Effects of alveolar hypoxia during partial mitral valve obstruction in unanesthetized sheep.

The effects of elevated left atrial pressure (Pla) on the pulmonary hemodynamic responses to hypoxia and infused prostaglandin-H2 analog (PGH2-A) were studied in 10 chronically instrumented unanesthetized sheep. Sheep were studied with isocapnic hypoxia (fraction of inspired O2, 0.12) or infused PGH2-A (0.2 to 1.0 micrograms X kg-1 X min-1 adjusted to increase pulmonary artery pressure (Ppa) by approximately 15 cm H2O) when Pla was normal or elevated to 10 or 20 cm H2O. The Pla was elevated by inflating a Foley catheter positioned in the mitral valve orifice. Elevation of Pla did not block the increase in Ppa or cardiac output (CO) caused by hypoxia but did block the increase in pulmonary vascular resistance (PVR). When Pla was elevated to 10 or 20 cm H2O, hypoxia caused Pla to increase further, and PGH2-A caused Ppa and PVR to increase whether Pla was elevated or not; PGH2-A did not cause CO to increase or Pla to increase further under any experimental condition. Neither hypoxia nor PGH2-A had any effect on left ventricular end-diastolic pressure under any experimental condition. We hypothesize that when Pla is elevated, the increase in CO may dilate the pulmonary circulation, obscuring hypoxic vasoconstriction. When Pla is elevated, the direct effects of hypoxic pulmonary vasoconstriction cannot overcome the increased intraluminal pressure, and PVR does not increase. The pulmonary vessels are still able to respond to a potent vasoconstrictor such as PGH2-A when Pla is elevated. We conclude that the further increase in Pla caused by hypoxia when Pla is elevated is primarily due to increased flow across a mitral valve behaving as a relatively fixed resistor.