Anaesthesiology Intensive Therapy
2014, vol. 46, no 2, 96–100
ISSN 1642–5758
DOI: 10.5603/AIT.2014.019
www.ait.viamedica.pl
REVIEWS
Methods to prevent intraoperative hypothermia
Bartosz Horosz, Małgorzata Malec-Milewska
Department of Anaesthesiology and Intensive Care Therapy, Medical Centre of Postgraduate Education,
W. Orłowski Hospital, Warsaw, Poland
Abstract
Inadvertent intraoperative hypothermia is by far the most commonly occurring anaesthesia-related complication. It can
increase the risk of unfavourable events perioperatively. Higher rates of surgical site infections and blood transfusions,
coagulation and drug metabolism disturbances are said to be the most relevant issues linked to this phenomenon.
Although they have been available for several years now, dedicated systems designed to prevent it are still not part of
routine anaesthesia conducted in Poland.
This review aims to discuss the factors which may potentially increase the risk of hypothermia, and to present tools that
are readily available and effective in perioperative temperature management
Key words: thermoregulation, disorders; anaesthesia, complications, intraoperative hypothermia; intraoperative
hypothermia, prevention
Anaesthesiology Intensive Therapy 2014, vol. 46, no 2, 96–100
Intraoperative hypothermia, defined as a core body
temperature lower than 36° C, can develop in over half of all
anaesthetic procedures and is associated with an increased
incidence of perioperative complications. This disorder of
homeostasis increases the risk of coronary incidents, increases the incidence rates of surgical site infections, causes
clotting disturbances and prolongs the action of the drugs
used during anaesthesia [1–3]. The loss of normothermia
during surgery results from several factors, such as the use
of drugs that impair thermoregulation, the low temperature
of the operating room and disinfection of the operative field.
With increased awareness of the essence and magnitude
of perioperative hypothermia, methods of its prevention
have aroused widespread interest. It is no longer sufficient
to ask whether hypothermia should be prevented; the question has become how to prevent hypothermia. Hypothermia should be prevented most effectively, considering the
efficacy and cost-effectiveness of the measures used, as
the maintenance of core body temperatures above 36°C
becomes one of the standards of perioperative care [4].
The human body exchanges heat with the environment by radiation (60%) as well as conduction, conversion
and vaporisation. The excessive loss of heat via each of the
above-mentioned methods is unfavourable and should be
eliminated. There are various methods to treat and prevent
96
hypothermia, which can be generally divided into active and
passive methods (Fig. 1). Passive prevention of hypothermia
involves a reduction in heat loss using “surgical drapes” (unheated) to isolate the patient’s body from the surroundings,
i.e., the trunk, limbs and head, and the placement of a heat
and humidity exchanger in the respiratory system of the
anaesthesia machine. Active methods include methods to
supply additional heat from the outside, which together
Heat and humidity
exchanger
Passive
Isolation
Warm infusion fluids
Methods to prevent
hypothermia
Warm air circulation
Active
Water mattress pads
and blankets
Electric mattress
pads and blankets
Figure 1. Hypothermia prevention methods
Bartosz Horosz, Małgorzata Malec-Milewska, Methods to prevent intraoperative hypothermia
with endogenous production aim to equalise heat loss. The
combination of several methods, which eliminates several
mechanisms leading to hypothermia, is the most effective
practice used.
TEMPERATURE OF THE OPERATING ROOM
Conditions in the operating room should be comfortable for all members of the team but mostly for the patients. The setting should ensure comfort before the induction of anaesthesia and should not be the factor increasing
the risk of complications during surgery and anaesthesia.
Thanks to the availability of air-conditioned rooms, hence
the possibility to provide complete control of air temperature and humidity, precise action can be taken. The optimal
temperature of the operating room is difficult to define,
mainly due to clashing needs of several groups. Surgeons
opt for one optimum range of temperature, whereas anaesthesiologists or scrub nurses are in favour of some other
range. The opinions regarding physical conditions in the
operating room have repeatedly changed since 1924, when
some started to advocate that traditional, warm and humid
operating rooms should be abandoned. In the days before
the introduction of widespread sterilisation, it was generally believed that warmer surroundings decreased the risk
of sepsis and pneumonia and should be provided despite
the inconveniencies experienced by those involved. [5]. In
the 1950s and 1960s, with the development of surgical and
anaesthetic techniques, the attitude towards the operating
room conditions during surgeries was reversed. The notion
of ergonomics as well as the comfort and effectiveness of
work became increasingly popular; therefore, the search
for optimal conditions inevitably focused on a compromise
between the surgeon’s comfort and patient’s cooling. Despite substantial differences in the range of temperatures
comfortable for the general population depending on the
geographical location, the range of temperatures desirable
for operating room teams appear to be globally similar:
21–24°C in the United States and 18–21°C in Great Britain [6].
