RESUMEN
We identified inspired gas conditions that result in no net respiratory heat loss, an isenthalpic condition, but induce a mucosal loss of water. Inspired gas at 37 degrees C with 47 mm Hg water vapor pressure, 56 degrees C with 38 mm Hg; and 78 degrees C with 27 mm Hg has the same heat content as fully saturated air at body temperature. In four normal subjects hyperventilating at a minute ventilation of 30 times their FEV1 for 6 min, expired temperatures at the mouth averaged 39 degrees, 43 degrees, and 43 degrees C for the three conditions. Retrotracheal esophageal temperatures did not fall in any subject, thereby demonstrating the absence of significant airway cooling. Nine subjects with exercise-induced bronchospasm were tested under the same conditions. Baseline functions showed an FEV1 of 85 +/- 10% of predicted (mean +/- SD), FVC, 98 +/- 13% of predicted, and FEV1/FVC, 79 +/- 4% of predicted. The asthmatic subjects demonstrated postchallenge mean falls in FEV1 of 3.4%, 6.2%, and 10.1% for the three conditions, with bronchospasm increasing as the temperature of the inspired air increased (p = 0.001). The amount of respiratory water lost from the respiratory mucosa significantly correlated with the resultant bronchospastic response as measured by the fall in FEV1 (p = 0.017), but the net respiratory heat lost did not significantly correlate (p = 0.113). This study demonstrates that bronchospasm can be induced without significant respiratory heat loss or airway cooling and suggests that it is proportional to the amount of water lost from mucosal surfaces.