Effects of a New Angiotensin Converting Enzyme Inhibitor, Enalapril, in Acute and Chronic Left Ventricular Failure in Dogs

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Abstract
Enalapril (MK‐421) is a new angiotensin converting enzyme inhibitor which effectively lowers elevated blood pressure and might also be useful in heart failure. Enalapril was infused into six awake dogs 2 hours after left circumflex coronary artery embolization (acute failure group) and into six other awake dogs two to six months after coronary embolization (chronic failure group). In the acute failure group 2 hours after embolization, increased left ventricular end‐diastolic pressure and reduced cardiac output remained unchanged during enalapril infusion. In the chronic failure group, increased left ventricular end‐diastolic pressure also remained unchanged during enalapril infusion, but cardiac output, which had fallen to 131.8 ± 11.9 (S.D.) from 165.8 ± 17.9 ml/min/kg (P < 0.01) by two to six months in this group rose during enalapril infusion to 154.5 ± 27.7 ml/min/kg (P < 0.05). Heart rate and blood pressure were not changed during enalapril in either group, but stroke volume rose (26.0 ± 5.9 to 29.2 ± 6.9 ml, P < 0.01) and systemic vascular resistance fell (58.5 ± 10.3 to 39.3 ± 4.3 units, P < 0.01) during enalapril only in the chronic failure group. Plasma renin activity after embolization was slightly but not significantly higher in the acute failure group. Thus, enalapril appears to be an arterial vasodilator in dogs with chronic but not acute left ventricular failure.