Journal of Applied Physiology, Vol 68, Issue 3 1092-1100, Copyright © 1990 by American Physiological Society
Inspiratory and expiratory muscle function during continuous positive airway pressure in dogs
J. D. Road and A. M. Leevers
Department of Medicine, University Hospital, University of British Columbia, Vancouver, Canada.
Continuous positive airway pressure (CPAP) is known to produce activation
of the expiratory muscles. Several factors may determine whether this
activation can assist inspiration. In this study we asked how and to what
extent expiratory muscle contraction can assist inspiration during CPAP.
Respiratory muscle response to CPAP was studied in eight supine
anesthetized dogs. Lung volume and diaphragmatic initial length were
defended by recruitment of the expiratory muscles. At the maximum CPAP of
18 cmH2O, diaphragmatic initial lengths were longer than predicted by the
passive relationship by 52 and 46% in the costal and crural diaphragmatic
segments, respectively. During tidal breathing after cessation of
expiratory muscle activity, a component of passive inspiration occurred
before the onset of inspiratory diaphragmatic electromyogram (EMG). At CPAP
of 18 cmH2O, passive inspiration represented 24% of the tidal volume (VT)
and tidal breathing was within the relaxation characteristic. Diaphragmatic
EMG decreased at CPAP of 18 cmH2O; however, VT and tidal shortening were
unchanged. We identified passive and active components of inspiration.
Passive inspiration was limited by the time between the cessation of
expiratory activity and the onset of inspiratory activity. We conclude that
increased expiratory activity during CPAP defends diaphragmatic initial
length, assists inspiration, and preserves VT. Even though breathing
appeared to be an expiratory act, there remained a significant component of
active inspiratory diaphragmatic shortening, and the major portion of VT
was produced during active inspiration.
