Constant flow- vs. constant pressure-perfusion for studies of pulmonary vasoactive responses

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Abstract
Pulmonary vascular responses to a standardized hypoxic vasoconstrictor stimulus (FIO2 [inspired O2 fraction] = 0.02) obtained during constant volume inflow, with pulmonary arterial pressure as the dependent variable, were compared to those obtained during constant inflow pressure, with flow as the dependent variable. Isolated rat lungs were perfused at different baseline transvascular pressures. The experimental arrangement allowed changes between the 2 types of perfusion. Hypoxia at constant pressure perfusion gave a higher percentage rise in pulmonary vascular resistance (PVR) at all pressure levels. This advantage was more than offset by the findings that vascular closure (total or partial) often occurred, particularly below arterial pressure of 3 kPa [kilo Pascal unit], making detection of graded responses impossible, and the control situation was rarely regained. Responses obtained during constant flow were less reduced by elevations in baseline transvascular pressure, and the control situation was rapidly and completely regained. High altitude edema may be caused by precapillary occlusion of a major part of the vascular bed, thereby subjecting still-perfused regions to very high pressures and flow.