of temperatures (F ig. 31.2).
A varie ty of covers, both disposable and reusable, are availab le . They have a
series of holes tha t al low the warm, f i l tered ai r to pass th rough. Another design
uses a fabric tha t al lows the heated air to fi l ter through the enti re pa tient s ide.
The shape of the cover varies (Figs. 31 .1, 31.3,31.4,31.5). A number of pediatric
blankets are availab le. A U-shaped tubular blanket that enci rc les the patient may
be usefu l in s ituations such as cardiac surgery, where much of the body cannot be
covered. However, i t is less effect ive than a b lanket p laced over the body
(13,22,105,106). I t is possible to cut some covers and seal the edges of the cut to
f i t the pat ient (107,108). Patien t and hea lth care provider gowns that can be
connec ted
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to a forced-air uni t are avai lable. Sleeves for warming an arm to faci l i tate IV
ca theter insert ion are avai lable.
View Figure
Figure 31.1 Forced air-warming device with low body blanket. (Picture courtesy of Arizant Healthcare, Eden Prairie, MN.)
View Figure
Figure 31.2 Control unit on a forced air device. A selection of temperatures is provided.
View Figure
Figure 31.3 Warming blanket for a small patient. Note the plastic cover over the child. (Picture courtesy of Arizant Healthcare, Eden Prairie, MN.)
Placing a blanket or sheet over the warming b lanket wil l resul t in inc reased heat
transfer (109). Lower body warming is sl ightly more effec tive than upper body
warming (110,111,112). Underbody blankets al low easy access to the pat ient (113).
However, they are probably only useful for very small pat ients .
A number of inst itut ions have used these units wi thout the warming blanket by
plac ing the hose e ither under the surgical drapes or between cotton blankets (free
hosing) (114,115,116). Free hosing may resul t in heated ai r blowing direct ly onto
only a small area of the pat ient 's skin and cause burns (117,118). Therefore, this
pract ice is no t recommended.
View Figure
Figure 31.4 Over-the-body warming blanket with an area in center removed to allow surgical access. (Picture courtesy of Arizant Healthcare, Eden Prairie, MN.)
Numerous stud ies have shown that forced-air warming is effective in maintaining or
increasing the pat ient 's ( including both materna l and baby) temperature , dec reasing
the incidence of shivering and increasing thermal comfort
(31,48,55,58,59,62,105,110,119,120,121,122,123,124,125,126,127,128,129,130,13
1,132,133,134,135,136,137,138,139). I t
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works wel l even when the available skin surface area to be warmed is restric ted , as
occurs during orthopedic , major vascular, or abdominal operat ions (9,109). Forced-
air warming is of ten used in conjunc tion wi th other warming methods (140).
View Figure
Figure 31.5 Cardiac access blanket. This provided localized warming to the legs while allowing access to both legs.
Although forced-air warming is often effective in ra is ing peripheral tempera ture,
core temperature may not rise (47,141,142,143,144,145,146,147). This may be
because o f l imi ted heat transfer between thermal compartments in vasoconstric ted
patients. In the patient with a neuraxial b lock, the vasodi lation may aid in heat
transfer from the peripheral to the core t issues.
Most studies have found forced-air warming superior to older-style l iquid-c i rcu lat ing
mattresses, warmed or unwarmed blankets , rad iant heat lamps, inhala tion
rewarming, passive insulat ion , electric blankets , negative-pressure warming
devices, o r warming IV f luids
(14,19,22,46,47,57,109,127,128,139,144,145,147,148,149,150,151,152,153,154,15
5,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174
,175,176,177,178,179,180,181,182). Newer l iqu id-c i rculating devices and resist ive
heating devices may be as or more eff icient than forced-air warming devices
(85,137,138,183,184,185,186,187,188). Some s tud ies have found that warming IV
f luids was as effective as forced-air warming in maintaining normothermia
(189,190).
Forced-air warmers are usual ly safe when properly used. There are a few reports o f
burns (191,192,193,194,195,196,197,198,199). Care should be exerc ised that the
hose does not come in contac t with the patien t's skin and that the exi t vents are
posit ioned away from the patient and the surg ical f ie ld. Special care should be
taken to avoid contac t wi th ischemic areas (i.e., dis tal to a vascular clamp).
Forced-air warming is simple, safe , effec tive , and inexpensive. The variety of
patient covers makes i t adaptable to many d if fe rent si tuations. Most nurs ing
personnel and family members prefer this method to radiant heat. Forced-a ir
warming prov ides more calories/cost than other modal it ies
(47,97,200,201,202,203). Fiberopt ic laryngoscopes can be warmed before use wi th
a fo rced-air warming dev ice (204). Warming these devices wil l prevent fogging
when the device is inserted in to the mouth for intubat ion. I t can be used to warm
the operat ing table before the patient is transferred to i t by plac ing the hose under
a sheet. It can also be used for cool ing (205,206). Another use is to rel ieve
c laus trophobia (207). A coiled tubing can be placed ins ide the hose from a forced-
air heating dev ice to heat IV f lu id (F ig . 31.14). However, this is effect ive only at low
f luid f low.
A disadvantage is tha t its electric power requirements make it unsui table for f ield
use (208). I t is somewhat cumbersome to transfer or se t up in a computed
tomography (CT) scanner (190). It must be removed f rom the pat ient to expose
covered areas. Another disadvantage is that many systems do not permit the
concurrent use of mul tiple b lankets (i .e., upper and lower body) wi thout using two
separate forced-air uni ts .
