Perioperative
Hypothermia
Karim Rafaat, MD
Introduction
The human thermoregulatory system usually maintains
core body temperature within 0.2℃ of 37℃
 Perioperative hypothermia is common because of the
inhibition of thermoregulation induced by anesthesia
and the patient`s exposure to cool environment
 Hypothermia complications:

 Shivering
 prolonged drug effect,
 coagulopathy
 surgical wound infection
 morbid cardiac event
Normal thermoregulation

Processing of thermoregulatory information:
afferent input
central control
efferent responses
Normal thermoregulation

Afferent input:
 cold signal-Aδ fiber
 warm signal-C fiber

Each of the following contribute 20% of the
total thermal input:
 Hypothalamus




other parts of brain
skin surface
spinal cord
deep abdominal and thoracic tissues
Normal thermoregulation

Primary thermoregulatory control center
 Hypothalamus

Control of autonomic responses is 80%
determined by thermal input from core
structures

In contrast, behavior response may
depend more on skin temperature
Normal thermoregulation

The inter-threshold range (core
temperatures that do not trigger
autonomic thermoregulatory responses)
is only 0.2℃

Each thermoregulatory response can be
characterized by a threshold, gain,
maximal response intensity

Behavior is the most effective response
Normal thermoregulation

Major autonomic defenses against heat:
1. sweating
2.cutaneous vasodilation

Major autonomic defenses against cold:
1.cutaneous vasoconstriction
2.nonshivering thermogenesis
3.shivering
Normal Thermoregulation
Sweating
Active vasodilation
37
Vasoconstriction
36
Shivering
Normal thermoregulation

Vasoconstriction occurs in AV shunts
located primarily in fingers and toes,
mediated by α-adrenergic sympathetic
nerves

Non-shivering thermogenesis is important
in infants, but not in adults (brown fat)

Shivering is an involuntary muscle activity
that increases metabolic rate 2-3 times
Thermoregulation during general
anesthesia

General anesthesia removes a pt’s ability to regulate
body temperature through behavior, so that autonomic
defenses alone are available to respond to changes in
temperature

Anesthetics inhibit thermoregulation in a dosedependent manner and inhibit vasoconstriction and
shivering about 2-3 times more than they restrict
sweating

Inter-threshold range is increased from 0.2 to 4℃ (20
times), so anesthetized pts are poikilothermic - with
body temperatures determined by the environment
Thermoregulation during general
anesthesia

The gain and maximal response intensity of sweating and
vasodilation are well preserved when volatile anesthetics are
given

However volatile anesthetics reduces the gain of AV-shunt
vasoconstriction, without altering the maximal response
intensity

Nonshivering thermogenesis dosen`t occur in anesthetized
adults

General anesthesia decreases the shivering threshold far
more than the vasoconstriction threshold
Anesthesia Impairs Regulation
38
Sweating
Constriction
36
34
Shivering
32
30
Threshold
(°C)
0.0
2.0
4.0
6.0
0.0
[Desflurane] (%)
0.1
0.2
0.3
[Alfentanil] (µg/ml)
38
36
34
32
30
0.3
0.6
0.9
[Dexmedetomidine] (ng/ ml)
0.0
2.0
4.0
6.0
8.0
[Propofol] (µg/ml)
Inadvertent hypothermia during
general anesthesia

Inadvertent hypothermia during general
anesthesia is by far the most common
perioperative thermal disturbance (due to
impaired thermoregulation and cold
environment)
Patterns of intraoperative hypothermia
Phase I:
Initial rapid decrease
0
-1
Phase II :
Slow linear reduction
∆Core
Temp
(°C)
-2
-3
Phase III:
Thermal plateau
0
2
4
Elapsed Time (h)
6
Patterns of intraoperative hypothermia
1.
Initial rapid decrease

heat redistribution

decreases 0.5-1.5℃ during 1st hr

Tonic thermoregulatory
vasoconstriction that maintains a
temperature gradient between the core
and periphery of 2-4℃ is broken

