AV Dissociation with Interference

Report:

Sinus rhythm 1

Borderline right atrial abnormality (RAA) 1

SVEB 1

Ventricular demand pacemaker rhythm 72/min 2

AV dissociation with interference 2

Fusion beat 2

Nonspecific ST/T changes 1

Comment:

The sinus P waves march through the first four cycles until an appropriately timed one captures the ventricle from the pacemaker. This is not (as sometimes said) a sinus capture, it’s a ventricular capture beat (by the sinus impulse). Another obvious thing that need repeating is that capture beats are always early! It’s all in Marriott (9th and later editions excepted). The capture is effected with 1o AV block, reflecting concealed retrograde conduction of the 5th paced beat to the AV node.

The AV dissociation – independent beating of the atria and the ventricles – isn’t complete. The terminology here is rich and confusing. I prefer to use the traditional – and descriptive – AV dissociation with interference.

The 9th complex is a fusion beat, with a preceding P wave, shorter-than-sinus PR interval and morphology intermediate between a pure ventricular and pure sinus QRS. In V1 it demonstrates its provenance (in part, anyway) from the right ventricle by having slurred S descent and a much sharper S ascent. The next beat is a pure paced beat but the lead transition prevents its proper display in V1.

Fusion beats can be regarded as partial dissociation or, perhaps, partial capture, or both!

The SVEB, which returns the control to sinus rhythm at the end of the tracing, is easy to overlook; without mapping out the P waves, it would look like another capture by sinus rhythm. The following sinus P wave is terminating the longest cycle in this ECG, being subject to post-ectopic SA depression.