Spatial distribution of hypoxic pulmonary vasoconstriction in the supine pig

Hypoxic pulmonary vasoconstriction (HPV) serves to maintain optimal gas exchange by decreasing perfusion to hypoxic regions. However, global hypoxia and nonuniform HPV may result in overperfusion of poorly constricted regions leading to local edema seen in high-altitude pulmonary edema. To quantify the spatial distribution of HPV and its response to regional PO2 (Pr-O2) among small lung regions, five pigs were anesthetized and mechanically ventilated in the supine posture. The animals were ventilated with an inspired O-2 fraction (FIO2) of 0.50 and 0.21 and then (in random order) 0.15, 0.12, and 0.09. Regional blood flow ((Q) over dot) and alveolar ventilation ((V) over dot A) were measured by using intravenous infusion of 15 mum and inhalation of 1-mum fluorescent microspheres, respectively. Pr-O2 was calculated for each piece at each FIO2. Lung pieces differed in their (Q) over dot. response to hypoxia in a manner related to their initial (V) over dot A/(Q) over dot with FIO2 = 0.21. Reducing FIO2 < 0.15 decreased (Q) over dot to the initially high (V) over dot A/(Q) over dot (higher Pr-O2) regions and forced (Q) over dot into the low (V) over dot A/(Q) over dot (dorsal-caudal) regions. Resistance increased in most lung pieces as Pr-O2 decreased, reaching a maximum resistance when Pr-O2 is between 40 and 50 Torr. Local resistance decreased at Pr O-2 < 40 Torr. Pieces were statistically clustered with respect to their relative (Q) over dot. response pattern to each FIO2. Some clusters were shown to be spatially organized. We conclude that HPV is spatially heterogeneous. The heterogeneity of (Q) over dot response may be related, in part, to the heterogeneity of baseline (V) over dot A/(Q) over dot.