View Figure
Figure 31.6 Liquid-circulating device. The patient contact part can be wrapped around various parts of the body. (Picture courtesy of Gaymar Industries, Inc.)
Liquid-circulating Devices A liquid-c i rculating device consis ts of a heat ing/cooling uni t and pat ient contact
device (mattress, pad, blanket, o r wrap) that is connected to the heating/coo ler uni t
by hoses (Fig . 31.6). Heated/coo led l iquid ci rcu la tes through the pat ient contact
device and then back to the heat ing/cool ing element. Some machines can supply
more than one pat ient contact device.
Other devices may be attached to the l iquid-c i rculating uni t (209). IV f lu ids can be
heated by using a water mattress pad applied to the tubing c lose to the pat ien t
(210).
A pad can be placed ei ther over or under the pat ient bu t is safer and more effec tive
when placed over the
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patient (154,211) (Fig . 31.7). Direct pat ien t contac t wi th the pad surface should be
avoided. Folds and creases in the pad should be avoided. These uni ts may
predispose pat ients to burns, so skin in tegrity mus t be assessed frequently
(212,213,214).
View Figure
Figure 31.7 The pad from a liquid-circulating device may be placed over the patient. (Picture courtesy of Augustine Biomedical.)
View Figure
Figure 31.8 Liquid-circulating device. The patient contact part adheres to the body surface. (Picture courtesy of Kimberly-Clark.)
Some newer l iquid -c i rculat ing systems use a disposab le thin pad tha t adheres
di rectly to the sk in and is made of a material that fac il i tates heat conduction toward
the pat ient. Dif fe rent shapes and s izes are avai lable , al lowing a ttachment to
various body surfaces (183,186,215) (F igs. 31.8, 31.9). Most incorporate a
microprocessor that controls the f low and temperature and a patient tempera ture
sensor so tha t the l iquid tempera ture can be ad justed to maintain the desired
patient temperature. One system operates under negative pressure so that i f the
pad is cut or punctured, ai r wi l l be pul led into the system rather than water spil l ing
out (104).
Older-style liqu id-c i rculating units are less effective than forced-air heating
(14,154,163,164,165,166,167,181,216,217,218). The newer-style uni ts may be more
ef fective than forced-air heat ing, because they can cover a larger surface area
(65,78,85,104,183,184,185,219,220,221,222).
View Figure
Figure 31.9 Close-up of liquid-circulating device. The backing is removed, revealing the sticky side of the pad that is attached to the patient.
The older-style mattresses are heavy and cumbersome. I t is d if f icult to mainta in
good contact with the s tiff mattress and to cover a large surface area (15). Their
use can lead to burns, especially over pressure poin ts where the pat ient contacts
the blanket (109,212,213,214,223,224,225). P lac ing them above the pat ien t may
decrease the l ikelihood of burns. Care should be taken that the tub ing does not
come into contact with the patien t.
Passive Coverings Apply ing passive insulat ion can decrease heat loss f rom convect ion, radiat ion,
conduc tion, and evaporation . Cotton blankets, surgica l drapes , towels and sheets ,
plast ic sheeting, p lastic bags, and spec ially designed ref lect ive compos ites
(thermal d rapes, space b lankets, ref lective blankets, metall ized plas tic sheets or
sheets, head coverings , blankets , socks, leggings, e tc.) are among the materials
that have been used (226,227,228,229,230,231,232) (Fig. 31.10). There are
minimal c l in ical dif ferences among the various coverings
(228,233,234,235,236,237). Warming the covers or adding addi tional layers of
insulat ion further reduces heat loss only s lightly and has not been found to be of
benef it in preventing shivering (238,239). These covers prov ide a transient sense of
warmth . Covering as much surface area as possible is more important than the type
of covering or speci fic area covered. Cos t and convenience should be major fac tors
when choos ing among covers. The costs of laundering and replacing cotton
blankets must be taken into account. There are no publ ished reports of pat ient
injury caused by warmed hospi tal blankets (240).
Passive insulat ion wi l l reduce cutaneous heat loss but wi l l not maintain
normothermia
(6,22,127,158,159,177,218,230,231,241,242,243,244,245,246,247,248,249).
Apply ing warmed co tton blankets to the patient has been a tradit ional ri tual in the
PACU, but is ineffective (250). However, plac ing a cotton
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blanket over the pat ient as soon as possible af ter the pat ient has entered the OR
wi ll reduce init ia l heat loss and resu lt in a higher body temperature when the
patient enters the PACU (231).
View Figure
Figure 31.10 Thermal head covering.
Insulated coverings have been found to be less effective than forced-air warming
for conserv ing body temperature
(22,47,57,127,128,149,150,151,152,153,155,158,159,160,161,172,251). Warmed
blankets were found to be inferio r to radiant heat in preventing shivering (248).
Passive covers are convenient, easy to use, l ightweight, and not subjec t to
electrical or mechanica l failures. There is no burn hazard . However, they may
inhibi t access to the patient. Most of them are combustible and should be used wi th
caution when a source of igni tion is present (252) (see Fires in Chapter 32).
Resistive Heating Resis tive electrical heating dev ices generate heat by passing low-vo ltage current
through semiconduc tive wires or carbon-f iber fabric . No danger results from
penetrat ing the fabric . The system cont inues to operate normal ly because the
current s imply f lows through ad jacent fabric.
The warming blanket or mattress is thermosta tical ly controlled by a computer to
maintain the contact surface at the se t tempera ture. This system can heat several
f ie lds independently . Blankets and mattresses are available in a large number of
configurations that can be used in various combinat ions to inc rease the heat ing
surface. They can be cleaned and disinfected after use.
