Abstract
In 11 mongrel dogs (14.5 to 26.3Kg) during complete heart block and right ventricular pacing, the cardiovascular dynamics were observed with the effect of isoproterenol infusion (8μg/min intravenously). During complete heart block, cardiac output 2.01±0.17L/min, heart rate 57.6±3.5beats/min, and stroke volume 35.5±2.8ml. Left ventricular (LV) residual fraction was 54±2%. LV end-diastolic volume (EDV) in which almost complete filling is performed due to the long filling period was 4.01±0.35ml/Kg by thermodilution method. Right ventricular (RV) residual fraction and EDV were 39±3% and 3.01ml/Kg, which were significantly smaller than the left. Infusion of isoproterenol increased cardiac output (2.67±0.21L/min), heart rate (66.9±3.5beats/min), and stroke volume (40.4±3.1ml), but both ventricular residual fractions and EDV's did not change significantly, although mean blood pressure decreased remarkably. During right ventricular pacing the effects ofisoproterenol were essentially the same as in complete heart block except no change of heart rate and relatively improved cardiac functions.The plots of derived force-velocity relationship expressed in normalized mean circumferential shortening rate (MCSR) and peak ventricular force (F) showed that isoproterenol increases the velocity of shortening of muscle contraction significantly without significant change of F, while increase of heart rate did not. It is concluded that isoproterenol affects the myocardium mainly through its direct action and the improvement of contractile state via positive chronotropic effect (Bowditch's effect) is relatively small
