Abdominal muscle and diaphragm activities and cavity pressures in pressure breathing

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.

Submitted on May 21, 1962

Note:

(With the Technical Assistance of J. R. Blumstein and D. Pennec)

Submitted on January 22, 1962

Diaphragm activity and thoracoabdominal mechanics during positive pressure breathing

The electrical activity of the diaphragm and the mechanical contribution of the thorax and the abdomen through the breathing cycle has been investigated in man during positive pressure breathing (PPB). The electrical activity of the diaphragm persists even at values at which inspiration should be completely passive according to the pressure volume diagram of the thorax and lung. The transdiaphragmatic pressure decreases as the value of PPB increases but is still appreciable at values of PPB at which the inspiration appears to be completely passive by an analysis based on transthoracic pressure measurements alone; the transdiaphragmatic pressure becomes zero only at PPB of about 30 cm H2O. The persistent activity of the diaphragm during PPB is counterbalanced by an activity of the abdominal muscles in excess of that of the expiratory muscles as given by transthoracic pressure measurements. Owing to the persistence of an abdominothoracic pressure gradient, the venous return and therefore the cardiac output should be less reduced than in the case of a passive inspiration.

Submitted on September 22, 1961

Circulating plasma volume changes in anesthetized dogs during positive pressure breathing

Various circulatory functions were measured in anesthetized dogs subjected to 18.5 mm Hg positive pressure breathing. Immediately upon raising the intrapulmonary pressure there occurred a five- to sixfold increase in venous pressure as well as a decrease in mean arterial pressure. Accompanying these pressure changes was a progressive decrease in circulating plasma volume as measured by the T-1824 method. A 30% decrease in plasma volume was recorded after 160 minutes of increased intrapulmonary pressure. Hemoconcentration was also indicated by the increased hematocrits, although calculated fluid loss was only 13%. All circulatory changes returned to prepressure breathing levels upon release of pressure breathing. Other changes, such as oliguria, periods of apnea and an alkaline urine accompanied positive pressure breathing. The data suggest that the decrease in plasma volume is the result of venostasis caused by the rapid increase in venous pressure.

Submitted on April 27, 1959