However, these data do not mean that individual medical
specialities prefer the same range of comfort; according to
the British study performed in the 1960s, anaesthesiologists
explicitly preferred significantly higher temperatures (by
over 1°C) than surgeons from the same operating room,
whereas the remaining personnel chose ranges between
those extremes. The operating room, however, remains the
place where the desired temperature oscillates approximately 20°C. This temperature is far from the comfort range
for patients who are deprived of suitable clothes before
the induction of anaesthesia. Making provisions for the
increased comfort of patients in the preoperative period is
another difficult task for anaesthetic personnel [7].
PASSIVE METHODS OF HYPOTHERMIA
PREVENTION
There are two basic types of thermally isolating coverings: mass coverings, isolating due to the arrest of air between the fibres of the material of which the covering was
made, and reflecting coverings, which decrease the heat
loss via radiation by re-reflection of heat radiation to the
warmer surface (body), allowing only slight dispersion. In
clinical practice, mass isolators include surgical drapes and
prefabricated surgical coverings. Air is an excellent physical
isolator; therefore, the larger the amount of air the covering
can hold, the better the thermal isolation. The air between
fibres directly affects the quality of isolation, whereas the
type of fibre seems of no importance [8]. Moreover, the fact
that a single-layer covering markedly reduces heat loss (by
33%) can be relevant; the use of additional layers results in
only slightly better results (additional 18% with three layers)
[9], which is undoubtedly attributable to air arrest between
the skin and the covering.
Considering the operating room temperature, the loss
of energy in the form of heat through exposed skin is always
substantial; the greater the difference in the skin and surrounding temperatures, the bigger the loss [10]. The covering
of the skin reduces this loss, largely via decreased radiation
and convection. The initial phase of core temperature decrease during anaesthesia results from its redistribution to the
peripheral compartment, whereas the loss through the skin
and airways remains constant (the heat reserves of the body
do not change). In the initial phase of hypothermia, thermal
isolation of integuments only slightly affects the development
of hypothermia and therefore is not capable of preventing it.
During the next phase of temperature decrease associated
with excessive loss and reduced production of heat, prevention of excessive heat transfer to the surroundings is essential
attaining the thermal plateau. The initial heat loss through the
skin is approximately 100 W during the first hour of anaesthesia, which can be reduced to approximately 70 W when the
entire surface is covered [8]. Because approximately 70 W are
produced by the mechanically ventilated patient and because
heat loss through the loss through the surgical field is only
approximately 5 W, theoretically, isolation methods should be
sufficient to achieve thermal equilibrium [11]. Unfortunately,
proper covering of the suitable body surface is typically infeasible, and the loss through the airway reaches 50% of the total
heat losses [12]. Therefore, passive methods are insufficient
to maintain normothermia.
ACTIVE METHODS OF HYPOTHERMIA PREVENTION
Thanks to the development of modern surgical and
anaesthetic techniques, long procedures lasting many hours
can be performed but are associated with difficulties in
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Anaesthesiol Intensive Ther 2014, vol. 46, no 2, 96–100
maintaining homeostasis when it becomes necessary to
support or replace the protective mechanisms abolished
by anaesthesia. With respect to temperature, it is easier to
prevent temperature changes than to treat undesirable
changes; an elevation in the core temperature of the patient
already cooled to the normal value under post-anaesthesia
in a surveillance room setting is considerably more difficult than the prevention of its decrease in the operating
room. The ineffectiveness of warming the body surface at
this stage likely results from the isolation of the peripheral
compartment due to shrinkage of the peripheral vessels
[8, 13]. However, prevention of hypothermia is relatively
effective, assuming that suitable devices are available and
properly used [14, 15].
The patient’s heat reserve decreases during surgery,
predominantly via the loss through the skin, the expenditure
of energy used for vaporisation from the surgical site and for
the warming of transfused intravenous fluids, and through
the airway (albeit to a minimum extent). Because the surgical site cannot be altered, hypothermia can be effectively
prevented only by the prevention of heat loss through the
skin and by transfusion of pre-warmed fluids. The passive
methods described above reduce the heat loss yet do not
eliminate it. Only the elimination or reversal of the gradient
of temperatures between the skin and its direct surroundings and the use of warmed infusion fluids can inhibit the
heat escape or even enable its supply to the body.
