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OUTCOMES RESEARCH
Providing the evidence for evidence-based medicine©
Thermoregulation & Heat Balance
Thermoregulation During General Anesthesia
Temperature Monitoring
Consequences of Hypothermia
Maintaining Normothermia
www.or.org
Normal Thermoregulation
37
36
Sweating
Active vasodilation
Vasoconstriction
Shivering
Anesthesia Impairs Regulation
0.0 2.0 4.0 6.0 8.0
[P ro p o f o l] (µ g/m l )
0.0 0.1 0.2 0.3
[A l fe n tan i l ] (µ g/m l )
30
32
34
36
38
0.0 2.0 4.0 6.0
[D e s f l u ran e ] (% )
C o n s tr ic ti o n
S h i v eri n g
T h re s h o l d
(°C )
S w eati n g
32
34
38
36
30
[D e x m ed e to m i d i n e ] (n g / m l)0.3 0.6 0.9
Hypothermia During Anesthesia
0 2 4 6
∆ C o r e
T e m p
( ° C )
E l a p s e d T i m e ( h )
- 1
- 3
- 2
0
Redistribution Hypothermia
Core37°C
Vasoconstricted
Periphery31-35°C
Anesthesia
Periphery33-35°C
Core36°C
Vasodilated
Redistribution Hypothermia
Intraoperative Heat Transfer
Evaporation
Conduction
Convection
Radiation
Vasoconstriction Constrains Heat
Summary: General Anesthesia
Central thermoregulatory inhibition•Dose-dependent increase in interthreshold range•More impairment of cold than warm responses
Intraoperative hypothermia•Redistribution of heat (initial decrease)•loss exceeding heat production (slow linear decrease)
Core Temperature Plateau•Decreased cutaneous heat loss; thermal steady-state•Separation of core and peripheral compartments
Temperature MonitoringCore Sites
•Pulmonary artery•Distal esophagus•Nasopharynx•Tympanic membrane thermocouple
Other generally-reliable sites•Mouth•Axilla•Bladder•Forehead skin with 2°C compensation
Sub-optimal•Infrared “tympanic”•Rectal
Benefits of Mild HypothermiaCerebral ischemia and hypoxemia
•1-3°C provides marked protection in animals
No benefit in major human trials•Brain trauma: Clifton, et al.•Anurysm surgery: Todd, et al.•Acute myocardial infarction: Dixon, et al.
Improves neurologic outcome after cardiac arrest•Bernard, et al.•Hypothermia after cardiac arrest study group
Improves neurologic outcome in asphyxiated neonates•Shankaren, et al.
Malignant hyperthermia•2-3°C delays or prevents triggering and decreases severity
Myocardial Outcomes: Frank, et al.
0Hypothermic
35.3 ± 0.1
P = 0.04
Normothermic36.7 ± 0.1
2
4
6
8
10
Core Temperature (°C)
Perioperative Blood Loss
16% less blood loss (95% CI 4-28%, P = 0.009)
Wound Infections: Kurz, et al.
Normothermia is more effective than antibiotics!
0
5
10
15
20
25
0
5
10
15
20
25
Core Temperature (°C)
Hypothermic34.7 ± 0.6
Normothermic36.6 ± 0.5
Hypothermic34.7 ± 0.6
Normothermic36.6 ± 0.5
P < 0.01P = 0.001
Duration of Vecuronium
6 0
2 0
4 0
8 0
D u r a t i o n
o f A c t i o n
( m i n )
H y p o t h e r m i c
( 3 4 . 6 ± 0 . 3 ° C )
N o r m o t h e r m i c
( 3 6 . 6 ± 0 . 1 ° C )
Recovery Duration
0
20
40
60
80
100
40 80 120 160
T im e
(m in )
F i t F o r D i s c h a rg e
F i t F o r D i s c h a r g e
&
≥ 36 ° CT c
h y p o t h e r -
m i c
n o r m o t h e r -
m i c n o rm o t h e r -
m i c
h y p o t h e r -
m i c
D is ch arg e
F ro m P A C U
(% )
40 80 120 160
T im e
(m in )
Thermal Discomfort
T h e r m a l
C o m f o r t
( m m )
0
2 0
4 0
6 0
0 1 2 3 4 5 6
T i m e ( h )
N o r m o t h e r m i c
H y p o t h e r m i c
Summary: Consequences
Benefits•Protects against cerebral ischemia (cardiac arrest)•Decreases triggering and severity of MH
Major complications•Morbid myocardial outcomes•Bleeding and increased transfusion requirement•Wound infections and prolonged hospitalization
Other complications•Decreased drug metabolism•Prolonged recovery duration•Thermal discomfort
Insulating Covers
P l a s t i c
P a p e r
C o t t o n
C l o t h
6 0
8 0
1 0 0
1 2 0
H e a t
L o s s
( W )
- 2 0 - 0 2 0 4 0 6 0
T i m e ( m i n )
Thermal-Drape
More Layers Do Not Help Much
T i m e ( m i n )
0
2 0
4 0
6 0
8 0
1 0 0
- 2 0 0 2 0 4 0 6 0
H e a t
L o s s
( W )
1 W a r m
1 U n w a r m e d
3 W a r m
3 U n w a r m e d
Active Warming
-3
-2
-1
0
0 60 120 180
Time (min)
Water Blanket
Air Warmer
Control
Humidifier
Δ T (° )C
Prewarming Prevents Hypothermia
T M
( ° C )
3 4
3 5
3 6
3 7
3 8
T i m e ( m i n )
- 6 0 0 6 0
N o W a r m i n g
P r e - W a r m i n g
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•90% of heat loss is from anterior skin surface
Cooling prevented by warming solutions•Type of warmer usually unimportant•Use high-flow systems for major trauma
The Rule: Monitor and Warm
Monitor core temperature•General anesthesia >30 minutes•Large procedures under neuraxial anesthesia
Maintain normothermia•Core temperature ≥36°C
Forced-air heating•Best combination of efficacy, cost, and safety
OUTCOMES RESEARCH
Providing the evidence for evidence-based medicine©
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