Chapter 31 Temperature Control Equipment Physiologic Temperature Control Numerous studies have shown that significant temperature changes routinely occur in anesthetized patients ( 1 ). Inadvertent hypothermia is by far the most common disturbance. Without specific interventions, up to 90% of patients entering the postanesthesia care unit (PACU) may be hypothermic ( 2 ). An exception may be patients undergoing magnetic resonance imaging (MRI) in whom the absorption of radio frequency radiation may partially offset heat loss. P.885 In the unanesthetized patient, vasoconstriction maintains a temperature gradient between the core and periphery of 2°C to 4°C. Core body temperature is normally maintained within a narrow range of 37 ±0.2°C. When core body temperature goes out of this range, physiologic mechanisms are initiated to reestablish the norm. Anesthesia alters the response threshold, allowing the body to experience greater variations in temperature before it attempts to reestablish a 37°C core temperature. Responses to altered temperatures are less effective under anesthesia. Hypothermia under anesthesia usually follows a characteristic pattern ( 1 , 3 , 4 ). Core body temperature usually decreases 0.5°C to 1.5°C during the first hour ( 5 , 6 , 7 ) as vasodilatation causes redistribution of body heat from the core to the periphery. Warming peripheral tissues before induction of anesthesia (prewarming) decreases the central-to-peripheral temperature gradient, thereby minimizing the redistribution of heat from the core to the periphery and reducing the initial decrease in core temperature ( 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ). This redistribution cannot be prevented by intraoperative skin surface warming ( 14 ). After the first hour, core temperature typically decreases at a slower rate as the body's heat loss exceeds the metabolic heat production. This is followed by a thermal plateau during which core temperature no longer significantly decreases. At this time, heat loss equilibrates with heat production and vasoconstriction constrains metabolic heat to the core compartment while allowing peripheral tissues to continue to cool. Patients with neuropathies have more severe hypothermia than other patients, possibly because the onset of vasoconstriction is delayed ( 20 ). A
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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.
P.885
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
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
,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
,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.
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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
418. Woon S. Possible risk of air emboli using hot l ine HL-900 f luid warmer.
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419. Anonymous . Use of pressure infusion wi th Level 1 technologies L-10 gas
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432. Smith CE, Kabbara A, Kramer RP. A new IV f luid and blood warming system to
prevent ai r embolism and compartment syndrome. Anes thesiology 2001;95:A549.
433. Comunale ME. A laboratory evaluat ion of the Level 1 rapid infuser (H1025)
and the Belmont Ins trument F lu id Management System (FMS 2000) for rapid
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434. Schechter L. Comparison between Level 1 and other fast f low f luid warming
systems. Anesth Analg 2004;99:301.
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435. Eaton MP, Dhil lon AK. Relative performance of the Level 1 and Ranger
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436. Phi l ip JH. Performance of Level I and microwave medical systems ai r
el iminators. Anesthes io logy 1999;91:A513.
437. Hemmerling TM, Fort ier JD. Falsely inc reased bispectral index values in a
series of pa tients undergoing cardiac surgery using fo rced-a ir warming therapy of
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438. Willsey D, Peterfreund R. Compartment syndrome of the upper arm af te r
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439. Luscombe M, Andrzejowski JC. Cl inical appl ications of induced hypothermia.
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440. Baranov D, Bumgardner J, Smith DS, et al . Rapid infusion of chil led IV
so lut ions for moderate hypothermia. Anesthes iology 1998;89:A316.
441. Bernard SA, Buist MD, Monteiro O, et al . Induced hypothermia using large
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442. Rajek A, Greif R, Sessler DI, et al . Core cool ing by central venous infusion of
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443. Baumgardner JE, Baranov D, Smith DS, et al . The effect iveness of rapidly
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446. Carhuapoma JR, Gupta K, Coplin WM, et al . Treatment of refractory fever in
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447. O'Donne ll JM, Axelrod P, Fisher C, et al . Use and effec tiveness of
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449. Creechan TA, Vollman KM, Kravutske M. Comparison of coo ling by convection
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cooling for induc tion of mi ld hypothermia during neurovascular procedures in
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451. Schmutzhand E, Engelhardt K, Beer R, e t al . Safety and eff icacy of a novel
intravascular cooling dev ice to control body tempera ture in neuro logic intensive
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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