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A rat model was used to assess the viscosity factor in pulmonary hypertension of high altitude. Rats exposed to 10% O2 for three weeks developed increased pulmonary vascular resistance (p.v.r.) and polycythaemia; the haematocrit (Hct) was 50-60%, values similar to those in normal men at high altitudes. The contribution of high Hct to the increased p.v.r. was assessed in isolated perfused lungs of chronically hypoxic rats perfused with their own high Hct blood, or normal Hct blood from control rats. Pressure/flow relationships were measured over a wide range and the slope (P/Q) of this relationship and its extrapolated intercept on the pressure axis were increased by high Hct blood. A return to low Hct blood did not restore initial conditions although a second perfusion with high Hct blood again increased p.v.r. and intercept. Lack of reversibility was attributed to changes with time in blood or lung. In a second experiment designed to eliminate time changes, chronically hypoxic or litter-mate control rats were each perfused with only one blood, their own or each other's and P/Q relations were rapidly measured. The P/Q slope and pressure intercept increased progressively in the following groups: control rats perfused with their own blood (Hct 34%), control rats perfused with chronically hypoxic blood (Hct 56%), chronically hypoxic rats perfused with control blood (Hct 35%) and chronically hypoxic rats perfused with chronically hypoxic blood (Hct 53%). To exclude factors in chronically hypoxic blood other than high Hct which might increase p.v.r., control rats were perfused with blood of different Hct obtained by centrifugation. High Hct again increased p.v.r. There was a significant relationship in all rats between pulmonary artery pressure (Ppa), which takes into account both P/Q slope, intercept and Hct. There was substantial batch variation which may reflect sensitivity to hypoxia. In chronically hypoxic rats with high Hct blood, Ppa varied from 29-47 mmHg; with low Hct blood the range was 26-38 mmHg. Comparable values for control rats were 21-29 and 17-20 mmHg. We conclude that the polycythaemic blood of chronic hypoxia contributes substantially to pulmonary hypertension. Where it is excessive, it may prejudice tissue blood flow.
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