Effect of volume and volume history of the lungs on pulmonary shunt flow

Abstract

Relative pulmonary shunt flow (Qs/Qt), was measured in denitrogenated open-chested cats during apnea over the full range of lung volumes. The particular lung volume and transpulmonary pressure were also measured. When completely collapsed lungs were inflated, Qs/Qt decreased sharply to 3% at total lung capacity (TLC). During deflation from TLC Qs/Qt was insensitive to changes in lung volume. Qs/Qt remained low during reinflation after deflation from TLC. These changes in shunt flow can be interpreted as due to either recruitment or collapse of gas exchange units during lung volume change. It appears that completely collapsed lungs inflate very unevenly but that deflation from TLC proceeds remarkably evenly. Reinflation after deflation from TLC also seems to proceed evenly, and the manifest pressure-volume hysteresis is most likely due to hysteresis of the surface-active properties of the alveolar lining material.