Posture determines the nature of the interaction between angiotensin converting enzyme inhibitors and loop diuretics in patients with chronic cardiac failure

Abstract The effects of inhibition of the renin angiotensin aldosterone system on the natriuretic and diuretic actions of an intravenous dose of frusemide 40 mg in patients with chronic cardiac failure maintained on oral diuretics were studied in the supine and erect positions. In the patients studied in the supine position the total 4 hour diuresis was decreased from 995 (92) ml to 668 (66) ml and the total 4 hour natriuresis fell from 105 (14) mmol to 67 (14) mmol following the administration of captopril. Creatinine clearance fell from 87 (8) ml/minute to 52 (15) ml/minute. In the patients studied in the erect position the total 4 hour diuresis was 596 (87) ml without captopril and 562 (83) ml with captopril. Total 4 hour natriuresis was 71 (13) mmol without captopril and 65 (9) mmol with captopril. Creatinine clearance was reduced by captopril from 82 (7) ml/minute to 47 (12) ml/minute. The reduction in the diuretic and natriuretic response to frusemide caused by captopril in the supine position is mediated through a fall in glomerular filtration rate. However, in the erect position, which is associated with even further increases in activity of the renin angiotensin aldosterone system, the reduction in diuresis and natriuresis that a fall in glomerular filtration rate would cause is offset by abolition of the rise in sodium retaining hormones, angiotensin II and aldosterone that mediate the antinatriuretic effect of the erect position.