Characteristics of ventricular extrasystoles and their prognostic importance: a reappraisal of their method of classification.
The concept of two different types of extrasystoles, parasystolic and coupled, depends upon two distinguishing characteristics of these beats. The characteristics of the parasystolic extrasystoles are the invariability of the ectopic cycle together with their independence from the basic rhythm. Coupled extrasystoles demonstrate a dependence upon the basic rhythm although they may express some degree of ectopic variability. The degree of variation of the interectopic interval or its common denominator measures the irregularity of the ectopic parasystolic rhythm. The variation of coupling intervals describes the dependence of the ectopic beat upon the basic rhythm. In a study of 719 electrocardiograms with ventricular extrasystoles, about one-third of the extrasystoles appeared intermediate between these types since they had both variable coupling intervals and variable interectopic intervals. Some of these had total variability of coupling intervals and of the interectopic intervals (random non-parasystolic coupling), and others had limited variation of coupling intervals when expressed in relationship to the total duration of electrical diastole (approximate non-parasystolic coupling). Both these ectopic types appeared to be associated with cardiac disease, and repetitive ventricular extrasystoles. Left bundle branch type extra-with fixed coupling. There were no obvious relationships between the contour and the type of coupling of ventricular extrasystoles. Left bundle branch type extrasystoles with vertical or right axis were the most frequent, particularly in normal subjects, but in the presence of cardiac disease there were more electrocardiograms with right bundle branch type extrasystoles. Extrasystoles in the presence of underlying conduction defects were usually of opposite configuration to this defect. The contour of uniform extrasystoles did not appear to predispose to serious ventricular arrhythmias but multiformity of extrasystoles was an important prognostic indicator. It is suggested that variability of contour and coupling are important signs of inhomogenous conduction and may precede the onset of severe ventricular arrhythmias. Random nonparasystolic coupling and marked multiformity indicate a more sinister arrhythmic state.