Intraoperative hypothermia is a common physiological event where a patient’s core body temperature drops below the normal range during surgery involving anesthesia. This temperature decrease occurs because anesthetic medications interfere with the body’s natural ability to regulate heat. The surgical team must actively manage this disruption to the body’s stable internal temperature (normothermia).
What Defines Hypothermia in Surgery
Intraoperative hypothermia is defined as a core body temperature falling below 36 degrees Celsius (96.8 degrees Fahrenheit). This threshold marks the point where adverse physiological effects become significant. Severity is categorized further: mild hypothermia falls between 34 and 35.9 degrees Celsius, while moderate hypothermia is between 32 and 33.9 degrees Celsius. Core temperature must be accurately monitored using specialized probes placed in sites that reflect true internal heat, such as the distal esophagus, nasopharynx, or urinary bladder. These devices provide continuous electronic readings, which are more reliable than standard skin or oral measurements.
Key Factors That Cause Temperature Loss
The primary cause of temperature loss is the effect of general anesthesia, which impairs the central thermoregulatory control in the hypothalamus. Anesthesia resets the body’s internal thermostat, causing vasodilation (the widening of peripheral blood vessels). This vasodilation allows warm blood from the core to rush to the cooler skin surface, where heat is rapidly lost. This initial, rapid drop of about 1 to 1.5 degrees Celsius in the first hour is known as redistribution hypothermia.
Continuous heat loss also occurs through environmental factors and the physics of heat transfer.
- Radiation (transfer of heat to cooler objects not in direct contact) accounts for the majority of heat loss, often up to 60 percent.
- Convection loss happens when cool air, such as the laminar air flow in the operating room, moves across the exposed skin surface.
- Evaporation occurs during surgical procedures as fluids from open body cavities or large skin preparation solutions vaporize.
- The administration of room-temperature intravenous fluids and blood products introduces cold content directly into the patient’s circulation, compounding the heat deficit.
How Hypothermia Impacts Surgical Recovery
Uncontrolled intraoperative hypothermia complicates recovery by disrupting multiple biological systems. A lowered body temperature impairs the coagulation cascade, increasing the risk of bleeding and potentially requiring more blood transfusions. This is partly due to the reduced efficiency of platelets and clotting enzymes at sub-optimal temperatures. Hypothermia also triggers peripheral vasoconstriction, which reduces blood flow and oxygen delivery to the tissues. This localized oxygen deprivation can impair immune function and wound healing, increasing the risk of surgical site infection.
The cardiovascular system is also stressed, potentially increasing the risk of arrhythmias and other cardiac events as the body attempts to compensate for the cold. In the post-anesthesia care unit, hypothermia often results in shivering, which increases the body’s metabolic rate and oxygen consumption. Furthermore, a lower core temperature slows the liver’s metabolism of anesthetic medications and muscle relaxants. This delayed drug clearance can prolong the patient’s time in the recovery room and extend the hospital stay.
Techniques Used to Maintain Core Temperature
The surgical team employs a multimodal approach using both passive and active methods to prevent and treat heat loss. Passive warming involves using simple insulating materials, such as cotton blankets or specialized surgical drapes, to minimize heat loss from the skin surface. These materials trap the heat the body naturally produces, reducing loss primarily through radiation and convection.
Active warming techniques introduce heat directly to the patient or their environment. Forced-air warming systems (e.g., “Bair Hugger”) are effective methods, blowing temperature-controlled warm air over the patient’s exposed skin through a specialized blanket. Other active methods include conductive warming mattresses placed beneath the patient on the operating table. Additionally, intravenous fluids, irrigation solutions, and blood products are administered through specialized warmers to prevent the infusion of cold substances that could rapidly lower the core temperature.