Introduction
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In homeothermic species, a thermoregulatory system coordinates defenses against environmental temperature to maintain internal body temperature within a narrow range, thereby optimizing normal body function•
The primary thermoregulatory control center in mammals is the hypothalamus•
Anesthetic induced thermoregulatory impairment and exposure to a cool environment makes the majority of unwarmed surgical patients hypothermic•
Core hypothermia results from redistribution of body heat from core to periphery•
Shivering is an important complication of hypothermia•
Although it occurs frequently, i.e. after 40-60% of volatile anesthetics, it remains poorly understood•
Shivering is present in about 50% of patients with a core temperature of 35.5 degrees C and 90% of patients with a core temperature of 34.5 degrees C•
Heat loss is normally regulated by cutaneous vasodilation or vasoconstriction, sweating, and shivering•
Shivering is a “last resort” defense that is activated only when behavioral compensations and maximal arterio-venous shunt vasoconstriction are insufficient to maintain core temperature•
It is an involuntary, oscillatory muscular activity that augment metabolic heat production up to 600% above basal levelRisk Factors
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Incidence appears to be related to duration of surgery and the use of high concentrations of volatile agent•
Three major risk factors for shivering have been identified: young age, endoprosthetic surgery, core hypothermia•
Age appears to be the most important risk factorConsequences of Shivering
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Can increase oxygen consumption and CO2 production by up to 200%•
Has been linked with an increase in adverse myocardial outcomes in high risk patients•
Can be intense enough to cause hyperthermia (38-39 C) and a significant metabolic acidosis•
Increases intraocular and intracranial pressures•
Increases the risk of incidental trauma, disrupts medical devices, and interferes with ECG and pulse oximetry monitoringPrevention and Treatment of Postoperative Shivering
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Shivering should be treated by warming the patient and then administering medication to inhibit it•
Treatment providers should not restrict themselves to combating shivering solely with drugs, as heat recovery will still be slower and the patient will be deprived of an important defense mechanism against core temperature loss•
Studies suggest intraoperative warming is faster than comparable postoperative warming•
Cutaneous heat loss can be decreased by covering the skin; single layer of an insulator reduces heat loss by 30% (adding additional layers does not proportionately increase the benefit)•
Forced air warming is the most effective available method•
Pharmacologic interventions:•
opioids (meperidine 25mg, alfentanil 250mcg, fentanyl, morphine)•
other centrally acting analgesics (tramadol, nefopam, metamizol)•
clonidine 150mcg (alpha 2-agonist)•
methylphenidate•
doxapram•
ketanserin 10mg (5HT-3 antagonist)•
magnesium sulfate•
Physostigmine (cholinesterase inhibitor)•
Ketamine (NMDA antagonist)
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