Resis tive heating has been found to be as or more effect ive than most other
technologies, inc lud ing forced-air warming
(9,57,85,101,109,137,138,208,253,254,255,256,257). A reported advantage is cost
saving compared wi th forced-air warming (57,254,255). I t is also quieter than
forced ai r. Resistive heating devices may be especially helpfu l fo r f ie ld treatment of
hypothermia (101,208). However, severe thermal injuries have been reported wi th
an electrical warming mattress (258). A f ire may resul t if an electric blanket is
folded and wires are broken.
Radiant Heaters Radiant warmers use spec ial incandescent bulbs or heated surfaces to genera te
infrared energy. The radiant heater can be a s imple lamp on a portable stand or a
more elaborate panel (145). The latter comes as a portable uni t or may be cei l ing
mounted on tracks (6,259,260).
The radiant device is most effect ive when i t heats areas high in a rteriovenous
anastomoses such as the fo rehead, nose, ears, hands , and feet (261). These can
di late in response to local heat ing and anesthesia and allow applied heat energy to
be transferred di rec tly to the core.
Rad iant warming can be used on exposed areas of the pat ient 's skin during
ca theter placement, sk in prepara tion, and during the surgical procedure , when
feasible (6). I t may be especial ly useful in the PACU (260). The heat should shine
on the pat ient's bare sk in or at most through a thin sheet. If placed too c lose to the
sk in, burns can resul t . The skin surface mus t be assessed frequently to detect early
s igns of burns. Skin exposure may resu lt in coo ling by convection currents, so i t is
importan t to eliminate draf ts in the envi ronment.
Rad iant heat can provide faster rewarming and reduced shivering
(6,248,259,262,263,264,265,266). It is more effect ive in small infants because of
thei r re lat ively la rge body surface area. Rad iant heating has been found to be less
ef fective than forced-air warming but superior to electric , warmed, or ref lect ive
blankets (145,149,154,178,180,248,260,267,268). It decreases heat loss before
sk in washing but increases i t during washing (269).
Rad iant heat lamps enab le the medical and nurs ing staff to have an unobstruc ted
v iew of and access to the pat ient. There are no d isposables and no patient contact.
However, the equipment is bulky and somewhat cumbersome. Exposure of the adul t
patient may be unacceptable to the pat ient and fami ly members (145). I t may cause
burns or hyperthermia if used for long periods of time, if the radian t heat source is
c lose to the patien t's sk in, o r if there is a problem with the sk in-temperature
measurement sensor (214,270). Because radiant warmers increase evaporat ive
heat loss, they may inc rease f luid requirements (271). Pat ients with poor periphera l
c i rculation may be more di ff icul t to heat. It is necessary to adjust the heater-sk in
distance if the operat ing tab le
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is raised or lowered. Radiant warmers may interfere with imag ing if used in an MRI
unit (272).
Heating and Humidifying Inspired or Insufflated Gases Evaporative heat loss f rom the ai rway can be prevented by us ing warm, humid if ied
gases (inhala tion rewarming). Devices fo r heating and humidifying inspired gases
are discussed in Chapter 11 . Min imal heat transfer occurs with this method, which
carries the risk of thermal in juries.
Some studies have found that heating insp ired gases is of minimal value in
rewarming, even when used in conjunction wi th o ther warming modali t ies
(14,163,259,273,274). Other studies found that heated humidif ica tion sys tems can
reduce the inc idence of shivering and resu lt in a more rapid return to normothermia
(263,275). Ai rway heat ing and humidif ica tion may be more effect ive in infants and
ch ildren than in adu lts (276,277,278), but cu taneous warming is far more effect ive
(279).
Heating and humidifying gases used for peri toneal insuff lation can prevent the heat
loss associated wi th this p rocedure (280).
Heat and Moisture Exchangers Heat and moisture exchangers (HMEs) are d iscussed in Chapter 11. They provide
ef fic ient humidif icat ion and work almost as we ll as heated humidif iers to prevent
respira tory heat loss but cost less and provide fi l trat ion. However, the amount of
heat preserved by this method is small (281,282). Heat conservat ion by a ll
availab le HMEs is comparable (38).
Low Fresh Gas Flows Using low fresh gas f lows reduces heat loss through the ai rways (282). However,
s tudies indicate that us ing low-f low anesthesia is ineffect ive in mainta in ing
intraopera tive normothermia (158).
Fluid Warming Fluid warmers are used to warm blood produc ts , IV solutions, irrigat ion solutions,
and insuff lating gases (283,284). Since these f luids are usually well below body
temperature , exposure to them can be a s ignif icant source of heat loss when large
volumes are used. Mos t fluid warmers have a tempera ture display and an alarm to
alert the operator if the heater temperature or temperature of the f luid is too high.
Some alarm if the heater temperature falls below a thresho ld.
Manufac turers wil l p rov ide information about the maximum f low rate tha t is possible
for the particular system. This f low rate wi l l not necessarily provide f luid at body
temperature . If there is a disc repancy, the manufacturer wi l l a lso prov ide the f low
rate that wi l l provide f luid warmed to body tempera ture.
Most uni ts are designed fo r use wi th dedicated disposable sets. Some o f these sets
are used in conjunction wi th a standard blood or IV administra tion set and
ex tension tubing, whi le o thers inc lude a Y-set as well as a pat ien t l ine . Some offer
a number of dif ferent disposable sets designed for spec if ic appl ications.
