Journal of Applied Physiology, Vol 70, Issue 4 1554-1562, Copyright © 1991 by American Physiological Society
Respiratory muscle coordination and diaphragm length during expiratory threshold loading
J. D. Road, A. M. Leevers, E. Goldman and A. Grassino
Department of Medicine, University of British Columbia, Vancouver, Canada.
Active expiration is produced by the abdominal muscles and the rib cage
expiratory muscles. We hypothesized that the relative contribution of these
two groups to expiration would affect diaphragmatic length and, hence,
influence the subsequent inspiration. To address this question we measured
the respiratory muscle response to expiratory threshold loading in
spontaneously breathing anesthetized dogs. Prevagotomy, the increase in
lung volume (functional residual capacity) and decrease in initial resting
length of the diaphragm were attenuated by greater than 50% of values
predicted by the passive relationships. Diaphragmatic activation
(electromyogram) increased and tidal volume (VT) was preserved.
Postvagotomy, effective expiratory muscle recruitment was abolished. The
triangularis sterni muscle remained active, and the increase in lung volume
was attenuated by less than 15% of that predicted by the passive
relationship. Diaphragmatic length was shorter than predicted. VT was not
restored, even though costal diaphragmatic and parasternal intercostal
electromyogram increased. During expiratory threshold loading with
abdominal muscles resected and vagus intact, recruitment of the rib cage
expiratory muscles produced a reduction in lung volume comparable with
prevagotomy; however, diaphragmatic length decreased markedly. Both the rib
cage and abdominal expiratory muscles may defend lung volume; however,
their combined action is important to restore diaphragmatic initial length
and, accordingly, to preserve VT.