WARMING OF INFUSION FLUIDS
The unfavourable effects of infusions of intravenous
fluids cooler than the patient’s body temperature were reported in the 1960s. A decrease in core temperature by
0.5 to 1.0° C was observed after transfusion of 500 mL of cold
blood, and massive transfusions were found to be associated
with marked hypothermia and a high risk of sudden cardiac
arrest (SCA) [16, 17]. In a 1964 study, transfusions of more
than 3000 mL of cold blood at a rate of 50–100 mL min–1 resulted in cardiac arrest in 12 out of 25 cases; transfusion of
the same volume of warmed blood led to SCA in 8 out of
118 cases [18]. Since that time, the attitude to therapy with
fluids and blood preparations has substantially changed,
which is visible in the guidelines published and periodically
updated; however, the physical properties of these drugs
and transfusion-induced thermoregulation disturbances
can persist in modern times [19, 20]. It is possible to estimate
energy expenditures related to the supply of cold fluids. To
increase the temperature of 1 kg of water by 1°C, 1 kcal of
energy is needed (the heat capacity of water is 1 kcal); thus,
the body needs 16 kcal of energy to “warm” 1 litre of crystalloids from room temperature (21°C) to body temperature
(37°C) (assuming that their heat capacity equals that of
water). Based on the above data, to warm 3.7 litres of crystal98
loids from room temperature, the anaesthetised patient’s
body has to use the energy it produces during one hour of
anaesthesia using substitutive ventilation (approximately
60 kcal h–1 ~ 70 W). The transfusion of 1 litre of full blood at
4° C requires approximately 30 kcal of energy for warming;
the transfusion of 2 litres is likely to decrease the core temperature by 1–1.5°C [21]. The available systems of warming
blood, blood preparations and transfusion fluids use various
technological solutions, e.g., a water bath (Hotline, Smiths
Medical), forced-air warming (Bair Hugger, 3 M, USA), conductive warming with metal surfaces (enFlow, GE Healthcare, USA) or microwave technology (MMS ThermoStat, Meridian Medical Systems, USA). Most of the systems provide
a wide range of temperatures in a marked range of flow
velocities and include devices preventing excessive warming and air-detection. A relevant issue is the upper limit of
temperature to which blood preparations and infusion fluids
can be warmed. Based on numerous observations related
to extracorporeal circulation, temperatures of 42°C are safe
for blood preparations [22]. Polish legal regulations allow
the warming of blood solely in the device designed for this
purpose with suitable systems for measuring temperature
and alarm systems; however, the safe limit of temperatures
has not been defined. The directive of the Minister of Health
nm September 19, 2005 recommended blood warming in
cases of transfusions of more than 50 mL min–1 in adults and
15 ml min–1 in children, in cases of exchange transfusions in
newborns and in recipients with clinically significant cold
antibodies [23]. The desirable temperature of crystalloids
and colloids should depend on the clinical situation; there
are, however, reports about the safe supply of crystalloids
of 54°C [24].
FORCED-AIR WARMING SYSTEMS
Active prevention of intraoperative hypothermia using
warmed air has been used for several decades. Such systems
were initially used postoperatively (to treat hypothermia);
gradually, these systems began to be used in operating
rooms (to actively prevent hypothermia) and preoperatively
(to reduce the risk of hypothermia) [25]. The systems operate
by forcing warmed air through the warming device to the
container, which is in direct contact with the patient’s skin,
(usually a two-layer blanket). The blanket shape is tailored
to the needs, i.e., it contacts the largest possible surface of
the body, depending on the patient’s position and location of the operative field. The air forced inside escapes
through the pores of blanket fabric, forming a specific, warm
microclimate around the individual being warmed. The
lack or reversal of the temperature gradient between the
environment (warming blanket) and the skin inhibits heat
loss through radiation as well as overheating. Additionally,
there is no loss through convection. The extent of heat
Bartosz Horosz, Małgorzata Malec-Milewska, Methods to prevent intraoperative hypothermia
transfer predominantly depends on the surface of contact
with the patient’s skin [26]; the small thermal capacity of the
carrier (air) is the limiting factor and necessitates its quick
exchange. Considering the operating conditions, a duvet
enabling warming of the upper body is most frequently used
intraoperatively; pre- and postoperatively, the systems warming the largest possible surface of the body can be used. The
effectiveness of these systems also depends on the gradient of temperatures between the blanket and skin surface.
