Abdominal muscle and diaphragm activities and cavity pressures in pressure breathing

¹ Department of Physiology, The University of Buffalo, Buffalo, New York

The respiratory contribution of the diaphragm and external oblique abdominal muscle has been assessed by recording from the anesthetized cat the integrated electromyograms during continuous pressure breathing. As the intrapulmonary pressure is progressively reduced from 0 to –12 cm H₂O, the diaphragm becomes increasingly active until it has no silent period during the respiratory cycle. Concomitantly, any respiratory activity in the abdominal muscle is completely silenced. A hyperactive diaphragm and relaxed abdominal wall can account for the constancy seen in the directly recorded intra-abdominal pressure even though the intrathoracic pressure falls. When the animal is subjected to pressures from 0 to +14 cm H₂O, the diaphragm is initially inhibited and the abdominal muscle becomes increasingly active. In every animal, on positive pressure the abdominal muscle becomes active during expiration and in 20% of the animals it is also active during inspiration. Active expiration continues throughout the pressure breathing and is sufficient to reverse the breath by breath abdominal pressure variations. During negative pressure breathing, respiration is an inspiratory act and only the thorax is subjected to stress. During positive pressure breathing, respiration is an expiratory act and both the thorax and abdomen are subjected to the stress.