RESPIRATORY HEAT-LOSS IS NOT THE SOLE STIMULUS FOR BRONCHOCONSTRICTION INDUCED BY ISOCAPNIC HYPERNEA WITH DRY AIR

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Abstract
It is uncertain if respiratory heat loss or respiratory water loss is the stimulus for bronchoconstriction induced by isocapnic hyperpnea or exercise with dry air in subjects with asthma. These 2 stimuli were partially separated by having 18 subjects with asthma breathe dry air (0 mg/l water content) at increasing ventilations by isocapnic hyperpnea while the increase in specific airway resistance (SRaw) was measured. The study was divided into 2 phases. In Phase 1, an apparatus with a single respiratory valve was used and the subjects'' responses at 3 different inspired temperatures (-8.4, 20.5, and 39.4.degree. C) was evaluated. Seven of the subjects had esophageal catheters with 2 thermocouples in place to measure retrocardiac and retrotracheal temperatures. In this phase, there were no significant differences in the ventilation required to cause a 100% increase in SRaw among the 3 different inspired temperatures (48.4 l/min, cold; 47.5 l/min, room temperature; 44.2 l/min, hot), even though the retrotracheal temperature fell more when the subjects breathed cold air at 40 l/min (2.1.degree. C) than when they breathed hot air (1.2.degree. C), suggesting greater airway cooling with the cold air. In Phase 2, in order to accurately measure inspired and exhaled temperatures and exhaled water content, 2 separate systems for delivering the inspired air and collecting the exhaled air at 2 different inspired temperatures (-21.4 and 38.9.degree. C) were used. Again, there was no significant difference in the ventilation required to cause a 100% increase in SRaw between the 2 different inspired temperatures (28.3 l/min, cold; 33.6 l/min, hot). When the subjects inhaled cold air, exhaled temperature was warmer than previously reported. Exhaled water content was close to completely saturated at the exhaled temperature under both conditions. Heat and water loss was calculated from the measurements of temperature and water content. When stimulus-response curves were constructed using the heat loss or water loss at each ventilation as the stimulus, a significant difference was found between the heat loss that resulted in a 100% increase in SRaw for hot and cold air. A significant difference was found between the water loss that resulted in a 100% increase in SRaw for the 2 different conditions. Thus, while respiratory heat loss does not appear by the sole stimulus for bronchoconstriction from isocapnic hyperpnea with dry air, it cannot be concluded that water loss is the sole stimulus either.