Effects of Tracheal Bypass Breathing on Heat Balance in Rabbits

Abstract

The effects of obstruction of the air passage through the nose on hypothalamic temperature (T_hy) and heat balance were studied in 37 conscious rabbits using a direct calorimeter.The obstruction of the air passage through the nose was made by forcing the animal to breathe through a tracheal bypass cannula at a calorimeter wall temperature (T_w) of either 17, 22, 26, or 28°C.By bypass breathing, T_hy and sensible heat loss (H_{L dry}) were increased without a significant change in colonic temperature in a warm environment. At T_w 28°C, T_hy in bypass breathing was 39.52±0.13°C while that in nose breathing was 39.30±0.11°C (p<0.05). H_{L dry} in bypass breathing was 29.3±0.8 W·m^-2 while that in nose breathing was 24.8±0.6 W·m^-2 (p<0.01).However, evaporative heat loss (H_Lwet) was not changed by bypass breathing, although the respiratory rate was increased slightly.These results indicate that the obstruction of air passing through the nose increases hypothalamic temperature and elicites a thermolytic response only through the increase in sensible heat loss in the T_w 28°C environment.Heat production (M) was not suppressed by bypass breathing at T_w 28°C, at which the value of M was already minimum (42.0±3.0 W⋅ m^-2).In the 17-26°C range of T_w, heat balance was roughly obtained in either nose or bypass breathing.At T_w 28°C, however, M was slightly higher than H_L, and a positive shift of heat storage was assumed particularly in nose breathing.