Pulmonary arterial flow-pressure characteristics in dogs--effects of hypoxia and nitroglycerin

To investigate the effects of hypoxia and nitroglycerin on pulmonary vascular tone, multipoint mean pulmonary arterial flow-pressure (Q-P relationship) plots were constructed by producing stepwise increments of pulmonary flow with a roller pump installed in a right ventricle-pulmonary artery shunt in 12 mongrel dogs under pentobarbital sodium anesthesia. The normal Q-P relationship was convex to the pressure axis when pulmonary flow was 0-0.2 L/min, but it became linear when pulmonary flow was over 0.2 L/min. The slopes and pressure intercepts extrapolated (RAPI) from linear regression fits to the linear parts of the Q-P relationship were determined to compare the effects of hypoxia and nitroglycerin on pulmonary vascular tone. Hypoxic ventilation (FIO2: 0.1) increased the slope from 8.3 +/- 3.3 to 12.5 +/- 3.6 mmHg/L (p less than 0.01), with no significant effect on PAPI. Nitroglycerin (1 microgram/min/kg as a continuous infusion) decreased the slope from 8.9 +/- 4.4 to 5.8 +/- 2.6 mmHg/L (p less than 0.05), again with no significant effect on PAPI. The results suggest that hypoxia and nitroglycerin, respectively, increased and decreased incremental pulmonary vascular resistance upstream to the site of pulmonary vascular closure.