Experimental Studies on Myocardial Contractility and Hemodynamics in Extrasystoles

Extrasystoles with a fully compensated pause were experimentally induced in dogs for analysis of their effects on myocardial contractility and hemodynam-ics. The first postextrasystolic beat showed improved hemodynamic measurements exceeding the respective values of the pre-extrasystolic beat of the basic rhythm, making some compensation for extrasystole-induced depression of hemodynamics, which was progressively enhanced as the coupling time be-came shorter. The coupling time, the site of origin, and even the frequency of the extrasystole did not affect the rate of compensation, which averaged 60%. It was confirmed that hemodynamics in extrasystoles was mainly affected by two independent factors; the blood volume in the ventricle at the onset of ventricular contraction with the resultant STARLING'S effect, and ventricular contractility for individual beats, the latter of which depends upon myocardial contractility and the sequence of ventricular contraction. Postextrasystolic potentiation was regarded as the positive inotropic effect of the premature activation on the myocardium in postextrasystolic beats.