Perioperative Hypothermia

1. PerioperativeHypothermia Karim Rafaat, MD

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 environment • Hypothermia complications: • Shivering • prolonged drug effect, • coagulopathy • surgical wound infection • morbid cardiac event

3. Normal thermoregulation • Processing of thermoregulatory information: afferent input central control efferent responses

4. Normal thermoregulation • Afferent input: • cold signal-Aδfiber • 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

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 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

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

9. 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

10. Thermoregulation during general anesthesia • General anesthesia removes a pt’sability 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 more than they restrict sweating • Inter-threshold range is increased from 0.2 to 4℃ (20 times), so anesthetized ptsare poikilothermic - with body temperatures determined by the environment

11. 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

12. Anesthesia Impairs Regulation

13. 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)

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

15. Patterns of intraoperative hypothermia • Initial rapid decrease • heat redistribution • decreases 0.5-1.5℃ during 1sthr • 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

16. 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

17. 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 ptis sufficiently hypothermic, plateau phase means activation of vasoconstrictionto 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

18. Regional Anesthesia • Regional anesthesia impairs both central and peripheralthermoregulation • Hypothermia is common in patients given spinal or epidural anesthetics

19. Control Epidural Thermoregulation • All thermoregulatory responses are neurallymediated • 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

20. 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

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 • Disturbs patients and care givers but produces relatively little heat because it is restricted to the small-muscle mass cephaladto the block • Treated by warming surface of skin or administration of clonidine / meperidine

24. 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

25. Potential Benefits of Mild Hypothermia • Improves neurologic outcome after cardiac arrest • 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 • Improves neurologic outcome in asphyxiated neonates • 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

26. 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

27. 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

28. Wound Infections: Melling, et al. Normothermia is more effective than antibiotics!

29. 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

30. Blood Loss 20% less blood loss per °C

31. Transfusion Requirement 22% less blood Transfusion per °C

32. Myocardial Outcomes: Frank, et al.

33. 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

34. Duration of Vecuronium

35. Recovery Duration Time (min)

36. 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

37. 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

38. 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 ↓

39. Prewarming Prevents Hypothermia

40. 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

41. 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 doesnot 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

42. 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

43. 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.

44. Insulating Covers

45. More Layers Do Not Help Much

46. Forced-Air vs. Circulating-Water

47. Over-body Resistive Warming Negishi, A&A 2003 Röder, BJA 2011

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

49. 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

50. 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 ℃