Differences among these se ts include the gauge and length of the tubing, the
number of bag spikes, and the presence of a blood f i l ter and/or degassing
mechanism.
Fluid warming can prevent the heat loss caused by infusion of cold f lu ids but
generally cannot transfer enough heat to p revent hypothermia or res tore
normothermia expedi t iously unless ex tracorporeal rewarming is used
(14,27,30,156,157,162,189,210,274,285,286,287,288,289,290,291,292,293,294,295
,296). Warming l iquids improves the f low through the administrat ion set by lowering
the v iscosity (297,298).
No clear guidel ines about when these devices should be used exist. Drawbacks
include the expense and the t ime needed to assemble the appara tus . It is generally
agreed that warming should be performed during massive and/or rap id transfusion,
in patien ts wi th co ld agglutinins, and fo r exchange transfusions in the neonate , but
i ts use for rout ine procedures is controversial . Variables that should be cons idered
include the ra te of infus ion, the total volume to be used, the temperature of the
f luid to be infused, and o ther pat ient warming techniques that are in use (283,299).
Factors Determining the Fluid Temperature at the Patient Temperature Controller Set Point Increasing the set temperature of the warmer resul ts in a h igher ou tlet and dis tal
f lu id temperature. The f luid may or may not reach the set poin t, depending on the
ef fic iency with which heat is transferred by the warmer and the speed tha t the f luid
transi ts the warmer.
Starting Fluid Temperature Fluids maintained at co ld temperatures such as b lood wil l require more heat to
warm them than f luids stored at room tempera ture or in warming cabinets .
Fluid Flow Rate Fluids lose heat whi le hanging and whi le f lowing to the pat ient. The heat loss
usual ly increases as the rate of infus ion s lows
(300,301,302,303,304,305,306,307,308,309,310,311,312). This is especial ly
importan t in pediatric patien ts where f low ra tes are usual ly relatively low. However,
i f the f low is very rapid, i t may exceed the abi li ty of the f luid warmer to heat it
adequately.
Length of Tubing between the Warmer and Patient Keeping the tubing between the warmer and the pat ient as short as possible wil l
reduce heat loss (200,279,302,306,313,314,315,316,317). Heating the tubing may
prevent heat loss. Placing the tubing under a warming blanket wi l l help to maintain
f luid temperature.
Methods of Fluid Warming Preuse Warming Fluids other than blood products can be warmed in an OR cab inet (316,318). A
maximum temperature of 43°C is recommended (319); some manufacturers
recommend lower tempera tures . This method
P.893
can be used to heat sal ine that is subsequently mixed wi th red blood cells
(320,321). Dextrose-contain ing solutions should not be heated because the
dextrose wi l l be al te red by heat. Containers should be marked wi th the date that
they were placed in the warmer.
A uni t of blood or bag of IV f luid may be immersed in a bowl of warm water before
administra tion (44,322). However, this is s low and associated with technical
problems (323). The bath water mus t not enter the b lood or IV so lu tion. Placing the
unit between two hot packs may resul t in overheating (324).
Whole bags of blood and blood components have been warmed by placing them in a
microwave oven. This pract ice was abandoned because the nonunifo rm dis tr ibution
of energy and the f ini te depth of penetrat ion resu lted in hot spots and overheat ing
(325,326,327,328,329). More recent ly , a microwave device specif ically designed fo r
thawing fresh f rozen plasma and warming packed red blood cells has become
availab le (330). During heat ing, the products are rotated within the device.
Syringes that contain f luid can be warmed by using a ci rcula ting water mattress or
forced-air warmer (323). Smaller syringes warm more rap idly than la rge ones. A
“c irc le sys tem” may be created to al low the f luid in the heated tubing to act as a
reservoir f rom which f luid can be wi thdrawn in to syringes (331).
Prewarming b lood or f lu ids is inexpensive and convenient. It is most effect ive with
rapid transfusion . At s lower f low rates or if transfusion is delayed, the f luid cools
rapidly (301). Insu la ting the f luid container or tubing wi l l reduce heat loss (300).
Warmed IV fluid bags or plastic containers of irrigat ion solut ions should no t be
applied to the patient 's sk in either as warming or posi tion ing devices , because this
has been associated wi th burns and is ineffec tive (38,214,332).
In-line Warming In-l ine warming devices heat f lu id as i t passes from the source (a solu tion bag or
infusion dev ice) to the pat ient. The warmer may be mounted on a s tandard infusion
pole, attached to a ded icated pole system, or frees tanding. A special disposable
administra tion set is usual ly used, but at least one unit warms the f luid in
conventiona l IV tubing. Some devices have a means for controll ing infusion rate
and may d isplay the infused volume in real t ime. Some al low the operator to set the
temperature . Some have a means to administer a bo lus of f lu id.
Desirable qual i ties include a low priming volume, large heat transfer a rea, low
pressure drop, and the abil i ty to heat eff icient ly at all f low rates (323,333).
Advantages of In-line Warmers
• They can generally be used with red blood cells , whole blood, o r IV or
i rr igation f luids .
• Once the warmer is set up, new f luid bags can be connec ted at once.
• Entry ports must be punc tured before warming, so warmed blood cannot be
mistakenly returned to the blood bank for issue to other pa tients (44).
Studies show that plast ic izer does not leach into heated IV tubing (210). One
drawback is that most devices can be used for only one l ine at a time (44). Loss of
body heat can be minimized by warming the f luid f lowing to one IV s ite whi le
keeping veins open at other available s ites. Mult iple heat ing devices could be used
for mul tiple IV infusions.
