Journal of Applied Physiology, Vol 52, Issue 1 236-244, Copyright © 1982 by American Physiological Society
Cardiac output increase and gas exchange at start of exercise
M. L. Weissman, P. W. Jones, A. Oren, N. Lamarra, B. J. Whipp and K. Wasserman
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.
