PAO2 and PVO2 interaction on hypoxic pulmonary vasoconstriction.

We sought to determine why large lung compartment hypoxic pulmonary vasoconstriction fails to redistribute blood flow at a low fraction of inspired oxygen (FIO2) level (0.06) when the remaining small lung compartment is ventilated with room air. In 10 pentobarbital-anesthetized dogs, we decreased large compartment FIO2 from 1.0 to 0.06 while the small compartment FIO2 was constant at 0.21, 0.3, 0.5, or 1.0. When small compartment FIO2 was 0.21 and 0.3, large compartment FIO2 decreases from 1.0 to 0.15-0.10 caused a disproportionate increase in large compartment pulmonary vascular resistance (PVR) and further large compartment FIO2 decreases from 0.15-0.10 to 0.06 caused a decrease in large compartment PVR while small compartment PVR continued to increase. When small compartment FIO2 was 0.5, large compartment FIO2 decreases caused an increase and then no change in large compartment PVR, while small compartment PVR remained constant. When small compartment FIO2 was 1.0, all large compartment FIO2 decreases caused increases in large compartment PVR, while small compartment PVR remained constant. When small compartment FIO2 was 0.21 and 0.3, small compartment alveolar oxygen tension (PAO2) and PVR were always inversely related. When small compartment FIO2 was 0.21, 0.3, and 0.5, large compartment PVR either decreased or remained constant whenever mixed venous oxygen tension (PVO2) was less than 30-32 Torr and large compartment PAO2 was less than 50-60 Torr. We conclude that both small compartment hypoxic pulmonary vasoconstriction and primarily failure of large compartment hypoxic pulmonary vasoconstriction occurred when large compartment FIO2 was low (0.06) and small compartment FIO2 was 0.21 or 0.3.