[Intraoperative heat conservation. A lot of hot air?]

Thermoregulation and its impairment by anaesthesia and surgery has recently been brought back into focus by researchers and clinicians. All volatile and IV anaesthetics, opioids, as well as spinal and epidural anaesthesia increase the inter-threshold range of thermoregulation from 0.2 degree C to 4 degrees C between vasodilation and vasoconstriction. Thermoregulatory vasoconstriction and shivering occurs in anaesthetized patients at lower core temperatures than in awake subjects. Following induction of general or spinal/epidural anaesthesia, core temperature decreases significantly due to internal redistribution of body heat from the core thermal compartment to peripheral tissues. About 1 h after induction of general anaesthesia and initial redistribution hypothermia, a real reduction in body heat occurs as heat loss exceeds metabolic heat production. Heat loss is further increased due to low operating room temperatures, evaporation from open body cavities, and cold IV fluids. Peripheral thermoregulatory vasoconstriction is triggered by core temperatures between 33 degrees C and 35 degrees C, and is able to slow heat loss. However, body heat content continues to decrease even though core temperatures remain nearly constant. During spinal or epidural anaesthesia thermoregulation remains intact in the unblocked body segments, leading to reduced real heat loss when compared to general anaesthesia. Inadvertent hypothermia markedly decreases drug metabolism. Coagulation is impaired by cold-induced defects of platelet function. Hypothermia reduces neutrophil phagocytosis and oxidative killing capacity, causing wound infections. Postoperative hypothermia represents an unnecessary stress for the circulatory system, elevating plasma catecholamines and leading to myocardial ischaemia and arrhythmias. These hypothermia-related morbidities therefore have consequences reaching fare into the postoperative period. Prevention of inadvertent hypothermia is always indicated. Forced-air warming is the most effective and safest method to prevent perioperative hypothermia.