1. Perioperative Hypothermia
The New England Journal of Medicine June 12, 1997
Daniel I.Sessler, M.D.
Senior clerk: 王桓奇 李兆翔 胡名孝

2. 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 enviroment
Hypothermia complication:
shivering,prolonged drug effect,coagulopathy,
surgical wound infection,morbid cardiac event

3. Normal thermoregulation
Processing of thermoregulatory information:
afferent input
central control
efferent responses
Core temperature measurements
pulmonary artery
tympanic membrane
distal esophagus
nasopharynx

4. Normal thermoregulation
Afferent input: cold signal-Aδ fiber
warm signal-C fiber
Each contribute 20% of the total thermal input:
hypothalamus
other parts of brain
skin surface
spinal cord
deep abdominal and thoracic tissues

5. 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

6. Normal thermoregulation
The interthreshold range(core temperatures not triggering 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

7. 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

8. Normal thermoregulation
Vasoconstriction occurs in AV shunts located primarily in fingers and toes, mediated by α-adrenergic symp. nerve.
Nonshivering thermogenesis is important in infants,but not in adults (brown fat)
Shivering is an involuntary muscle activity that increase metabolic rate 2-3 times

9. Thermoregulation during general anesthesia
General anesthesia removes a p`t 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 dose-dependent manner and inhibit vasoconstriction and shivering about 2-3 times as they restrict sweating
Interthreshold range is increased from 0.2 to 4℃(20 times), so anesthetized p`t are poikilothermic with body temperatures determined by the environment

10. Thermoregulation during general anesthesia
The gain and maximal response intensity of sweating and vasodilation are well preserved when volatile anesthetics is 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

11. Thermoregulation during general anesthesia

12. 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)
Heat transferred from p`t to environment:
radiation > convection >>conduction & evaporation

13. Patterns of intraoperative hypothermia
Phase I:
Initial rapid decrease
Phase II :
Slow linear reduction
Phase III:
Thermal plateau

14. Patterns of intraoperative hypothermia
Initial rapid decrease
heat redistribution
decreases ℃ 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

15. 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

16. Patterns of intraoperative hypothermia
3. Thermal plateau
After 3-5 hrs,core temperature stops decreasing
It may simply reflect a steady state that
heat loss=heat production in well-warmed p`t
If a p`t 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

17. Regional Anesthesia
Regional anesthesia impairs both central and peripheral thermoregulation
Hypothermia is common in patients given spinal or epidural anesthetics

18. Thermoregulation All thermoregulatory responses are neurally mediated
Spinal and epidural anesthetics disrupt nerve conduction to more than half the body
The peripheral inhibition of thermoregulatory defense is a major cause of hypothermia during RA

19. 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
The thermoregulatory system’s incorrect evaluation of skin temperature in the blocked area

20. Undetected hypothermia
The core temperature is rarely monitored by medical personnel during spinal and epidural anesthesia
Patients usually do not feel cold

21. 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

22. Subsequent hypothermia (Phase II)
Loss of heat exceeds production
Patients given SA or EA cannot reestablish core-temperature equilibrium because peripheral vasoconstriction remains impaired
Hypothermia tends to progress throughout surgery

23. Shivering Occurs during spinal and epidural anesthesia
Disturb patients and care givers but produced relatively little heat because it is restricted to the small-muscle mass cephaled to the block
Treated by warming surface of skin or administration of clonidine / meperidine

24. Consequences of Hypothermia
Advantages
Provide substantial protection against cerebral ischemia and hypoxia
Slows the triggering of malignant hyperthermia and reduce its severity
Appear to facilitate recovery and reduce mortality from septic ARDS

25. Disadvantages
Wound infection---the most common serious complication, due to
Impaired immune function
decreased cutaneous blood flow
protein wasting
decreased synthesis of collagen

26. 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
Just 1.5 ℃ of core hypothermia triples the incidence of VT and morbid cardiac events

27. 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

28. 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

29. 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 ↓

30. Airway heating and humidification
Less than 10% of metabolic heat is lost through respiratory
Two thirds of heat in humidifying inspiratory gases
Passive or active airway heating and humidification contribute little to thermal management

31. Intravenous fluids
1L of IV fluids at ambient temperature or 1 unit of refrigerated blood decreases the mean body temperature 0.25 ℃
Heating fluids to near 37 ℃ helps prevent hypothermia and is appropriate if large volumes are being given

32. 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

33. 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.

34. The Relative Effects of Warming Methods on Mean Body Temperature.

35. 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 produce both peripheral and central inhibition

36. 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 ℃