ASA Monitor 2 2019, Vol.83, 44-45.
What is the most significant type of heat loss encountered by the exposed surgical patient prior to surgery?
- (A) Conductive
- (B) Radiative
- (C) Convective
- (D) Evaporative
Mechanisms of heat transfer (Figure 1) should be considered when trying to prevent or reverse heat loss.
Radiative heat loss is the major type of heat transfer affecting the exposed patient in the operating room (O.R.). The walls, ceiling and equipment in the O.R. absorb heat from the patient’s body in proportion to the fourth power of the temperature difference between the two surfaces.
Convective heat loss occurs when air moves over the body, transferring heat to molecules that quickly move away. This is the second-largest mechanism of heat transfer encountered by the exposed surgical patient in the O.R. Covering the patient with a blanket prior to surgical draping, and the surgical drapes themselves, significantly reduces heat transfer due to convection.
Conductive heat loss occurs between two touching surfaces of different temperatures, such as the patient’s skin and the O.R. table. The use of foam pads, blankets and other insulating materials on the O.R. table protects the patient from significant heat loss through conduction. Conductive heat loss does not contribute greatly to overall heat loss in exposed surgical patients prior to surgery.
Evaporative heat loss may be produced by evaporation of patient sweat, evaporation of the surgical preparation solution, evaporation from exposed tissues during surgery and through the respiratory system. Evaporative heat loss does not contribute significantly to the heat loss experienced by the exposed patient prior to surgery.
Covering the patient upon entry to the O.R. and coverage with the surgical drapes helps to prevent radiative loss. The use of forced air warming has been shown to decrease heat loss through convective and conductive heat transfer. Warming of intraoperative fluids prevents declines in body temperature and may be helpful in increasing body temperature. Prevention of evaporative heat loss through humidification and warming of the respiratory gas contributes little to warming the patient. Passive warming using a reflective heating blanket is ineffective.
Miller RD , ed. Miller’s Anesthesia. 8th ed. Philadelphia, PA: Elsevier Saunders; 2015:1627-1628.
Hernandez M, Cutter TW, Apfelbaum JL . Hypothermia and hyperthermia in the ambulatory surgical patient. Clin Plast Surg. 2013;40(3):429-438.