The loss of heat from the body to
environment is little

Heat redistribution decreases core
temperature, but mean body temperature
and body heat content remain
unchanged
Patterns of intraoperative hypothermia
2. Slow linear reduction

decreases in a slow linear fashion
for 2-3hrs

Simply because heat loss
>metabolic heat production

90% heat loss through skin surface
by radiation and convection
Patterns of intraoperative hypothermia
3. Thermal plateau

After 3-5 hrs, core temperature stops decreasing

It may simply reflect a steady state of heat loss=heat production

If a pt is sufficiently hypothermic, plateau phase means activation
of vasoconstriction to reestablish the normal core-to-peripheral
temperature gradient

Temperature plateau due to vasoconstriction is not a thermal
steady state and body heat content continues to decrease even
though temperature remains constant
Regional Anesthesia

Regional anesthesia
impairs both central
and peripheral
thermoregulation
38
Sweating
37

Hypothermia is
common in patients
given spinal or
epidural anesthetics
Threshold
(°C)
Vasoconstriction
Shivering
36
35
Control
Spinal
Thermoregulation

All thermoregulatory
responses are neurally
mediated
600
Control

Spinal and epidural
anesthetics disrupt nerve
conduction to more than
half the body
400
VO2
(ml/min)
Epidural
200

The peripheral inhibition of
thermoregulatory defense
is a major cause of
hypothermia during RA
0
35
36
37
Core Temperature (°C)

RA also impairs the central control of
thermoregulation
 The regulatory system incorrectly judges the
skin temperature in blocked areas to be
abnormally high

It fools the regulatory system into
tolerating core temperatures that are
genuinely lower than normal without
triggering a response
Heat Balance and Shivering
Initial hypothermia (Phase I)

Redistribution of heat from core to periphery

Primarily caused by peripheral inhibition of
tonic thermoregulatory vasoconstriction

Although the vasodilatation of AV shunts is
restricted to the lower body, the mass of the
legs is sufficient to produce substantial core
hypothermia
Subsequent hypothermia (Phase II)

Loss of heat exceeds production

Patients given SA or EA cannot reestablish coretemperature equilibrium because peripheral
vasoconstriction remains impaired

Hypothermia tends to progress throughout
surgery
Shivering

Occurs during spinal and epidural
anesthesia

Disturbs patients and care givers but
produces relatively little heat because it
is restricted to the small-muscle mass
cephalad to the block

Treated by warming surface of skin or
administration of clonidine / meperidine
Temperature Monitoring

Core Sites





Pulmonary artery
Distal esophagus
Nasopharynx
Tympanic membrane
thermocouple
Other generally-reliable sites
 Mouth
 Axilla
 Bladder

Sub-optimal




Forehead skin
Infrared “tympanic”
Infrared “temporal artery”
Rectal
Anesth Analg 2008
Potential Benefits of Mild
Hypothermia

Improves neurologic outcome after cardiac arrest






Improves neurologic outcome in asphyxiated neonates





Bernard, et al.
Hypothermia after cardiac arrest study group
Now recommended by European and American Heart Associations
Number needed to treat: ≈6
Hypothermia recommended by International Liaison Committee
Shankaren, et al.
Gluckman, et al.
Eicher, et al.
Number needed to treat: ≈6
No benefit in major human trials
 Brain trauma in adults (Clifton, et al.) or children (Hutchison, et al.)
 Anurysm surgery: Todd, et al.
 Acute myocardial infarction: Dixon, et al
Complications of Mild
Hypothermia

Many!

Well documented
 Prospective randomized trials
 1-2°C hypothermia

Effects on many different systems
 Most patients at risk for at least one complication
Complications of Mild
Hypothermia
Wound infection---the most common
serious complication due to

Impaired immune function

decreased cutaneous blood flow

protein wasting

decreased synthesis of collagen

Wound Infections: Melling, et al.
Wound Infection (%)
16
P = 0.001
12
8
4
0
Hypothermic
Normothermic
Normothermia
is more
effective than
antibiotics!
 Coagulopathy

Hypothermia reduces platelet function and
decreases the activation of the coagulation
cascade

