Effect of facial cooling during heat acclimation process on adaptive changes in sweating activity

On assumptions that tympanic temperature (Tty) reflects brain temperature and that the latter can be lowered by cooling of the face, effect of facial cooling during acclimation process on adaptive changes in sweating activity was examined, in comparison with the results of our previous studies on heat acclimation with controlled hyperthermia. Face fanning, by which Tty was clamped at approximately 37.1 degrees C, was combined with either of the following 9-day acclimation procedures: 90-min heating in a "Sauna box," keeping mean skin temperature slightly above 40 C, or 90-min exercise on a bicycle, clamping rectal temperature (Tre) at approximately 38 degrees C. Each procedure was imposed on the same four male subjects on different occasions, two of whom had participated in our previous experiments. Sweat tests, carried out before and immediately after the completion of the procedure, consisted of measurements of local sweat rates, whole body sweat rate, Tre, Tty, and skin temperatures on 5 areas, and of calculations of mean body temperature (Tb) and the rate of sweat expulsions (Fsw, as an indicator of central sudomotor activity). No or only a slight increase in sweating activity was observed following the acclimation procedures with face fanning, whereas similar procedures without face fanning had resulted in substantial enhancement of sweating activity in most of the cases, which had been attributed mainly to adaptive changes in central sudomotor activity (as indicated by a shift of the regression line relating Fsw to Tb). Similar results were obtained in an additional series of experiments, where the effects of 9-day 90-min exercise in heat, clamping Tre at approximately 38.2 degrees C, with and without facial cooling, were compared with each other in a subject. From the above results it is inferred that Tty reflects brain temperature and that enhancement of sweating activity induced by repeated heat load is strongly impeded, if not accompanied, by an elevation of brain temperature.