RESPIRATORY EFFECTS OF WARM AND DRY AIR AT INCREASED AMBIENT PRESSURE

We have measured in 7 divers forced vital capacity (FVC), forced expir ed volume in 1 s (FEV1), and forced midexpiratory flow rate (FEF25-75% ) before and after exposure to dry or humid breathing gas of 35.3-degr ees-36.8-degrees-C (air) when diving to pressures of 117-600 kPa. The response was compared with the subjects reactivity to pharmacologic br onchoprovocation with methacholine. Base-line FEV1 and FEF25-75% decre ased in accordance with increasing gas density. Relative to baseline, there was a significant reduction after the dives in FEV1 of 4.0 +/- 6 .1% (P < 0.05) and in FEF25-75% of 8.6 +/- 9.7% (P < 0.01) with exposu re to dry breathing gas. By analysis of variance the reduction in the lung function variables below baseline were related to the breathing g as characteristic (dry/humid)(P < 0.01), bronchial hyperreactivity (P < 0.02), and ambient pressure (P < 0.02) independently of each other. There was no significant change in FVC after the exposures. Humid brea thing gas was considered more comfortable than dry breathing gas, and the upper comfort limit for breathing gas temperature was higher with humid breathing gas. Convective respiratory heat loss was negligible i n these experiments, indicating that dry gas itself had a significant bronchoconstrictive effect. Bronchial hyperreactivity may cause increa sed risk of development of bronchial obstruction and air trapping duri ng diving.