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Hypoxia has a greater effect than exercise on the redistribution of pulmonary blood flow in swine

¹Division of Physiology, ⁵Department of Surgery, University of California, San Diego, La Jolla, California; ²Department of Anesthesiology and Critical Care Medicine, Innsbruck Medical Univeristy, Innsbruck, Austria; Departments of ³Physiology and Biophysics and ⁴Medicine, University of Washington, Seattle, Washington; and ⁶The Mountain-Whisper-Light Statistical Consulting, Seattle, Washington

Submitted 16 March 2007 ; accepted in final form 23 August 2007

Strenuous exercise combined with hypoxia is implicated in the development of high-altitude pulmonary edema (HAPE), which is believed to result from rupture of pulmonary capillaries secondary to high vascular pressures. The relative importance of hypoxia and exercise in altering the distribution of pulmonary blood flow (PBF) is unknown. Six chronically catheterized specific pathogen-free Yorkshire hybrid pigs (25.5 ± 0.7 kg, means ± SD) underwent incremental treadmill exercise tests in normoxia (FI_O2 = 0.21) and hypoxia (FI_O2 = 0.125, balanced order), consisting of 5 min at 30, 60, and 90% of the previously determined O_2max. At steady state (4 min), metabolic and cardiac output data were collected and fluorescent microspheres were injected over 30 s. Later the fluorescent intensity of each color in each 2-cm³ lung piece was determined and regional perfusion was calculated from the weight-normalized fluorescence. Both hypoxia and exercise shifted PBF away from the ventral cranial lung regions toward the dorsal caudal regions of the lung, but hypoxia caused a greater dorsal caudal shift in PBF at rest than did near-maximal exercise in normoxia. The variance in PBF due to hypoxia, exercise, and vascular structure was 16 ± 4.2, 4.0 ± 4.4, and 59.4 ± 11.4%, respectively, and the interaction between hypoxia and exercise represented 12 ± 6.5%. This observation implies that there is already a maximal shift with in PBF with hypoxia in the dorsal-caudal regions in pigs that cannot be exceeded with the addition of exercise. However, exercise greatly increases the pulmonary arterial pressures and therefore the risk of capillary rupture in high flow regions.

fluorescent microspheres; ventilation; perfusion; gas exchange