Journal of Applied Physiology, Vol 70, Issue 5 1983-1990, Copyright © 1991 by American Physiological Society
Oxygen cost of inspiratory loading: resistive vs. elastic
S. J. Cala, P. Wilcox, J. Edyvean, M. Rynn and L. A. Engel
Thoracic Medicine Unit, Westmead Hospital, Sydney, New South Wales, Australia.
We measured the O2 cost of breathing (VO2resp) against external inspiratory
elastic (E) and resistive loads (R) when end-expiratory lung volume, tidal
volume, breathing frequency, work rate, and pressure-time product were
matched in each of six pairs of runs in six subjects. During E, peak
inspiratory mouth pressure was 65.7 +/- 1.8% (SD) of the maximum at
functional residual capacity. However, during resistive runs, peak
inspiratory mouth pressure was 41.1 +/- 2.8% of the maximum at functional
residual capacity. In 36 paired runs, where both work rate and
pressure-time product were within 10%, VO2resp for E was less than for R
(81 and 96 ml/min, respectively; P less than 0.01). During loaded and
unloaded breathing with the same tidal volume, we measured the changes in
anteroposterior diameter of the lower rib cage in five subjects. In four
subjects we also recorded the electromyograms of several fixator and
stabilizing muscles. During E and R, the change in anteroposterior diameter
of the lower rib cage was -116 +/- 5 and -45 +/- 4% (SE), respectively, of
the unloaded value (P less than 0.01), indicating greater deformation
during E. Although the peak electromyographic activity was 72 +/- 16%
greater during E (P less than 0.01), there was no difference between the
loads for area under the electromyogram time curve (P greater than 0.05).
However, the time to 50% peak activity was less during R (P less than
0.02). We conclude that, even when work rate and pressure-time product are
matched, VO2resp during R is greater than that during E. This difference
may be due to preferential recruitment of faster and less efficient muscle
fibers.
