Volume-pressure relationships during pressure breathing and voluntary relaxation

Lung volumes during pressure breathing (PB) were measured in six subjects by two methods: 1) The subject was seated completely enclosed in a body plethysmograph breathing room air through a tube; pressures were applied to this tube and volume changes were recorded from the plethysmograph. 2) The subject was seated enclosed in a body box except for his head which protruded through a respirator collar; pressures were applied around the body and lung volume changes were recorded from a bag-box system. In addition, relaxation pressure (RP) measurements were obtained. Volume-pressure relationships by PB 1 closely approximated those by RP, whereas PB 2 tended to yield smaller volume changes for a given pressure than the other methods. The extrathoracic airways are subjected to the applied pressure in PB 1 and RP, but not in PB 2. The different results may relate to this difference.