Ventricular Fusion Beats

Report:

Sinus rhythm

First degree AV block

PR 0.28”

VEBs, possible parasystole

Ventricular fusion beats

Left bundle branch block

QRS 0.20”

Comment:

The criteria for diagnosing ventricular fusion beats were formulated by Marriott et al18 in 1962:

Contour and duration must be intermediate between those of the competing pacemakers; Timing is consistent with inherent rhythmicity of both pacemakers19; P-S or P-J time must not be shorter than the PR interval of the sinus beat, which must at lest reach the ventricles before they are fully activated by the ectopic beat; The PR interval of the fusion beats must be equal to, or shorter, than the sinus PR interval, but shorter by not more than 0.06” – time required to depolarise the ventricles entirely by the ectopic beat. Fusion can only take place during the first 0.06” of the ectopic beat, which by then reaches the His bifurcation and thereby blocks the sinus impulse. When the PR of the fusion beat is shorter than the sinus PR interval, the initial vector of the fusion beat will be different from that of the sinus beat: fusion beat is initiated by the ectopic beat. If the PR intervals are of same duration, the initial vector may or may not be different from that of the sinus beat. The terminal vector is always different.

The exceptions occur with in the presence of a bundle branch block with VEBs arising at the site of the block; with varying sinus PR intervals; and with parietal intraventricular conduction delays.

This trace has only one “pure” VEB: the second complex in the bottom strip, preceded by an obviously dissociated P wave. All the others are fusion beats. The longer their PR intervals, the more they resemble the LBBB sinus beats: the 4th beat in the top strip has the same 0.30” PR interval as the sinus beats and resembles them closely. Its initial vector is the same as that of sinus beats, too.

The shortest fusion PR interval is associated with the 4th beat in the bottom strip: 0.18”; the flanking sinus beats are conducted with PR intervals of 0.28”. The fusion PR is shorter by a hefty 0.10”. This is one of the exceptions: the VEBs take more than 0.06” to completely depolarise the ventricles. Both the broad LBBB and the multinotched VEBs indicate advanced myocardial disease in this patient.

More examples are seen in Fig 39a. Another strip is shown further below (Fig 39b): its 6th complex (arrow) is near-normal in duration.

Fig 39a. Further examples of fusion.

Fig 39b.

Fig 40. 62 year old man following repair of ruptured abdominal aortic aneurysm.