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.

References 1. Schwartz AJ . Anesthetic issues related to body temperature (ASA Refresher

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.

3. Sessler DI. Perioperat ive heat balance. Anes thesiology 2000;92:578–596.

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4. Schwartz AJ . Anesthetic issues related to body temperature (ASA Refresher

Course). Park Ridge, IL: ASA, 2000.

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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8. Hynson JM, Sessler DI, Moayeri A, e t al . The effects of preinduc tion warming on

temperature and blood pressure during propofol/ni trous oxide anes thesia.

Anesthesiology 1993;79:219–228.

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9. Just B, Trevien V, Delva E, et al. Prevention of intraopera tive hypothermia by

preoperat ive skin-surface warming. Anes thesiology 1993;79:214–218.

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10. Glosten B, Hynson J, Sessler DI, et a l. Preanesthet ic skin surface warming

reduces redis tr ibution hypothermia caused by epidural block . Anes th Analg

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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