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

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 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 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

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 Phase II : Slow linear reduction Phase III: Thermal plateau

Patterns of intraoperative hypothermia 1. Initial rapid decrease  heat redistribution  decreases ℃ during 1 st 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  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 Control Epidural

 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 core- temperature 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 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

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

 Wound infection---the most common serious complication due to  Impaired immune function  decreased cutaneous blood flow  protein wasting  decreased synthesis of collagen Complications of Mild Hypothermia

Wound Infections: Melling, et al. 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

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

 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

More Layers Do Not Help Much

Forced-Air vs. Circulating-Water

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 ℃