Mechanisms of fixed coupled ventricular ectopic complexes.

Abstract

Mechanisms of fixed coupled ventricular ectopic complexes (VECs) were studied in the monitoring ECGs of 7 patients with the aid of a special purpose computer. Over 80% of the 4500 fixed coupled VECs studied had coupling intervals restricted to a range of 120 ms. The actual coupling interval varied between subjects. No special part of diastole exhibited the property of fixation. In five patients there was evidence of parasystole despite the fixed coupling. In two of these the ectopic period was clearly different from the sinus period. In the other three a difference was evident between inter-ectopic and sinus intervals, but the ectopic period could not be determined due to absence of short inter-ectopic intervals. Absence of some intervals was due to exit block or consistent coincidence of ectopic stimuli with ventricular refractoriness. In all five patients the inter-ectopic intervals showed progressive adjustment so as to become equal to a multiple of the sinus period. The coupling interval, which indicated the preferred phase relationship of the two rhythms, was altered by small changes in the sinus period. These adjustments were greater manifestations of the synchronising effects, already described, of sinus rhythm on a ventricular parasystolic rhythm. By contrast the VECs of two other patients exhibited very different characteristics. In one, the VECs restricted to 40 ms after the T wave in early diastole were probably due to ventricular re-excitation. In the other, runs of idioventricular rhythm and VECs in late diastole were due to an escape pacemaker rhythm.