Abstract
Profound hypothermia involves the cooling of a patient from 37°C to 13-15°C, using the patient's own blood as a secondary heat transfer medium. The problem presented is one of heat transfer in viscous flow, demanding apparatus of absolute reliability and ensuring thermal and hydrodynamic conditions which would in no way damage blood cells. Apparatus must be easily sterilized and all blood contact surfaces readily examined. The paper presents a solution using an annular heat exchanger design capable of cooling a patient weighing 10 stone in a period of 30 minutes, the same heat exchanger being suitable for body weights of between 8 and 186 lb. The heat transfer theory presented compares published work in this field, in particular the Graetz result for viscous flow. In the paper it is suggested that measurement of local heat transfer coefficients under similar conditions of both heat flux and velocity gradients gives a better approximation for design than lengthy arithmetic analysis of theoretical equations. Blood is a complex non-Newtonian fluid and consideration is given to viscosity variations with both temperature and rate of shear. Finally, there is a description of automatic controls installed with the heat exchanger unit now operating at the Westminster Hospital.
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