Hypothermia is defined as a patient with a core temperature below 36.0°C. The difference between a positive patient outcome and a complicated recovery can be a matter of degrees. Approximately 90% of heat loss is through the skin surface, with convection and radiation usually contributing more to the process than evaporation or conduction.
Heat from the patient is transferred to the environment in four ways: conduction, radiation, convection, and evaporation. Conduction is the transfer of the patients heat through physical contact with other objects, such as a bed or theatre table. Radiation (radiated heat) is the heat generated and emitted due to the patients metabolism. Convection is the air that moves around the patient taking the heat away. Evaporation is an effective means of removing heat from a patients body, and occurs constantly as the patient breathes.
Unintended perioperative hypothermia is a frequent, yet preventable, complication of surgery. It can increase the rate of surgical site infection (SSI), extend recovery time and length of stay, and increase mortality rates. However, hypothermia can be easily prevented when temperature is monitored and an active warming measure is instituted throughout the perioperative process — starting before the induction of anaesthesia.
Maintaining normothermia can help avoid the potential cascade of negative outcomes associated with unintended hypothermia, including:
- Increased risk of SSIs
- Blood loss and transfusion requirements
- Adverse cardiac events
- Decreased drug metabolism
- Shivering and thermal discomfort
Hypothermia in trauma victims is a frequently observed phenomenon in acute care. Known complications of hypothermia are impaired wound healing, cardiac complications, hemodynamic instability, impaired immune function and increased blood loss.
Ways to Maintain Normothermia in Patients
Forced air warming - Air is sucked in from the surroundings and warmed using electric coils. The blower circulates the warm air through a blanket that warms the patient through convection.
Resistive heating - Conductive polymer fibre sheet that produces heat and warms the patient through conduction.
Warm Blankets - Cotton Blankets warmed in a temperature-controlled incubator.
Circulating water garment - Heat pump circulates warm water through a patient worn garment.
Water mattresses - Thermostatically controlled water-filled mattress that warms patient through conduction.
Heated gel mattress - Thermostatically controlled gel-based mattress that warms patient through conduction.
Electric blankets - Blanket with inbuilt heating device.
Radiant heaters - Electric heater that employs infrared radiation.
Exothermic pads - Releases warmth through exothermic heat released when pads are exposed to air.
Heated humidifiers - Warms and moistens air that passes over a heated water reservoir.
The most common patient warming device in use in the NHS is forced air warming, such as the Bair Hugger. The Bair Hugger Normothermia System is a forced-air warming, or FAW, system designed to prevent hypothermia and keep patients at a normal core body temperature during all three phases of orthopedic surgery; pre-operative, intra-operative and post-operative. The warming blankets stabilise core temperature ahead of surgery, then maintain it during the procedure and immediately afterward as the patient recovers.
Forced-air warming systems filter air and then force the warm air through a disposable blanket, which cover patients before, during and after surgery. The blankets are designed to use pressure points on the patient’s body to prevent heat from reaching areas at risk for pressure sores or burns.
Up to 64 percent of the patient's body surface may be recruited for heat transfer, depending on which Bair Hugger blanket is used. The blankets also include drain holes where fluid passes through the surface of the blanket to linen underneath, which is supposed to reduce the risk of skin softening and reduce the risk of unintended cooling because of heat loss from evaporation. The blankets are disposable to reduce the chance of infection transmission from one patient to another. They are not designed to enter sterile fields during surgery.
Inadvertent perioperative hypothermia is a common but preventable complication of perioperative procedures, which is associated with poor outcomes for patients. Inadvertent perioperative hypothermia should be distinguished from the deliberate induction of hypothermia for medical reasons.