The higher the gradient in favour of the warming blanket
(air warmer than the skin), the bigger the heat flow to the
body surface is. The systems based on this technology have
been and continue to be the basic methods of perioperative
hypothermia prevention; therefore, numerous studies have
assessed their efficacy and safety. The findings demonstrate
their satisfactory efficacy in preventing intraoperative hypothermia once several conditions are met. The choice of
a properly shaped warming blanket, its appropriate placement and the use of a suitably high warming temperature
are not always sufficient. An important factor is the initial
warming of the patient before induction, which substantially
reduces the redistribution-induced decrease in core temperature [27]. The effectiveness of the hypothermia-prevention
system is markedly lower when used after the induction of
anaesthesia [28-30]. Over the past 20 years, the warm airflow
systems have been used in over 100 million cases and remain
the most readily chosen systems in Western Europe.
MATTRESSES AND ELECTRIC BLANKETS
An alternative for forced-air warming systems are systems using electric resistance for heat production. These systems belong to the group of devices whose effectiveness is
based on conduction. Therefore, the systems are sufficiently
effective only when the warm surface directly contacts the
surface to be warmed, as opposed to warm airflow systems,
in which a heat carrier leaves the warming blanket and
delivers heat to the body surface. The physical elements in
favour of such devices include the following: the small heat
capacity of air requires a continuous supply of warmed air to
maintain its pre-set temperature, whereas the material used
as a heat carrier in electric mattresses and blankets warms
and cools more slowly. Moreover, these conduction systems
operate noiselessly, which for many working in operating
rooms is the key argument weighing in favour of electric
systems compared with forced-air warming systems (even
modern warming devices emit noise at a level of 45–50 decibels). It should be stressed that the use of electric mattresses
alone is not recommended, as the loss through the body
surface contacting the operating table is negligible, and the
amount of heat that can be supplied using such mattresses
low [26, 31, 32]. To prevent intraoperative hypothermia
effectively by reducing the loss and supplying heat from
the outside, it is necessary to secure the largest possible
surface through which the body can potentially loose heat.
Therefore, the electric systems presently produced include
warming blankets of various shapes made of carbon fibre or
polymers. The forms of the blankets are adjusted to surgical
needs. The study findings regarding their efficacy vary from
poor efficacy in earlier studies [33] to efficacies comparable
with those of forced-air warming systems in recent studies
[15, 31, 34–36]. The economic implications associated with
their use are complex. On the one hand, the systems can be
used any number of times; on the other hand, variants of
high efficacy are extremely expensive.
WATER MATTRESSES AND COVERS
The idea of a heat carrier in continuous motion in the
warming system is also used when water is used as a carrier. Mattresses filled with warm water have been applied
for many years, but their use is associated with numerous
technical problems, and their efficacy in preventing hypothermia is found to be low. However, because both warm
and cold fluids can be used for their filling, these mattresses
have found application in special situations when the body
temperature is reduced intentionally (Intensive Cardiac Care
Units, cardiac surgery) [37]. Because water has a markedly
higher heat capacity than air, it can be potentially assumed
that when water circulates in the warming system, the
amount of heat supplied can be large. The only condition
is the provision of direct contact with the largest possible
skin surface; thus, specially shaped covers filled with warm
water were designed in which the limbs and intraoperatively
available trunk parts are wrapped (Energy Transfer Pads,
Kimberly Clark, USA; Allon ThermoWrap, MTRE Advanced
Technologies, Israel). Such a system was found to be more
effective than forced-air and electric systems [38–40] (particularly in extensive procedures in which the peritoneal
cavity is opened) yet comparable to the combined use of
force-air warming systems and the water mattress [41]. Unfortunately, the factor limiting the widespread application of
the system is its price and technical problems related to its
use — large dimensions and potentially extremely troublesome consequences of malfunction during use.
SUMMARY
Perioperative hypothermia is only one of a number of
potential problems perioperative medicine has to face but
is nonetheless critically important. The available study findings and guidelines provide some instructions useful in the
prevention of hypothermia:
—— Active methods of hypothermia prevention should be
used in surgeries longer than 30 min.
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Anaesthesiol Intensive Ther 2014, vol. 46, no 2, 96–100
—— Warming should be applied before the induction of
anaesthesia.
—— In long procedures and in high-risk patients, heat loss
should be prevented with more than one active method.
The use of several systems of hypothermia prevention
during one surgery is neither time-consuming nor troublesome; in the majority of cases, it is also not excessively
costly. Thus, there is hope that their routine use is only
a matter of time.
24.
25.
26.
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Corresponding author:
Bartosz Horosz, MD
Department of Anaesthesiology and Intensive Therapy
Medical Centre of Postgraduate Education
W. Orłowski Hospital
ul. Czerniakowska 231, 00–416 Warszawa, Poland
tel.: +48 22 584 12 20, fax: +48 22 584 13 42
e-mail: [email protected]
Received: 4.12.2013
Accepted: 9.01.2014