Pulmonary arterial and venous pressures measured with small catheters in dogs.

To further subdivide the pressure drops across the arterial, middle, and venous segments obtained from the occlusion technique, we compared the pressures at the distal end of the arterial segment (Pa') and the proximal end of the venous segment (Pv') measured with this technique, with the pressures measured with 1.2-mm catheters in a small artery (Psa) and a small vein (Psv) of left lower lobes of dogs perfused in situ. The arterial (Pa) and venous (Pv) pressures were monitored and blood flow kept constant. Under control conditions the mean Pa, Psa, Pa', Pv', Psv, and Pv were 18.1, 13.0, 11.3, 9.2, 8.4, and 0.7 mmHg, respectively, suggesting that 29 and 44% of the total pulmonary vascular resistance (PVR) were in arteries and veins with diameters larger than at least 1.2 mm. Serotonin and histamine increased the pressure drops in arteries and veins, respectively, both larger and smaller than 1.2 mm. The small catheter pressures increased with flow rate. Microvascular pressures calculated from occlusion and small catheter measurements were higher than those calculated from the formula assuming 40% of PVR on the venous side. Our data suggest that a substantial proportion of resistance in the lung may lie in larger arteries and veins, the fraction of resistance in vessels under 1.2 mm increases with vasoconstrictors, the arterial and venous segments include muscular vessels under 1.2 mm, and calculation of microvascular pressures assuming 40% of the resistance in the veins may be misleading, especially when vasomotor tone is increased.