Diaphragm contraction as a limiting factor to maximum expiration

¹ Istituto di Fisiologia umana, Università di Milano, Milano, Italy

Electrical activity of the diaphragm has been studied by means of a bipolar esophageal lead to investigate the high abdominal thoracic pressure difference found at the end of maximum expiration. An esophageal balloon, placed alongside the electrodes and moderately inflated, overcomes the difficulty previously found in using an esophageal lead to study diaphragm activity at small lung volume. At the end of maximum expiration, intense activity of the diaphragm has been recorded. Contraction of the diaphragm at the end of maximum expiration balances the muscle action of the abdominal wall, preventing further reduction of lung volume. Analysis of factors that may set the lower volume limit suggests that contraction of antagonist muscles is the main factor, at least on the abdominal boundary of the thorax. The possible afferent impulses leading to a reflex contraction of the diaphragm at full expiration are discussed.