Types of In-line Warmers Dry Heat In a dry heat exchanger, the f luid passes through a tubing, cassette, or bag that is
placed around or wi th in a heated block or plate(s ) (308,334) (Fig. 31 .11). I t may
also be heated by a magnetic induct ion heater (335,336) (Fig. 31.12). Inf ra red
lamps can a lso be used to heat the IV f luid (284). Some can be placed in an x -ray
cassette on certain OR tables .
Studies differ on the effectiveness o f dry heat warmers at high f low rates
(284,312,334,337,338,339,340,341). Newer models may perform bette r than older
ones. Because the f luid is usua lly fo rced through long, constric ted plast ic tubes or
channe ls, high res is tance may limi t the f low rate (297,340). Current leakage has
been reported wi th some units (342,343,344,345). Overheat ing with burning of the
plast ic disposable tub ing has been reported (336).
Microwave Although bu lk microwave warming devices were abandoned some t ime ago, recent
studies show tha t in -l ine microwave blood warming is not assoc ia ted wi th
s ignif icant damage to b lood (298,326,346,347,348,349,350,351,352,353).
One in-l ine mic rowave warmer employs a disposable cartridge that contains a short
length of IV tubing co iled around a plas tic bobb in . Temperature monitoring is
carried out by microwave radiometry, which measures the temperature wi th in the
lumen of the tubing without di rect contact wi th the f luid (350,436). The amount of
microwave power is then automatica lly adjusted unti l the measured temperature
matches the target temperature .
Microwave warming uni ts provide rapid heat ing and accura te temperature control
(298,346).
Water Immersion Water bath uni ts warm a f luid as it passes through a bag or coi ls of tubing
immersed in heated water (354). One or more IV extens ion sets can be used in
place of the coil or bag (355). Mos t water ba th warmers monitor and display only
the water bath tempera ture (44). Some uni ts agitate the water to improve heat
transfer. I f this is done, i t is important that inject ion ports no t become contaminated
and that the connec tions are secure.
Studies show that these dev ices are ineff ic ien t at high f low rates (312,334,356).
Some units take a long t ime to warm the water, so they must be turned ON
sometime prior to use.
P.894
View Figure
Figure 31.11 A: Dry heat warmer. A disposable cassette through which fluid flows is placed inside the device. B: Disposable cassette.
Hazards associated with water baths include leaking co ils , blockage, hemolysis
resul ting f rom overheat ing, leakage curren ts , and septicemia secondary to
contamination of the water that may enter the IV tubing
(357,358,359,360,361,362,363,364,365). The long tubing may offer a high
resistance that could l imit f low. A large priming volume may be needed.
This type of device is impractical for f ie ld or ambulance use. I t may require more
maintenance than other types of f lu id warmers (299). Blood or IV f luid can leak in to
the water bath solut ion.
Countercurrent Heat Exchangers Countercurrent heat exchangers use a countercurrent f low of heated water with a
tube containing the IV f luid ins ide (44,283,284,334,339,366,367,368). Single-
channe l and mul tichanne l countercurrent heat exchangers are available (353) (Fig.
31.13).
View Figure
Figure 31.12 Disposable set from a magnetic induction heater.
Most f luid warmers that use countercurrent technology heat more effectively than
other f luid warmers (284,305,307,308,309,333,334,339,366,369), so they may be
appropriate for s ituat ions where rapid volume resusci tation is necessary (335).
However, their effec tiveness may decrease as f low rate increases (306). Continual
countercurrent warming of f lu ids in the tubing leading to the pat ien t dec reases the
loss of heat dis tal to the warmer. The resistance to f low may be lower than wi th
water ba th warmers (305).
Other IV tubing can be p laced ins ide the tub ing between the patien t and a convect ive
warming device (156,299,370,371). One fo rced-air uni t manufac turer offers a
special disposable co il with IV tubing to be placed ins ide the hose that goes from
the warming un it to the b lanket (Fig. 31 .14). A folded water mattress pad can be
applied to the tubing c lose to the patient (210). The use of insula tion s lows heat
loss f rom tubing but is of l imi ted effec tiveness (210,315,369). All of these warming
methods result in reduced access to the tubing for drug injection.
Negative-pressure Warming Devices This dev ice (Fig. 31 .15) consists of a thermal exchange chamber tha t provides
negat ive pressure when ai r is exhausted from the chamber (175,372). A seal
around where the wrist enters the chamber ensures negative
P.895
pressure . Heat is suppl ied by an electrical warming dev ice.
View Figure
Figure 31.13 Countercurrent heat exchangers. Different arrangements can be used for IV fluid and warming fluid flow.
The theory behind this dev ice is that peripheral vasoconstric tion can hinder the
ef fectiveness of warming therapies applied to the sk in . If the subcutaneous
vascular s tructure of the hand of a hypothermic indiv idual can be dilated by using
subatmospheric pressure applied to the sk in , a thermal l ink between the sk in and
the body core would be c reated, al lowing transfer of appl ied heat to the core .
Some studies indica te that this may be a useful technique fo r rewarming
hypothermic indiv idua ls (373,374,375). Other studies have fai led to f ind any
s ignif icant benef its (175,176,377,378,379).
Esophageal Warming Devices This dev ice cons is ts of a disposab le double-lumen esophageal tube and a base uni t
wi th water heater, c irculating pump, and moni to r/alarm module (380,381). Steri le
dis t il led water is heated and then ci rculated through the esophageal tube.
This dev ice is expens ive and somewhat invasive. Mos t studies have found it to be
of l imited effectiveness (131,266,382,383).
