Relationship of pulmonary arterial and venous pressure to diffusing capacity

Single-breath carbon monoxide diffusing capacity (DlCO) was determined in ten isolated perfused cat lungs at 37 C during a) forward (arterial) perfusion through the pulmonary artery and b) reverse (venous) perfusion through the left atrium. Blood flow, inflow and outflow pressure, lung volume, and transpulmonary ventilating pressure were approximately equal in a and b, but in all ten lungs DlCO was greater in b than a. In five lungs during forward (arterial) perfusion blood flow was increased from a mean of 62–180 ml/min while left atrial outflow pressure was maintained about zero mm Hg. At the higher blood flow DlCO was not significantly changed although vascular resistance decreased a mean of 34% and arterial pressure increased a mean of 98%. We conclude that a) transmural pressure in the pulmonary veins is more important than that in the arteries in determining the size of the capillary bed as measured by DlCO, and b) the size of the capillary bed and total vascular resistance can vary independently. When pulmonary edema occurred in five lungs DlCO did not change significantly. pulmonary capillary bed size; pulmonary edema and lung diffusing capacity; pulmonary blood flow and lung diffusing capacity Submitted on February 11, 1963