Significance of coupling intervals of ventricular extrasystoles

Extrasystoles may have fixed or variable coupling intervals. The latter may be divided into those with coupling intervals related to and following the T wave of the preceding QRS (approximate coupling) and those which occur throughout electrical diastole with no obvious relationship to the T wave of the dominant rhythm (random coupling). Extrasystoles with random and approximate coupling may or may not be parasystolic. The relationship of extrasystoles to the dominant rhythm may alter. In particular, fixed coupling may precede or alternate with approximate coupling. These groups cannot be considered distinct, in view of transitions from one type of extrasystole to another. Extrasystoles with both approximate coupling and fixed coupling tend to occur just after the T wave and often occurred on the apex of the U wave. Approximate non-parasystolic coupling was infrequent in electrocardiograms which were otherwise normal. Random non-parasystolic coupling and parasystole only occurred in patients with cardiac disease. In view of these findings, it is proposed that extrasystoles should be described by their degree of dependence on each other (regularity of interectopic intervals) and their degree of dependence on conducted beats (regularity of coupling intervals),