Cryogen Packs Hot-cold c ryogen packs have been used to treat local ized areas of the body. Many
burns have occurred wi th their use.
Hot-water Containers Plast ic containers of i rr iga tion or IV f luid are f requently kept in warmers or ovens
near ORs, sometimes a t qui te elevated tempera tures (211). It may be tempting to
try to warm patients by posi tioning these containers in areas of high b lood f low
such as the axi lla. Th is prac tice, however, is both ineffec tive and dangerous. The
lack of eff icacy resu lts because the surface area involved is small (38). The danger
is tha t burns may resul t f rom high local t issue temperatures (214).
View Figure
Figure 31.14 Disposable fluid-warming coil that fits inside the hose of a forced-air warmer.
P.896
View Figure
Figure 31.15 Negative-pressure warming device. The seal around the wrist is not shown. (Picture courtesy of Dynatherm Medical, Inc.)
Increased Operating Room Temperature Increasing ambient temperature in the OR, especial ly whi le the patient is being
prepped and draped for surgery, wil l decrease the loss of body heat by reducing the
radiation and convect ion gradients (384). The room can be cooled af ter the patient
is draped, and other means of temperature control a re ini tiated. The temperature
can be raised again during emergence from anes thesia when the surgery is f in ished
(385). Sys tems are available that wi l l keep surg ical personnel cool regardless of
the tempera ture of the room.
Endovascular Devices Continuous arteriovenous rewarming uses percutaneously placed femoral arterial
and venous catheters and the patient 's own blood pressure to c reate an
arte riovenous f is tula that diverts a port ion of the cardiac output through a heparin-
bonded heat exchanger (43). The heat exchanger consists of an inner chamber
through which hot water is pumped and an outer chamber through which the
patient 's blood f lows in a countercurren t di rec tion. These devices can inc rease core
temperature by 1.5°C to 2.5°C per hour (386).
Central venous heat exchange catheters are discussed under the Cooling Devices
section . These catheters can also be used for heat ing. While they are highly
ef fective , they are h ighly invasive and expens ive.
Lavage Peri toneal , bladder, o r gastric lavage with warmed l iquids can be performed.
Cost-effectiveness The inf luence of warming on perioperat ive costs depends on the pat ient 's condi tion ,
surgical procedure , and inst itut ional factors related to cost accounting (59).
Avoid ing the negative effects associated with the cold patient may reduce
expenses. Blood loss and transfus ion requirements, time to extubation, the need fo r
drugs , and the number of blankets and the length of s tay in the PACU may be
reduced (53 ,59,155,387,388). The normothermic pat ient is more hemodynamically
s table, requ iring less intensive nursing care.
In looking at the dif ferent methods of providing warmth to pa tients, i t was
determined that the old s tyle of water mattress and insulat ing covers have the
lowest return on a cost basis; IV fluid warmers were more effective bu t not as
economical as fo rced-a ir warmers (47 ,200,201,202). Elec tric blankets may be more
cost-effective than forced-air dev ices (57). I f the blankets are reusable, the
reprocessing costs must be considered.
Hazards Softened Tracheal Tubes Heat supplied by a convective warming device has been shown to soften a polyv iny l
ch loride tracheal tube (389,390). This may make the tube more l ikely to k ink and
possibly obstruct (391).
Infection The possibi l i ty of bac terial disseminat ion f rom forced-air devices has caused some
to be uncomfortable wi th their use. Env ironmenta l contamination of the in tensive
care uni t (ICU) is wel l known. However, s tudies indica te that there is no increased
risk assoc ia ted wi th forced-air warming devices (392). Al l forced-ai r uni ts inc lude
f il te rs that remove bacteria from the heated ai r. Recommendations to avoid this
problem include using a f i l ter in the hose, changing the f i l ter regularly , using on ly
manufac turer-recommended b lankets, s teri l izing the detachable hose, and not
reusing coverle ts (393,394,395).
A water bath can ac t as a source of infec tion (358,396,397,398). IV injection ports
and tubing connec tions should be kept out of the water (44). The water shou ld be
discarded after use and the reservoir cleaned and disinfected.
I f a leak develops in a countercurrent f lu id warming sys tem, unsteri le water may be
infused into the patient (399). To avoid this problem, a leak check should always be
performed before connec ting the l ine of the warming set to the pat ient. Since the
pressure in the IV l ine may
P.897
be higher than tha t in the water chamber, blood or f lu id may enter the water bath.
Sedation Patients under regional anesthes ia have al tered thermal perception and behav ioral
responses that may counter the ac tion of sedative drugs. Warming may reverse this
ef fect wi th resul tant sedat ion (155,299,400).
Burns A report from the American Associat ion of Anes thesiologists (ASA) c losed-claims
database showed 54 patient burns out of 3000 tota l c laims (214). Eigh teen burns
were caused by bags or bott les that had been heated and placed next o r c lose to
the pat ient's sk in. Other cases of this type have been reported (401,402). Of the
eight burns from electrically-powered warming equipment, f ive resul ted from
ci rculating-water mattresses. Other burns resul ted f rom a warming l ight and a
heated humidif ier tubing. In only one case was the heat ing device found to be
defec tive. There are reports of burns wi th fo rced-air warming
(191,192,193,194,195,196,197), radiant warmers in infants (403), and resist ive
warming mattresses (258,404). Unfortunately, burns are not usua lly recogn ized
unti l af te r surgery has been completed.
