Entrainment of ventricular tachycardia by atrial depolarizations

Four patients in whom rapid atrial pacing resulted in transient entrainment of a sustained ventricular tachycardia (VT) are reported. Ventricular fusion was seen in 3 of the 4. The following new observations were made: (1) A single atrial depolarization resulted in ventricular fusion and resetting of a VT, while atrial pacing at a faster rate entrained the same VT but without detectable fusion. This suggests that fusion during entrainment may be a rate-dependent phenomenon. (2) The interval between the last paced beat and the first nonpaced VT beat was different from the pacing cycle length in 3 patients. Two mechanisms accounted for this: the initial forces of each entrained QRS occurring as a result of the pacing wavefront, with fusion taking place only during the terminal forces, and the last entrained cycle exceeding the pacing cycle length by an amount related to the nonfused portion of the QRS, and delay in the presumed reentrant circuit responsible for the tachycardia. One VT was entrained with atrial pacing while ventricular pacing at the same rate resulted in termination, suggesting "site specificity" for termination. It is concluded that entrainment can occur without the criteria previously described as characteristic of it and that additional phenomena may be observed after stimulation that further support reentry as the mechanism of VT.