Factors Influencing the Time of Appearance of Premature Systoles (Including a Demonstration of Cases with Ventricular Premature Systoles due to Re-entry but Exhibiting Variable Coupling)

The genesis of the most common cardiac arrhythmia, that due to premature systoles, still remains obscure. Observations made upon two patients with ventricular premature systoles are reported because of some unusual features of their timing which shed some light upon the mechanism behind the appearance of premature systoles, regardless of the extrasystolic impulse. The theories of the extrasystolic irregularities are reviewed with particular attention to the criterion of "fixed coupling" generally used to distinguish between premature systoles explained by a re-entry mechanism and those explained by a parasystolic pacemaker.