A common patient fac tor in many burns is poor cutaneous blood f low. In juries are
usual ly mos t severe in areas overlying bony prominences . The risk of tissue injury
is further increased when heat or pressure is combined with chemical irri tation such
as that p roduced by many skin-c leaning so lut ions, especially those contain ing
iodine (38). Age is another factor. The elderly often have thin, delica te sk in that is
especial ly suscept ib le to injury. The skin on newborn pat ients has a reduced
thickness compared wi th adul ts . This diminishes protection against ex ternal
noxious events (402). Patients with ischemic t issue or those who undergo
procedures involv ing cardiopulmonary bypass are l ikely to be at increased risk of
thermal inju ry. Heating devices should not be used distal to a tourniquet or arterial
c lamp or during cardiopu lmonary bypass.
When a warming device is used for a pat ient wi th compromised c ircu lat ion, the
patient 's sk in condi t ion should be moni tored f requently and the uni t's maximum
sett ing not used (192,211,405). Constant v igi lance mus t be exerc ised to ensure
that port ions of heating devices not meant for d irect pat ien t contac t, such as tubing
for a water blanket or the hose of a fo rced-a ir mattress, do not come in contac t with
the pat ient (211). Solut ion and blanket-warming cabinet temperature should be
l imi ted to 43°C (319,406).
When a burn occurs , the pattern of the lesion can help to identify the cause (407).
I f a warming dev ice has been used and the lesion conforms to that device's edges
but no other area of the sk in is involved, then i t is l ikely that the warming device
caused the lesion.
Increased Transcutaneous Medication Uptake An increase in transdermal drug uptake may occur when the skin is heated
(408,409). For th is reason, transdermal med ication should be applied in a locat ion
that wi l l not be warmed or shou ld be discont inued during heat ing (410).
Hemolysis Blood may hemolyze if overheated (44,324,336,345,359). Al terations in red ce ll
integri ty do not occur below a temperature of 46°C, and f rank hemolys is does not
occur un ti l 48°C (411,412). Packed red b lood cel ls remaining stat ionary wi thin
microwave or countercurrent heating cartridges may show evidence of hemolysis
(353). I f water from a f luid-warming sys tem leaks into blood, hemolysis may resul t
(399).
Current Leakage Liquid bath and dry heat exchangers must be wel l grounded. They can leak
electrical current into the f luid path (44).
Air Embolism A hazard wi th f luid warmers is the poss ib il i ty of infusing ai r into the pat ient—either
as a resul t of bubbles created as the f luid is warmed (outgass ing), air entrained
through an infusion sys tem, or by del ivering a ir contained in the f luid source
(283,299,413,414,415,416,417,418,419,420,421,422,423,424,425,426,427,428,429)
. The danger is greates t with the use of pressure infusers; when f luids are infused
by a pump; and when rapid, h igh-volume f luid admin is trat ion is necessary.
Solut ion manufacturers typica lly put 50 to 75 mL of ai r into each solution container
(299,421). Th is should be removed f rom the container and the tubing checked for
bubbles before the start of an infusion. Part ially emptied fluid bags should not be
reattached to the IV system (427,428).
Many systems provide a warning feature to alert the operator to ai r in the IV l ine,
and traps that co llect bubbles in the f luid are incorporated into many disposab le
se ts. If the trap is ins talled ups ide down, a ir may be transmitted to the patient
(430). Many of these traps cannot be eas ily vented; once the trap becomes full, a ir
may be de livered to the patient. Some gas-e liminating devices use a microporous
membrane that a llows the gas to escape without any user inte rvent ion (431).
Another design uses a mechanism that s tops the f luid f low when ai r is detected
(432,433,434). Some sys tems al low f luid containing ai r to be reci rcula ted to the
reservoir chamber (310). However, no gas-e liminat ing device can rel iably remove
large amounts of ai r. Automatic ai r de tec tion devices may fail
(335,413,421,435,436).
P.898
Interference with Bispectral Index Monitoring Falsely elevated bispectral index values have been reported in pat ients receiv ing
forced-air warming around the head (437). Temporary interrup tion of the warm air
f low may be required to ge t accura te readings .
Pressurized Infiltration The use of a f luid-warming system that pressurizes the f luid can resul t in
ex travasat ion of f lu id. A compartment syndrome could occur (438).
Cooling Devices Patients may require coo ling fo r a varie ty of reasons, including treatment of
mal ignant hyperthermia and possible cerebral p rotective effec ts during neurolog ic
or card iac surgery or fol lowing head trauma or cardiac arrest.
Chilled Intravenous Fluids Hypothermia can be induced by rapid infusion of chi lled f lu ids or IV f luids at room
temperature (440,441,442,443). One l iter of crystalloid at ambient temperature or
one unit of refr igera ted blood adminis te red through a peripheral si te reduces mean
body tempera ture by approx imately 0.25°C in adul ts (444). Admin istering the f luid
central ly wi l l dec rease core tempera ture more rapidly (442). This method is
restric ted by the volume tha t can be admin is tered wi thout overloading the
cardiovascular sys tem but may help to compensate for ant icipated diuresis and help
to maintain cerebral perfusion pressure . It is less effect ive than immersion in ice
water (206).
Circulating Water Units Newer c i rcu la ting water un its have been found to be effective for contro ll ing fever
in neurologic pat ients (445,446). Older uni ts were less effective (447).
Forced-air Cooling Cooling can be performed by using a fo rced-a ir uni t. This needs to be combined
wi th other methods to achieve ef fective cooling (206,448). Also, mos t uni ts have no
feedback control . Personal ai r-condi tioning systems tha t attach to the medical a ir
l ine are available.
