No protective role for hypoxic pulmonary vasoconstriction in severe hypergravity-induced arterial hypoxemia

Supine subjects exposed to hypergravity show a marked arterial desaturation. Previous work from our laboratory has also shown a paradoxical reduction of lung perfusion in dependent lung regions in supine subjects exposed to hypergravity. We reasoned that the increased lung weight during hypergravity caused either direct compression of the blood vessels in the dependent lung tissue or that poor regional ventilation caused reduced perfusion through hypoxic pulmonary vasoconstriction (HPV). The objective of this study was to evaluate the importance of HPV through measurements of arterial oxygenation during exposure to hypergravity with normal and attenuated HPV. A further increased arterial desaturation during hypergravity with attenuated HPV would support the hypothesis that HPV contributes to the paradoxical redistribution of regional perfusion. In a two-phased randomized study we first exposed 12 healthy subjects to 5 G while supine during two single-blinded conditions; control and after 50 mg sildenafil p.o.. In a second phase, 12 supine subjects were exposed to 5 G during three single-blinded conditions; control, after 100 mg sildenafil p.o. and after inhalation of 10 μg iloprost. There was a substantial arterial desaturation by 5-30% units in all subjects with no or only minor differences between conditions. The results speak against HPV as a principal mechanism for the hypergravity-induced reduction of lung perfusion in dependent lung regions in supine humans.