Physical principles of heat transfer

Body heat content determines body temperature. To maintain a constant body temperature the rate of metabolic heat production must equal the rate of environmental heat loss, 85% of which is from the skin. There are four mechanisms of heat loss: conduction, convection, radiation and evaporation. Clinically, conduction can be ignored. The heat transfer coefficient, h, for each mechanism defines the magnitude of heat transfer: convection, hc=8.2v0.5W/m2/°C, where v=air velocity in metres/second (m/s); radiation, hR=6.4 W/m2/°C; evaporation, hE=134v0.5W/m2/k P a. An exposed, dry surface loses heat by radiation and convection, and the combined radiation and convection heat transfer coefficient, hRC, is 10 W/m2/°C, at v=0.2 m/s. Insulation is the reciprocal of the heat transfer coefficient, and the SI units are m2/°C/W. An alternative insulation unit is the ‘clo’ (1 clo=0.155 m2/°C/W). There are three levels of insulation: (1) tissues (ITISS); (2) air (IAIR); and (3) material (IMAT), which add in series so that total insulation, ITOT=ITISS+IAIR+IMAT. The insulation required of materials to prevent heat loss in the operating room (OR) is approximately 1.13 clo, but no materials regularly found in an OR provide this. Materials designed to trap air and exploit its high insulation quality can provide adequate insulation. Adding layers increases insulation, but the effectiveness of layering is proportional to the intrinsic insulation of a single layer of the material.