Cardiac output increase and gas exchange at start of exercise.

To determine the rapidity of increased gas exchange resulting from increased cardiac output (Q) following exercise onset, subjects performed multiple rest-exercise transitions on a cycle ergometer: the early dynamics of pulmonary gas exchange were measured during 1) rhythmic breathing with ventilation kept constant at the resting level (controlled ventilation) and 2) prolonged constant airflow exhalation. With controlled ventilation, PACO2 increased and PAO2 decreased, typically beginning in the first exercise breath. After 15 s, PACO2 had increased and PAO2 decreased by 4.5-6.2 and 8.7-12.1 Torr, respectively, graded within these narrow ranges as functions of work rate (0-100 W). Exercise starting during a prolonged exhalation caused the slopes of the alveolar phases for O2 and CO2 to increase immediately or within 2-5 s following exercise onset. Work rate had little effect on the delay or the change of alveolar gas tension slope during the subsequent 10-15 s. Thus, increased gas exchange due to increasing Q occurred very rapidly following exercise onset so that it would coincide with the first or second breath of exercise in free-breathing subjects.