Application of heat flux transducers to determine perioperative heat exchange

Abstract

Perioperative hypothermia is a common problem in anaesthesia. To maintain a constant body heat content it is necessary to achieve a steady state of metabolic heat production and external heat exchange. During anaesthesia and surgery this balance is disturbed. The heat production is decreased to a level below resting metabolic heat production while heat losses increase due to the surgical procedure. As a consequence, heat losses exceed metabolic heat production and patients get hypothermic. Even during large abdominal operations the major source of heat loss (about 85%) is the radiative and convective heat loss from the skin. Because during anaesthesia heat production cannot participate in regulation, heat loss is of major importance. The four mechanisms of heat exchange are convection, conduction, radiation and evaporation. While evaporation is proportional to the difference of partial pressure of water vapour, the heat exchange by convection, conduction and radiation can be described as follows: Q ˙ A −1 = h( T Skin − T Env); Q ˙ : heat flux (W), A: area (m 2); h: heat exchange coefficient (W m −2 °C −1); T Skin: skin temperature (°C); T Env: environmental temperature (°C). The driving force of heat exchange is the temperature difference. The heat exchange coefficient describes the efficacy of heat exchange for a given temperature difference. For operating room conditions the heat exchange coefficient for radiation and convection can be combined ( h RC). To determine the heat exchange coefficient for a given heat exchange mechanism the temperature gradient (Δ T) between environment and skin has to be varied and the resulting heat flux per unit area (W m −2) measured by heat flux transducers. The heat exchange coefficient can be determined by linear regression analysis as the slope of the heat flux per unit area versus the temperature gradient. Several measures have been established to prevent perioperative hypothermia. Insulation reduces heat loss from skin about 30–80% depending on the material. Insulation reduces the h RC and is described as 1/ h RC. Heat transfer from the environment to the body can be achieved by active warming as forced air warming, conductive warming or radiative warming devices. The related heat flux is influenced by the h of each system, the temperature gradient between the skin and the blanket and the treated area. If the heat exchange coefficient is known, the heat exchange can be predicted by the measurement of the temperature gradient. This provides an estimation of intraoperative heat exchange, where heat flux measurement is difficult or impossible.