Left and right ventricular volumes in A-V block, and their changes induced by the infusion of isoproterenol

In mongrel dogs (14.5 to 26.3 Kg) during complete heart block and right ventricular pacing, the cardiovascular dynamics were observed with the effect of isoproterenol infusion (8 mug/min intravenously). During complete heart block, cardiac output 2.01 plus or minus 0.17 L/min, heart rate 57.6 plus or minus 3. 5 beats/min, and stroke volume 35.5 plus or minus 2%. Left ventricular (LV) residual fraction was 54 plus or minus 2%. LV end-diastolic volume (EDV) IN WHich almost complete filling is performed due to the long filling period was 4.01 plus orminus 0.35 ml/Kg by thermodilution method. Right ventricular (RV) residual fraction and EDV were 39 plus or minus 3% and 3.01 ml/Kg, which were significantly smaller than the left. Infusion of isoproterenol increased cardiac output (2.67 plus or minus 0.21 L/min), heart rate (66.9 plus or minus 3.5 beats/min), and stroke volume (40.4 plus or minus 3.1 ml), but both ventricular residual fractions and EDV's didnot change significantly, although mean blood pressure decreased remarkably. During right ventricular pacing the effects of isoproterenol 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 increased the velocity of shortening of muscle contraction significantly without significant change of F, while increase of heart rate didnot. It is concluded that isoproterenol affects the myocardium mainly throughits direct action and the improvement of contractile state via positive chronotropic effect (Bowditch's effect) is relatively small.