Regulation of contractile force during ventricular arrhythmias

Causes of fluctuations in ventricular pressure during arrhythmias were studied in anesthetized dogs. Excitability and contractility were measured throughout the cardiac cycle and their relationship to the problem was assessed. The strength of premature contractions was found to vary inversely with prematurity. Weakness of premature beats could not be ascribed to subnormal excitability nor to insufficient ventricular filling. Contractility was virtually zero at the end of the total refractory period, increased progressively thereafter and was fully restored only at the very end of the cardiac cycle. Sensitivity of contractile force to increments in ventricular filling paralleled the return of contractility. The strength of the postcompensatory contraction was always enhanced, this potentiation increasing as a linear function of the extrasystole's prematurity. Postextrasystolic potentiation was attributed to the extrasystole itself rather than to the pause which follows it or to increases in ventricular filling and distention. It was therefore concluded that pressure fluctuations during ventricular arrhythmias are not due to subnormal excitability and would occur independently of variations in ventricular filling.