Effects of tracheal bypass breathing on heat balance in rabbits.

The effects of obstruction of the air passage through the nose on hypothalamic temperature (Thy) 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 (TW) of either 17, 22, 26, or 28 degrees C. By bypass breathing, Thy and sensible heat loss (HL dry) were increased without a significant change in colonic temperature in a warm environment. At TW 28 degrees C, Thy in bypass breathing was 39.52 +/- 0.13 degrees C while that in nose breathing was 39.30 +/- 0.11 degrees C (p less than 0.05). HL 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 (Hl wet) 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 elicits a thermolytic response only through the increase in sensible heat loss in the TW 28 degrees C environment. Heat production (M) was not suppressed by bypass breathing at TW 28 degrees C, at which the value of M was already minimum (42.0 +/- 3.0 W . m-2). In the 17-26 degrees C range of TW, heat balance was roughly obtained in either nose or bypass breathing. At TW 28 degrees C, however, M was slightly higher than HL, and a positive shift of heat storage was assumed particularly in nose breathing.