Perioperative Hypothermia
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Transcript of Perioperative Hypothermia
PerioperativeHypothermia
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 fiberwarm signal-C fiber
Each of the following contribute 20% of the total thermal input: Hypothalamusother parts of brainskin surfacespinal corddeep abdominal and thoracic tissues
Normal thermoregulation
Primary thermoregulatory control centerHypothalamus
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 0.5-1.5 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 thermoregulationThe 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 arteryDistal esophagusNasopharynxTympanic membrane
thermocouple Other generally-reliable sites
MouthAxillaBladder
Sub-optimalForehead 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 documentedProspective randomized trials1-2°C hypothermia
Effects on many different systemsMost 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
BenefitsImproves neurologic outcomes after cardiac arrestImproves neurologic outcomes after neonatal asphyxia
Major complicationsIncreases morbid myocardial outcomesPromotes bleeding and increases transfusion requirementIncreases risk of wound infections and prolonges
hospitalization Other complications
Decreased drug metabolismProlonged recovery durationThermal discomfort
Treating and Preventing Intraoperative HypothermiaPreventing 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 temperature0.25°C per unit of blood from refrigerator
Fluid warming does not prevent hypothermia!Most core cooling from redistribution80% of heat loss is from anterior skin surface
Cooling prevented by warming solutionsType 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 ℃