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

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

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

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

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 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

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 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

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

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

Thermoregulation during general anesthesia

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

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

Patterns of intraoperative hypothermia 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 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

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

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

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

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

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 core-temperature equilibrium because peripheral vasoconstriction remains impaired Hypothermia tends to progress throughout surgery

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

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

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

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

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

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

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 ↓

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

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

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.

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 produce 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 ℃