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Spatial distribution of hypoxic pulmonary vasoconstriction in the supine pig

Departments of ¹Physiology and Biophysics and ²Medicine, University of Washington, Seattle 98195-6522; and ³The Mountain-Whisper-Light Statistical Consulting, Seattle, Washington 98112-2913

Submitted 28 February 2003 ; accepted in final form 18 November 2003

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 O₂ fraction (FI_O2) of 0.50 and 0.21 and then (in random order) 0.15, 0.12, and 0.09. Regional blood flow () and alveolar ventilation (A) were measured by using intravenous infusion of 15 µm and inhalation of 1-µm fluorescent microspheres, respectively. Pr_O2 was calculated for each piece at each FI_O2. Lung pieces differed in their response to hypoxia in a manner related to their initial A/ with FI_O2 = 0.21. Reducing FI_O2 < 0.15 decreased to the initially high A/ (higher Pr_O2) regions and forced into the low A/ (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 PrO₂ < 40 Torr. Pieces were statistically clustered with respect to their relative response pattern to each FI_O2. Some clusters were shown to be spatially organized. We conclude that HPV is spatially heterogeneous. The heterogeneity of response may be related, in part, to the heterogeneity of baseline A/.

hypoxia; pulmonary circulation; high-altitude pulmonary edema; heterogeneity; ventilation-perfusion; fluorescent microspheres

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