1. Reducing packed cell volume has been advocated as a therapeutic procedure in patients with polycythaemia secondary to hypoxic cor pulmonale. The aim of this investigation was to evaluate the effects of this manoeuvre on resting pulmonary haemodynamics and tissue oxygenation in 12 such patients.

2. The subjects were studied whilst they were breathing air (n = 12), after breathing 35% oxygen for 30 min (n= 11) and breathing air 30 min after isovolaemic reduction in packed cell volume, from 0.61 ±0.02 to 0.50 ±0.02 (mean ± sem), by erythrapheresis (n = 12). Initial values for the group were: Pao2 6.5 ± 0.3 kPa; red cell mass 152 + 12% predicted; mean pulmonary artery pressure (PAP) 41 ± 3 mmHg; cardiac index 3.1 ± 0.3 1 min−1 m−2.

3. Breathing 35% oxygen reduced PAP by 3.1 ± 1.0 mmHg (P < 0.02), cardiac index by 0.28 ± 0.121 min−1 m−2 (P < 0.05) and right ventricular stroke work by 0.05 ± 0.01 J (P < 0.01). Systemic vascular resistance was unchanged. Systemic oxygen transport increased and peripheral oxygen consumption was unaltered.

4. Erythrapheresis reduced blood viscosity at shear rates 23 s−1 and 230 s−1. PAP fell by 2.4 ± 1.1 mmHg (P < 0.05) and cardiac index increased by 0.32 ± 0.09 1 mm−1 m−2 (P < 0.01), but right ventricular stroke work was unchanged. Systemic vascular resistance was reduced by 25 ± 7 kPa s l−1 (P < 0.01). Systemic oxygen transport decreased but peripheral oxygen consumption was unchanged.

5. These results confirm the effect of oxygen in reducing PAP in patients with hypoxic cor pulmonale. Reducing blood viscosity by reversing polycythaemia also improved pulmonary haemodynamics but unlike oxygen its effects were not confined to the pulmonary circulation.

6. Although erythrapheresis reduced the oxygen carrying capacity of the blood, tissue oxygenation was not impaired at rest. However, oxygen transport during exercise was not assessed in this study.

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