Hypoxic pulmonary vasoconstriction is heterogeneously distributed in the prone dog

Hypoxic pulmonary vasoconstriction (HPV) is thought to protect gas exchange by decreasing perfusion to hypoxic regions. However, with global hypoxia, non-uniformity in HPV may cause over-perfusion to some regions, leading to high-altitude pulmonary edema. To quantify the spatial distribution of HPV and regional P O 2 ( P R O 2 ) among small lung regions (∼2.0 cm 3), five prone beagles (∼8.3 kg) were anesthetized and ventilated (PEEP ∼ 2 cm H 2O) with an F I O 2 of 0.21, then 0.50, 0.18, 0.15, and 0.12 in random order. Regional blood perfusion ( Q), ventilation ( V A) and calculated P R O 2 were obtained using iv infusion of 15 μm and inhalation of 1 μm fluorescent microspheres. Lung pieces were clustered by their relative blood flow response to each F I O 2 . Clusters were shown to be spatially grouped within animals and across animals. Lung piece resistance increased as P R O 2 decreased to 60–70 mmHg but dropped at P R O 2 ’ s < 60 mmHg. Regional ventilation changed little with hypoxia. HPV varied more in strength of response, rather than P R O 2 response threshold. In initially homogeneous V A/ Q lungs, we conclude that HPV response is heterogeneous and spatially clustered.