Diaphragm and abdominal muscle responses to elevated airway pressures in the cat.

Abstract

This is a study of respiratory muscle responses in a Dial-anesthetized cat when the pressures during inspiration and expiration are elevated independently. The results demonstrate that the abdominal muscles respond selectively to pressure opposing expiration whereas diaphragm activity depends on the pressures during both inspiration and expiration. Electromyograms show that abdominal muscle activity increases in proportion to the pressure applied during expiration regardless of the pressure during inspiration. In contrast, diaphragm activity is progressively augmented by a pressure opposing expiration but this augmentation is depressed if lung inflation is aided by positive pressure during inspiration. Whenever expiration is opposed, thorax and lung volume is increased in spite of the abdominal contractions, but continuous pressure inflates the thorax more than a pressure elevated only during expiration. For a given pressure opposing expiration, tidal volume is decreased equally by pressure breathing and by expiratory loading. Since the thoracic inflation is different in the 2 procedures, tidal volume seems to be determined by the pressure opposing expiration rather than lung volume. Vagotomy abolishes abdominal respiratory activity under all conditions as well as the inhibition of the diaphragm caused by positive pressure during inspiration but does not interrupt the diaphragm augmentation in response to a pressure opposing expiration.