Exercise ventilatory mechanics at increased ambient pressure

Abstract

Transpulmonary pressure, lung volume, and flow rate were recorded in two healthy subjects performing graded exercise between 1 and 10 ATA. At simulated depths greater than 4 ATA, exercise was terminated by severe choking dyspnea at levels of work, oxygen consumption, heart rate, and ventilation significantly lower than during maximum exercise at 1 ATA. Comparison of exercise ventilatory mechanics with corresponding maximum expiratory flow-volume and expiratory isovolume pressure-flow (IVPV) curves demonstrated that the reduced aerobic capacity was associated with expiratory flow limitation. We conclude that dynamic airways compression limited aerobic capacity at these depths by causing a persistent cough making it seem difficult to continue exercise. Analysis of the IVPV curves suggested that maximum expiratory flow was reduced at depth below the rate allowing adequate exercise ventilation because increased gas density raised the resistance downstream from equal pressure points. At each level of sumbaximal exercise, end-expiratory position and alveolar CO2 tension increased with ambient pressure due primarily to the density dependence of airways resistance. In these respects, healthy subjects breathing dense gas resemble patients with obstructive lung disease.