Immersion in Ice Water Cooling by conduc tion may be less eff icien t than using fo rced air and is more like ly
to interfere with pat ient care (449,450).
Central Venous Heat Exchange Catheters Dev ices are available that have heat transfer elements that can be inserted into the
central venous system (451,452,453,454). Pat ien t temperature is regu la ted through
a c losed-loop controller. These devices can lower core temperature rapidly (455).
The ir invasiveness is a drawback in terms of ease of applica tion and risk of
complica tions . The t ime lag before cool ing can be in it iated is a potent ial drawback.
Lavage Peri toneal , bladder, o r gastric lavage with iced solutions can be performed.
Peri toneal lavage is invasive, and bladder lavage provides minimal cooling.
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Course #222). New Orleans: ASA, 2001.
2. Stewart S, Lujan E, Ruff C. Innovations and excellence: the incidence of adul t
hypothermia in the postanesthesia care unit. Periop Nurs Q 1987;3 :57–62.
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5. Carl i F, Macdonald IA. Perioperative inadvertent hypothermia : what do we need
to prevent? Br J Anaesth 1996;76:601–603.
6. Lil ly RB. Signif icance and recovery room management of pos tanesthes ia
hypothermia and shivering . Anes th Cl in North Am 1990;8:365–374.
7. Matsukawa T, Sessler DI, Chris tensen R, et al . Heat f low and distribution during
epidural anes thesia. Anesthesiology 1995;83:961–967.
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10. Glosten B, Hynson J, Sessler DI, et a l. Preanesthet ic skin surface warming
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Questions For the fol lowing quest ions, select the correct answer
1. What is the normal core body temperature?
A. 38°C ± 0.2°C
B. 38°C ± 1°C
C. 37°C ± 0.6°C
D. 37°C ± 0.2°C
E. 37°C ± 0.6°C
View Answer2. How much does core body temperature normally decrease during the firs t hour after the beginning of surgery without using measures to reduce heat loss? A. 0.5°C to 2°C
B. 0.5°C to 1.5°C
C. 0.5°C to 1°C
D. 1°C to 1.5°C
E. 1°C to 2°C
View Answer3. What is the major mechanism of heat loss during anesthesia?
A. Evaporat ion
B. Conduc tion
C. Convection
D. Radia tion
E. Si te of surgery
View Answer4. What percent of the intravascular volume may be lost to the extracellular space during each degree of hypothermia? A. 1.85%
B. 2%
C. 2.5%
D. 2.75%
E. 3%
View Answer5. What is the highest temperature above which a lterations in red cell integrity occur? A. 44°C
B. 45°C
C. 46°C
D. 47°C
E. 48°C
View Answer6. How much does a liter of crysta lloid at ambient temperature or one unit of refrigerated blood infused through a peripheral site reduce the mean body temperature? A. 1°C
B. 15°C
C. 0.2°C
D. 0.25°C
E. 0.3°C
View Answer7. Which factors determine the temperature of a fluid as it enters the vein? A. The temperature set poin t on the warming dev ice
B. Starting temperature of the infusate
C. Rate of f lu id f low
D. Length of the tub ing between the patien t and the warmer
View AnswerFor the following quest ions, answer
• i f A, B, and C are correct
• i f A and C are correct
• i f B and D are correct
• i f D is correct
• i f A, B, C, and D are correct.
8. Which conditions prevent an equilibration between heat loss and heat production during surgery? A. Thoracic surgery
B. Peripheral neuropathies
C. Intra-abdominal surgery
D. Regional anesthetic blocks
View Answer9. Which factor(s) determine the severity of hypothermia? A. Length of the surgical p rocedure
B. Amount of cool fluids
C. Si te of surgery
D. Sex
View Answer10. Which metabolic changes occur as a result of hypothermia? A. Shif t of the oxyhemoglobin d issociation curve to the right
B. Lower umbilical pH in babies whose mothers were warmed
C. Higher hemoglobin saturation
D. Accumulat ion of metabolic products
View Answer11. Which problems are associated with shivering in the postoperative period? A. Increased metabolic demand
B. Increased cardiovascular work
C. Increased in traocu lar pressure
D. Decreased intrac ran ial pressure
View Answer12. Which patient(s) are most susceptible to burns from heating devices?
A. Those wi th poor cu taneous blood f low
B. Diabetics
C. The e lderly
D. Infants
View Answer13. What are the cardiovascular effects of hypothermia? A. Decreased need fo r vasoact ive drugs
B. Cardiac dysrhythmias
C. Increased contract il i ty
D. Increased catecholamine product ion
View Answer14. The most effective way(s) to uti l ize a water heating pad include A. Plac ing the heating pad over the pat ient
B. Plac ing the heating pad under the pat ient
C. Us ing a thin pad attached to the pat ient
D. Heating af te r the patien t is in place
View Answer15. Effective use of passive coverings includes A. Covering as much of the body surface as possib le
B. Applying in the PACU to rewarm the pat ient
C. Placing a warm b lanket over the patient as soon as he enters the operat ing room
D. Us ing ref lec tive coverings
View AnswerP.906
16. Advantages of radiant heat lamps include A. Decreased heat loss f rom the skin during washing
B. Unobstructed access to the pat ient
C. Decreased f lu id requirements
D. Mos t effec tive in areas of arte riovenous anastomoses such as the forehead,
hands, and feet
View Answer17. Indication(s) for fluid warming include
A. Patients with cold agg lut inins
B. Exchange transfusion in neonates
C. Rapid infusion
D. Restoring normothermia af te r surgery
View Answer
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