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1 Physiology and Biophysics, University of Washington, Seattle, WA, USA; Medicine, University of Washington, Seattle, WA, USA
2 Medicine, University of Washington, Seattle, WA, USA
3 The Mountain-Whisper-Light Statistical Consulting, Seattle, WA, USA
4 Medicine, University of Washington, Seattle, WA, USA; Physiology and Biophysics, University of Washington, Seattle, WA, USA
* To whom correspondence should be addressed. E-mail: hlastala{at}u.washington.edu.
Hypoxic pulmonary vasoconstriction (HPV) serves to maintain optimal gas exchange by decreasing perfusion to hypoxic regions. However, global hypoxia and non-uniform HPV may result in over-perfusion of poorly constricted regions leading to local edema seen in high altitude pulmonary edema (HAPE). To quantify the spatial distribution of HPV and its response to regional PO2 (PRO2) among small lung regions, five pigs were anesthetized and mechanically ventilated in the supine posture. The animals were ventilated with an FIO2 of 0.50 and 0.21, and then 0.15, 0.12 and 0.09 in random order. Regional blood flow (Q) and ventilation (V) were measured using iv infusion of 15µm and inhalation of 1µm fluorescent microspheres, respectively. PRO2 was calculated for each piece at each FIO2. Lung pieces were statistically clustered with respect to their relative flow response to each FIO2. Clusters were shown to be spatially organized. Reducing FIO2 below 0.15 decreased flow to the initially high V/Q (higher PRO2) regions and forced flow into the low V/Q (dorsal-caudal) regions. Resistance increased in most lung pieces as PRO2 decreased reaching a maximum at PaO2 between 40 - 50 mm Hg. Local resistance decreased at PRO2 < 40 mmHg. We conclude that hypoxic pulmonary vasoconstriction is spatially heterogeneous. This heterogeneity may be related, in part, to spatial heterogeneity of V/Q .
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