CONSEQUENCES OF MILD INTRAOPERATIVE HYPOTHERMIA

Perianesthetic hypothermia produces potentially severe complications as well as distinct benefits. Thermal management thus deserves the same thoughtful analysis of potential risks and benefits as other therapeutic decisions do.

Benefits

Substantial protection against cerebral ischemia and hypoxia is provided by just 1°C to 3°C hypothermia in animals.^{[90] [91] [92]} Similar benefits have been demonstrated for acute myocardial infarction in human-sized pigs. ^[93] Mild hypothermia reduces intracranial pressure, but randomized studies have yet to demonstrate that therapeutic hypothermia improves outcomes in patients with brain trauma, stroke, or subarachnoid hemorrhage.

Hypothermia for brain trauma was initially claimed to be therapeutic based on a post hoc subgroup analysis of a study that overall showed no benefit. ^[94] A subsequent large randomized trial was unable to demonstrate any benefit overall or in subgroups—although the study was somewhat limited by serious protocol violations in some patients.^[95] Most recently, hypothermia was shown to be beneficial in a nonrandomized but nonetheless fairly convincing study: brain trauma patients with elevated intracranial pressure refractory to conventional treatment were assigned to therapeutic hypothermia. Despite being sicker than the comparison patients (who had low intracranial pressure), the hypothermic patients demonstrated improved outcomes.^[96] The only situation in which therapeutic hypothermia has been unequivocally shown in randomized trials to improve outcome is during recovery from cardiac arrest.^{[97] [98]}

Protection was initially thought to result from the approximately 8%/°C linear reduction in tissue metabolic rate. However, the efficacy of mild hypothermia far exceeds that of treatments such as high-dose isoflurane or barbiturate coma, which comparably reduce the metabolic rate.^[99] These data suggest that other factors (e.g., decreased release of excitatory amino acids) explain the protective action of hypothermia. As such, there is no reason to expect protection to decrease linearly with temperature, and in animals it appears that much of the total benefit from moderate hypothermia occurs within the first couple of degrees.

The potential protection afforded by mild hypothermia is so great that reduced core temperature (i.e., ≅34°C) is increasingly being used during neurosurgery and other procedures in which tissue ischemia can be anticipated. Mild hypothermia may also be useful for the treatment of acute myocardial infarction. ^[93] The difficulty is that currently, few outcome data in humans have substantiated this extrapolation from animal data. Furthermore, the appropriate target temperature for therapeutic hypothermia has yet to be established in humans, and there are currently little data on which to base such decisions.

Acute malignant hyperthermia is more difficult to trigger in mildly hypothermic swine than in those kept normothermic. Moreover, once triggered, the syndrome is less severe.^{[100] [101]} These data suggest that active warming should be avoided in patients known to be susceptible to malignant hyperthermia; instead, they should be allowed to become slightly hypothermic during surgery.