From in vitro studies, it increased the loss
of blood and the need for allogenic
transfusion during elective primary hip
arthroplasty
Blood Loss
20% less blood loss
per °C
Transfusion Requirement
22% less blood
Transfusion per °C
Myocardial Outcomes: Frank, et al.
 Drug metabolism

Mild hypothermia decreases the metabolism
of most drugs

Propofol ---during constant infusion, plasma
conc. is 30 percent greater than normal

Atracurium---a 3 ℃ reduction in core temp.
increase the duration of muscle relaxation by
60 percent

Significantly prolongs the postoperative
recovery period
Duration of Vecuronium
80
Duration
of Action
(min)
60
40
20
Normothermic
(36.6 ± 0.1°C)
Hypothermic
(34.6 ± 0.3°C)
Recovery Duration
Time (min)
 Thermal comfort

Patients feel cold in postoperative period,
sometimes rating it worse than surgical
pain

Shivering occurs in ~40 percent of
unwarmed patients who are recovery
from GA
Summary: Consequences of Hypothermia

Benefits
 Improves neurologic outcomes after cardiac arrest
 Improves neurologic outcomes after neonatal asphyxia

Major complications
 Increases morbid myocardial outcomes
 Promotes bleeding and increases transfusion requirement
 Increases risk of wound infections and prolonges
hospitalization

Other complications
 Decreased drug metabolism
 Prolonged recovery duration
 Thermal discomfort
Treating and Preventing
Intraoperative Hypothermia
Preventing redistribution hypothermia

The initial reduction in core temperature
is difficult to treat because it result from
redistribution of heat


Prevent by skin-surface warming
→
→
Peripheral heat content ↑
Temperature gradient ↓
Redistribution of heat ↓
Prewarming Prevents Hypothermia
38
37
TM
36
(°C)
35
No Warming
Pre-Warming
34
-60
0
Time (min)
60
Airway heating and humidification

Less than 10% of metabolic heat is lost
through respiratory route

Passive or active airway heating and
humidification contribute little to thermal
management
•

Fluid Warming
Cooling by intravenous fluids
 0.25°C per liter crystalloid at ambient temperature
 0.25°C per unit of blood from refrigerator

Fluid warming does not prevent hypothermia!
 Most core cooling from redistribution
 80% of heat loss is from anterior skin surface

Cooling prevented by warming solutions
 Type of warmer usually unimportant
 Cutaneous Warming

The skin is the predominant source of
heat loss during surgery, mostly by
radiation and convection

Evaporation from large surgical incisions
may be important

An ambient temp. above 25℃ is
frequently required, but this is
uncomfortable for gowned surgeons

Heat loss can be reduced by covering the
skin( with surgical draps, blankets, or plastic
bags……)

Insulator

Forced-air warming

Typically, forced-air warming alone or combined
with fluid warming is required to maintain normal
intraoperative core temp.
Insulating Covers
120
100
Heat
Loss
(W)
Plastic
Cotton
Cloth
80
Paper
60
-20
-0
20
40
Time (min)
60
ThermalDrape
More Layers Do Not Help Much
100
80
1 Unwarmed
60
1 Warm
3 Unwarmed
Heat
Loss 40
(W)
3 Warm
20
0
-20
0
20
Time (min)
40
60
Forced-Air vs. Circulating-Water
38
Forced-Air
36
3.5°C
Temp
(°C) 34
Circulating-Water
0
4
8
Time (h)
12
Over-body Resistive Warming
Negishi, A&A 2003
Röder, BJA 2011
The Relative Effects of Warming Methods on Mean
Body Temperature.
Conclusions

Temperatures throughout the body are
integrated by a thermoregulatory system

General anesthesia produces marked,
dose-dependent inhibition of
thermoregulation to increase the
interthreshold range by roughly 20-fold

Regional anesthesia produces both
peripheral and central inhibition

The combination of anesthetic-induced
thermoregulatory impairment and exposure to
cold operating rooms makes most surgical
patients hypothermic

The hypothermia initially results from a
redistribution of body heat and then from an
excess of heat loss

Perioperative hypothermia is associated with
adverse outcomes, including cardiac events,
coagulopathy, wound infections……

Unless hypothermia is specially indicated, the
intraoperative core temperature should be above
36 ℃