Effect of intrathoracic pressure on pressure volume characteristics of the lung in man

Quasi static pressure volume (PV) curves in normal seated human subjects were determined with pressure at the airway opening (Pao) set below (negative pressure), above (positive pressure) or equal to ambient pressure. Dynamic compliance (Cdyn) during controlled continuous negative pressure breathing (CNPB) was also studied. Quasi static PV curves at negative pressure were decreased in slope, reflected a decrease in total lung capacity, and intersected the PV curve obtained at ambient Pao. At positive pressure, the PV curves showed an increase in slope and an increase in total lung capacity. During CNPB, a fall in Cdyn was found. The fall in Cdyn was rapid and persisted for the duration of CNPB. Cdyn promptly returned to control levels when Pao was adjusted to ambient pressure.