Absence of a General Critical Closing Pressure in the Isolated Perfused Lung

¹ From the Department of Physiology, The Medical School, University of Minnesota, Minneapolis, Minnesota

Evidence for a critical closing pressure in the lung blood vessels is contradictory. Some of it is obscured by the use of a positive expiratory pressure during ventilation of the lungs. In the present study, the isolated lungs of dogs or cats were perfused with homologous blood or dextran, and the pressure gradient was measured while the lung weight was followed with a strain-gauge weighing device. Expiratory pressure was allowed to fall to atmospheric levels. Pressure-flow curves were obtained which included points for low flows. In addition, the pressure decay curve and rate of weight loss (venous outflow) were followed after arrest of the arterial inflow. It was found that at low flows the pressure-flow curve turned towards the origin. When the arterial inflow was completely stopped the pressure drop fell to zero (0.1 mm Hg) as the venous outflow approached zero. It was concluded that a general critical closing pressure is not evident in the isolated perfused dog or cat lung. The findings of previous workers concerning the general shape of the pressure-flow curve for the lung were confirmed.