[Respiratory thermal exchanges in man as a function of inspired gas temperature]

1. Measurements of respiratory water vapor loses (MEH20) and of respiratory evaporative (WEV) and convective (WCV) heat losses were made on four of five subjects at three levels of inspired gas temperatures (T1): 10 degrees C, 22 degrees C, 40 degrees C, and at almost constant water vapour pressure (8.5 to 11 torrs). For T1 = 22 degrees C and P1H20 = 8.5 torrs, the mean value of MEH20 is 29.5 +/- 1.6 mg of water per dm3 of ventilated gas (BTPS), while the values of WEV and WCV are respectively 8.1 +/- 1.9 watts and 1.6 +/- 0.4 watts. 2. When T1 increases, MEH20 and WEV increases and WCV decreases. Results show that WCV changes in sign and becomes a thermal gain when T1 is higher than core temperature. The total respiratory heat drain, convective plus evaporative, involved of the conditioning of respiratory gases is a small part of the total body heat balance (approximately 15%). However this heat drain which slowly decreases when T1 increases, represents the largest energy expenditure of the human organism in respiratory function.