Effects of selective brain cooling on mechanisms of respiratory heat loss.

Experiments (n = 36) in three conscious goats were performed at 35 degrees C air temperature and low (LH) or high (HH) humidity. Prior to the experiments the animals received carotid loops and an arteriovenous shunt, which made it possible to increase the temperature of the blood flowing to head and trunk (series A), or to increase the temperature of the trunk at constant carotid blood and hypothalamic temperature (Thyp), respectively (series B). Owing to the smaller cooling power of the inspired air in HH, the slope of respiratory evaporative heat loss versus aorta blood temperature (Taor) was reduced in series A and B. In series A the slopes of respiratory minute volume (VE) and respiratory frequency (RF) versus Taor were larger in HH than in LH. The effects were caused by a reduction of selective brain cooling in HH, which resulted in higher levels of Thyp. This is concluded from the results of series B, in which Thyp was equal in LH and HH, and the slopes of VE and RF over Taor showed no differences. Thus, selective brain cooling contributes to counteract the deterioration of the gain of the respiratory heat loss mechanism, which occurs during exposure to humid air.