Longitudinal distribution of pulmonary vascular resistance with very high pulmonary blood flow

Dog left upper lobes (LUL) were perfused in situ via the left lower lobe artery. Lobe weight was continuously monitored. Increasing lobar flow from normal to 10 times normal had little effect on left atrial pressure, which ranged from 1 to 5 mmHg. There was a flow threshold (Qth) below which lobar weight was stable. Qth ranged from 1.1 to 1.55 l/min (mean 1.27) corresponding to four times normal LUL blood flow. Above Qth, step increases in lobar flow resulted in progressive weight gain at a constant rate that was proportional to flow. The effective pressure at the filtration site (EFP) at different flow rates was estimated from the static vascular pressure that resulted in the same rate of weight gain. From this value and from mean pulmonary arterial (PA) and left atrial (LA) pressures, we calculated resistance upstream (Rus) and downstream (Rds) from filtration site. At Qth, Rds accounted for 60% of total resistance. This fraction increased progressively with flow, reaching 83% at Q of 10 times normal. We conclude that during high pulmonary blood flow EFP is closer to PA pressure than it is to LA pressure, and that this becomes progressively more so as a function of flow. As a result, the lung accumulates water at flow rates in excess of four times normal despite a normal left atrial pressure.