Implications of Progressive Aberrancy Versus True Fusion for Diagnosis of Wide Complex Tachycardia

At the onset of wide complex tachycardia, beats with intermediate morphologies sometimes occur between the normally conducted beats and the wide complex tachycardia QRS. Intermediate beats could be true fusion; however, progressive aberrancy has been reported to mimic true fusion. To evaluate the incidence of progressive aberrancy, wide complex tachycardia tracings were collected in which an intermediate beat was noted at the onset. When the associated electrocardiographic findings were diagnosed as supraventricular tachycardia, the beat was identified as progressive aberrancy. When diagnosed as ventricular tachycardia, the intermediate beat was identified as true fusion. Electrocardiographic criteria were then identified from this cohort to identify the distinguishing features between progressive aberrancy and true fusion. Of 24 episodes of wide complex tachycardia, 17 (71%) were identified as true fusion and 7 (29%) as progressive aberrancy. The QRS duration of the intermediate and wide complex tachycardia beats were shorter with progressive aberrancy than with true fusion (109 ± 23 ms vs 131 ± 20 ms, p <0.023; and 139 ± 21 ms vs 177 ± 24 ms, p <0.001, respectively). In progressive aberrancy (n = 3), the PR interval of the intermediate beat was always greater than the PR interval of the normally conducted beat. In contrast, in true fusion (n = 11), the PR interval of the intermediate beat was always less than the PR interval of the normally conducted beat. Multiple intermediate beats were present in 4 of 7 cases of progressive aberrancy and in 0 of 17 cases of true fusion. In conclusion, true fusion is the most common explanation for intermediate beats, but progressive aberrancy occurs a significant proportion of the time (29%). The identified criteria will be helpful in differentiating ventricular tachycardia from supraventricular tachycardia with aberrancy as a cause of wide complex tachycardia.