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OBJECTIVES--To investigate the role of prostaglandins in maintaining circulatory homoeostasis in chronic heart failure and the hypothesis that an increase in vasodilatory prostaglandin synthesis may contribute to the actions of angiotensin converting enzyme inhibitors in heart failure. DESIGN--Randomised, double blind, placebo controlled studies. Cardiac output and renal and limb blood flow were measured after oral indomethacin 50 mg or placebo followed by "open" intravenous infusion of prostaglandin E2 (study A). In a second study the same measurements were made after oral indomethacin 50 mg or placebo was given 30 min before "open" captopril (study B). METHODS--Blood pressure was measured using a mercury sphygmomanometer. Cardiac output was determined by Doppler interrogation of blood flow in the ascending aorta and echocardiographic measurement of aortic root diameter. Renal blood flow was calculated from the effective renal plasma flow measured by p-aminohippurate clearance and the haematocrit, and glomerular filtration rate by endogenous creatinine clearance. Limb blood flow was measured by venous occlusion plethysmography using mercury in silastic strain gauges. The concentration of plasma prostaglandin E2 was measured by radioimmunoassay. SETTING--University department of cardiovascular medicine. PATIENTS--12 patients with chronic stable heart failure before starting treatment with angiotensin converting enzyme inhibitors. RESULTS--Indomethacin resulted in adverse effects on cardiac output, systemic vascular resistance, renal blood flow, glomerular filtration, urinary sodium excretion, and calf vascular resistance. Changes were reversed with infusion of prostaglandin E2. Pretreatment with indomethacin resulted in the attenuation of the acute increase in cardiac output and decrease in systemic vascular resistance that occurred with captopril. Similarly, an increase in renal blood flow with captopril was attenuated by indomethacin. CONCLUSIONS--The acute adverse effects of indomethacin on central and peripheral haemodynamic and renal function suggest that prostaglandins have a significant role in the regulation of peripheral blood flow and renal function in patients with stable chronic heart failure. The attenuation by indomethacin of captopril induced improvements in haemodynamic function and renal blood flow is consistent with the hypothesis that captopril may act in part via an increase in prostaglandin synthesis.