Journal of Applied Physiology, Vol 65, Issue 2 760-766, Copyright © 1988 by American Physiological Society
Influence of inspiratory flow rate and frequency on O2 cost of resistive breathing in humans
D. S. Dodd, P. W. Collett and L. A. Engel
Thoracic Medicine Unit, Westmead Hospital, Sydney, Australia.
We examined the combined effect of an increase in inspiratory flow rate and
frequency on the O2 cost of inspiratory resistive breathing (VO2 resp). In
each of three to six pairs of runs we measured VO2 resp in six normal
subjects breathing through an inspiratory resistance with a constant tidal
volume (VT). One of each pair of runs was performed at an inspiratory
muscle contraction frequency of approximately 10/min and the other at
approximately 30/min. Inspiratory mouth pressure was 45 +/- 2% (SE) of
maximum at the lower contraction frequency and 43 +/- 2% at the higher
frequency. Duty cycle (the ratio of contraction time to total cycle time)
was constant at 0.51 +/- 0.01. However, during the higher frequency runs,
two of every three contractions were against an occluded airway. Because VT
and duty cycle were kept constant, mean inspiratory flow rate increased
with frequency. Careful selection of appropriate parameters allowed the
pairs of runs to be matched both for work rate and pressure-time product.
The VO2 resp did not increase, despite approximately threefold increases in
both inspiratory flow rate and contraction frequency. On the contrary,
there was a trend toward lower values for VO2 resp during the higher
frequency runs. Because these were performed at a slightly lower mean lung
volume, a second study was designed to measure the VO2 resp of generating
the same inspiratory pressure (45% maximum static inspiratory mouth
pressure at functional residual capacity) at the same frequency but at two
different lung volumes. This was achieved with a negligibly small work
rate.(ABSTRACT TRUNCATED AT 250 WORDS)
