Journal of Applied Physiology, Vol 71, Issue 5 1787-1794, Copyright © 1991 by American Physiological Society
Decrease in functional residual capacity during inspiratory loading and the sensation of dyspnea
Y. Kikuchi, W. Hida, T. Chonan, C. Shindoh, H. Sasaki and T. Takishima
Department of Internal Medicine, Tohoku University School of Medicine, Sendai, Japan.
The purposes of the present study were to determine the changes in
functional residual capacity (FRC) during inspiratory loading and to
examine their mechanisms. We studied seven normal subjects seated in a body
plethysmograph. In both graded inspiratory elastic (35, 48, and 68 cmH2O/l)
and resistive (21, 86, and 192 cmH2O.l-1.s) loading, FRC invariably
decreased from control FRC and phasic expiratory activity increased. The
reduction in FRC was greater with greater loads. A single inspiratory
effort against an inspiratory occlusion at three different target mouth
pressures (-25, -50, and -75 cmH2O) and durations (1, 2, and 5 s) also
resulted in a decrease in FRC with an increase in expiratory electromyogram
activity in the following expiration. The decrease in FRC was greater with
greater target pressure and duration. This decrease in FRC is qualitatively
similar to that during inspiratory loaded breathing, and we suspect that
the same mechanisms are at work. Because neither vagal nor chemoreceptor
reflex can account for these responses, we suspect conscious awareness of
breathing or behavioral control to be responsible. In an additional study,
the sensation of discomfort of breathing during elastic loading decreased
with a decrease in FRC. These results suggest that the reduced FRC may be
due to behavioral control of breathing to reduce the sensation of dyspnea
during inspiratory loading.
