Haemodynamic effects of amrinone in the anaesthetized pig

The vasodilator and inotropic actions of amrinone were investigated in mini-pigs under pentobarbitone anaesthesia. Left ventricular volume was determined angiocardiographically under afterload and isovolumetric conditions. Furthermore, aortic flow, left ventricular pressure and aortic pressure were measured. In some of the animals, the beta-adrenergic receptors were blocked with propranolol prior to the administration of amrinone. Without blockade of the beta-receptors, amrinone (2 mg kg-1) caused a significant reduction in mean aortic pressure. Due to less end-diastolic ventricular filling, stroke volume decreased, and thus ejection fraction remained constant. Since heart rate increased under amrinone, cardiac output remained constant. At the same time, the maximum rate of pressure rise increased, despite less end-diastolic ventricular filling. After blockade of the beta-adrenergic receptors, aortic pressure, end-diastolic ventricular filling, and stroke volume also decreased with amrinone. In contrast, heart rate remained practically constant, so that cardiac output declined. The maximum rate of pressure rise also declined due to less end-diastolic ventricular filling. It can be concluded from these results that, in situ, the primary action of amrinone occurs on vascular smooth muscle and that a positive inotropic activity with a normal dosage of amrinone is only an indirect outcome of reflex activation of the sympathetic system. Analysis of isovolumetric mechanograms and the ejection phase does not indicate a direct positive inotropic effect of amrinone. In the failing heart, however, beneficial effects can be expected, since the maxima curves follow a flatter course. Thus a reduction in afterload can lead to a significant increase in stroke volume, provided that aortic pressure does not fall below the critical coronary perfusion pressure.