Flow-volume characteristics in the pulmonary circulation

Isolated ferret and canine lungs were used to validate a method for assessing determinants of vascular volume in the pulmonary circulation. With left atrial pressure (Pla) constant at 5 mmHg, flow (Q̇) was raised in steps over a physiological range. Changes in vascular volume (ΔV) with each increment in Q̇ were determined as the opposite of changes in perfusion system reservoir weight or from the increase in lung weight. At each level of Q̇, the pulmonary arterial and left atrial cannulas were simultaneously occluded, allowing all vascular pressures to equilibrate at the same static pressure (Ps), which was equal to the compliance-weighted average pressure in the circulation before occlusion. Hypoxia (inspired PO₂ 25 Torr) in ferret lungs, which causes intense constriction in arterial extra-alveolar vessels, had no effect on the slope of the Ps-Q̇ relationship, interpreted to represent the resistance downstream from compliance (control 0.025 ± 0.006 mmHg·ml^-1·min, hypoxia 0.030 ± 0.013). The Ps-axis intercept increased from 8.94 ± 0.50 to 13.43 ± 1.52 mmHg, indicating a modest increase in the effective backpressure to flow downstream from compliant regions. The compliance of the circulation, obtained from the slope of the relationship between ΔV and Ps, was unaffected by hypoxia (control 0.52 ± 0.08 ml/mmHg, hypoxia 0.56 ± 0.08). In contrast, histamine in canine lungs, which causes constriction in veins, caused the slope of the Ps-Q̇ relationship to increase from 0.013 ± 0.007 to 0.032 ± 0.006 mmHg·ml^-1·min (P < 0.05) and the compliance to decrease from 3.51 ± 0.56 to 1.68 ± 0.37 ml/mmHg (P < 0.05). We conclude that the relationships among flow, volume, and Ps provide valuable information pertaining to the distribution of resistance and compliance in the pulmonary circulation that contributes to the determination of vascular volume.