Lean Children of Fat Parents

Report:

Sinus arrhythmia

? Sino-atrial exit block, Möbitz I

Right bundle branch block

Pacemaker escape beats

Ventricular fusion beats

‘Normalisation’ of QRS

Comment:

Both ‘normal’ complexes (one in each strip) and the small RBBB complex (6th in the top strip) occur exactly when both the sinus and the escape (pacemaker) beats are likely to capture the ventricle. This is what fusion is: incomplete capture or its obverse - incomplete dissociation! The two broad complexes fuse to synchronise the ventricular activation sufficiently for narrow(er) beats to occur. Two wrongs making a right, as Schamroth would put it.

This superficially resembles rate-dependency for the RBBB. It would pass for one, were it not for the LBBB-like pacemaker escape beats occurring at le moment critique. The escape cycle is longer than the pacing R-R interval: the unit is programmed for rate hysteresis, allowing for more time in sinus rhythm.

The patient showed no teal rate-dependency of his RBBB during long pauses (Fig 7a) recorded before the pacemaker insertion.

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