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What mechanisms lead to hypothermia in a patient under general anesthesia in the operating room?
In the first phase, the main mechanism is redistribu-tion hypothermia. The core temperature decreases by 0.5–1.5°C during the first hour after induction of general anesthesia due to redistribution from the core to the periphery.
In the second phase, which lasts for 2–3 hours, heat loss exceeds metabolic heat production, which is decreased by about 20% after induction of general anesthesia.
Heat loss is multifactorial: 60% by radiation, 20% by evaporation (humidification of inspired gases and fluid loss from the surgical field, especially if the abdomen or thorax is opened), 5% by conduction (contact with the operating room table), and 15% by convection (warming of air flowing over the skin).
The core temperature will plateau after 3–4 hours because of peripheral vasoconstriction triggered by a core temperature of 33–35°C, while the peripheral temperature will continue to drop.
In other settings, hypothermia can be the result of loss of central thermoregulation (e.g., stroke, head trauma, or spinal cord injury), drug-related (e.g., alcohol or barbi-turate overdose), or secondary to metabolic changes (e.g., hypoglycemia, hypothyroidism, sepsis, burns, or hepatic failure).
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