1. Polycythaemia was corrected by erythrapheresis in ten patients with hypoxic cor pulmonale who were stable on regular diuretic therapy. Renal haemodynamics, renal function and the renin-angiotensin-aldosterone system were assessed before and afterwards.

2. Before erythrapheresis effective renal plasma flow (ERPF) was reduced (63% predicted) but glomerular filtration rate (GFR) was preserved (88% predicted) by a rise in filtration fraction (FF) (138% predicted). A negative correlation existed between ERPF and packed cell volume (r = −0.723; P < 0.02) and also between ERPF and Paco2 (r = − 0.710; P < 0.05). Polycythaemia was sufficient to maintain renal oxygen delivery (97% predicted).

3. After erythrapheresis systemic blood pressure, blood volume and blood viscosity all decreased. ERPF increased by 18% (P < 0.02). FF fell by 11% (P < 0.05) and GFR was unchanged. Renal oxygen delivery diminished by 25% (P < 0.001).

4. Plasma renin activity was increased in five patients and plasma aldosterone increased in two patients before erythrapheresis. No sustained fall occurred in plasma renin activity or plasma aldosterone, possibly because the haemodynamic consequences of the procedure had opposing actions on renin secretion.

5. Although the reduction in FF would per se tend to enhance renal sodium and water excretion, a diuresis or natriuresis did not occur consistently.

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