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Unintentional postoperative hypothermia is common.
Anesthesia impairs normal thermoregulatory mechanisms and promotes heat loss.
Forced air warming is the most effective intervention to correct postoperative hypothermia.
Human body temperature is tightly regulated to within 0.4 °C. Thermoregulation is controlled through the body’s regulatory action and can be characterized by three general phases: (1) afferent sensory input, (2) central regulation mainly through the hypothalamus, and (3) efferent responses. Aδ and C fibers act as thermal receptors and can be found throughout the body, including the skin, spinal cord, and hypothalamus. Efferent responses to reduction of body temperature include behavior change (seeking warmer environment, donning more clothing), vasoconstriction, non-shivering thermogenesis, and shivering thermogenesis. The administration of anesthetics impairs these efferent responses and promotes the development of unintentional hypothermia. Mild hypothermia may be defined as a body temperature between 33.0 °C and 36.4 °C.
The administration of general anesthetics decreases the threshold for response to hypothermia from 37 °C to between 33 °C and 35 °C. With the induction of anesthesia, the body’s voluntary behavioral defenses are lost, and involuntary autonomic compensatory mechanisms are activated in a predictable pattern as core temperature decreases, beginning with vasoconstriction, then non-shivering thermogenesis, and finally shivering thermogenesis.[1] The initial decrease in core body temperature by 0.5 °C to 1.5 °C after anesthesia-induced vasodilation results from redistribution of heat from central compartment to peripheral tissues down the temperature gradient.[2] Exposure of the patient to a cold operating room environment further contributes to the development of intraoperative hypothermia.
Mechanisms of heat loss include radiation, conduction, convection, and evaporation. Radiation constitutes the largest portion of heat lost and occurs through the emission of infrared rays. Conductive heat loss occurs through direct skin transfer by the loss of kinetic energy, while convective loss occurs through the removal of heat by air currents. Evaporative heat loss occurs through the vaporization of water.[3] About 90% of heat loss can be accounted for by cutaneous heat loss, which is proportional to the exposed body surface area. The remaining 10% of metabolic heat loss is thought to be from respiration when patients are ventilated with dehumidified gas.
Clinical stages of hypothermia
Unintentional perioperative hypothermia occurs when heat loss to the environment exceeds metabolic heat production. Mild hypothermia (between 33.0 °C and 36.4 °C) is associated with mild central nervous system depression. Peripheral vasoconstriction and tachycardia may ensue. Vasoconstriction decreases cutaneous heat loss and conserves heat in the core. Post-anesthetic shivering is another common presentation of hypothermia in the Post-Anesthesia Care Unit (PACU), especially in young patients who have mild hypothermia.[4] Shivering is a potentially serious complication, which can double or even triple oxygen consumption and carbon dioxide production. Shivering results from hypothalamic regulation to increase endogenous heat production during emergence from general anesthesia.[3] In patients with pre-existing cardiopulmonary conditions such as coronary artery disease, low cardiac output, intrapulmonary shunts, or respiratory derangement, increases in metabolic requirement due to shivering may predispose to myocardial ischemia or respiratory failure.[5]
Moderate hypothermia (31 °C to 32.9 °C) results in further depression in the central nervous system and decreased motor activity. Patients may have depressed consciousness and unstable vital signs. Cardiac dysrhythmias may follow. Unintentional perioperative hypothermia is rarely this severe, but when it occurs, surgical patients with moderate hypothermia are generally kept intubated and mechanically ventilated until the body temperature is corrected.
Severe hypothermia (<30.9 °C) is associated with significantly depressed vital signs and cardiac impulse conduction. Ventricular fibrillation and asystole generally occur in core temperature below 23 °C. Respiratory arrest usually follows cardiac arrest. Patients may be comatose and areflexic at this point. If left untreated, such a profound level of hypothermia will soon lead to death.
Adverse effects of hypothermia
Postoperative hypothermia is a common complication after surgery and it prolongs PACU length of stay by an average of 40 to 90 minutes.[6,7] Although mild hypothermia of brief duration is usually well tolerated by patients, it can represent a significant clinical risk if it remains unrecognized and uncorrected. Postoperative hypothermia increases sympathetic nervous system activity. Levels of circulating epinephrine and norepinephrine rise, resulting in tachycardia, hypertension, and systemic vasoconstriction. Imbalance between oxygen demand and supply may lead to a higher incidence of postoperative myocardial ischemia. Unintentional hypothermia on arrival to PACU is associated with a significantly higher incidence of myocardial ischemia, angina, and PaO2 less than, 80 mmHg during the early postoperative period.[8] Tissue hypoperfusion may occur and can lead to metabolic acidemia. This acidemia can worsen tissue hypoperfusion by shifting the oxygen–hemoglobin dissociation curve to the left and, therefore, compromise tissue oxygen unloading.
Coagulopathy may ensue even from mild hypothermia causing platelet sequestration and platelet function impairment. Enzymes of the coagulation cascade are directly attenuated by hypothermia.[9,10,11] Many randomized trials and subsequent meta-analysis report increased blood loss and transfusion requirements in hypothermic patients.[12]
CNS = central nervous system, SVR = systemic vascular resistance, HR = heart rate
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