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.
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.
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