Basic, Clinical Sciences: Anesthesia Procedures, Methods, and Techniques
Hypothermia during general anesthesia is the most common preoperative thermal disturbance. It can be caused by a variety of factors. Besides a cold operating room, impaired thermoregulation tends to be the biggest cause. Heat transfer from a patient to the environment can be carried out by 4 ways. These include (1) radiation, (2) conduction, (3) convection, and (4) evaporation. Out of these four, radiation and convection contribute the most to preoperative heat loss. Hypothermia during general anesthesia is typically characterized by an initial rapid decrease in core temperature followed by a slow linear reduction.
What does this mean for the patient? There are adverse effects as well as potential benefits. Benefits of mild hypothermia may include protection against cerebral ischemia and hypoxia. This can be provided by just 1oC to 3oC; however, no target temperature has been clearly established for potential neuro-protection with ischemia. Overall, hypothermia can reduce a patients metabolic rate by approximately 8%/oC to approximately half the normal at 28oC. Also, hypothermia has been shown to be of possible benefit to heart muscle. Studies have been able to reduce infarct size after myocardial infarction in animal models, but this has yet to translate to humans. Other potential benefits of the hypothermia for the heart include, decrease in heart rate, increased contractility and maintained stroke volume.
Unfortunately, hypothermia has many more adverse effects. Coagulation is impaired even by mild hypothermia with the most important factor being cold-induced platelet dysfunction. Drug metabolism is also markedly reduced during preoperative hypothermia. Common drugs affected include non-depolarizing muscle relaxants and propofol. Hypothermia can also alter the pharmacodynamics of volatile anesthetics leading to a reduced minimum alveolar concentration (MAC) estimated at 5% per degree Celsius in change. Wound infections can lead to serious complications with anesthesia and surgery. Hypothermia contributes to this by both directly impairing healing and indirectly by triggering thermoregulatory vasoconstriction that can impair wound oxygen delivery. Patient distress can also be associated with preoperative hypothermia. This has been termed Thermal Discomfort, which can lead to elevated blood pressure, heart rate and plasma catecholamine concentrations. These factors have been shown to give a threefold increase in morbid myocardial outcomes. Furthermore, post anesthetic shivering-like tremor has been estimated to occur in 40% of patients. This can lead to profound increases oxygen consumption. It can also increase intracranial and intraocular pressure with uncontrolled shivering. Finally, hypothermia can decrease blood flow to the kidney’s by increasing renovascular resistance.