Abstract
Increases in cardiac output were directly related to heart rate and inversely related to the control heart rate; the stroke volume generally remained constant. These changes were associated with a fall in total peripheral resistance, arteriovenous O2 difference, and left and right atrial pressures, a rise in venous O2 content and left ventricular minute work, and no change in the left ventricular stroke work and arterial blood pressure. Isoprenaline effectively lowered the pulmonary vascular resistance when raised. The administration of higher doses frequently resulted in arrhythmias; the appearance of nodal rhythm or ectoplc beats caused a fall in cardiac output desplte the increased isoprenaline dosage. The circulatory effects of isoprenaline following homograft replacement of the aortic valve are predominantly chronotroplc and weakly inotroplc.