Un Unusual Morphology

Report:

Sequential AV pacemaker (known ICD) rhythm 59/min 2

Composite (? Fusion) QRS morphology 8

Comment:

The ventricular complexes start as expected, with an initial slur of myocardial activation lasting approximately 0.08”. The second half of each complex, for another 0.08”, appears like a completely normally conducted QRS! This is best seen in leads V1 and V2, where presumptive septal r waves initiate the normal half of each QRS. Even the repolarisation changes could “pass” for those of a supraventricular QRS complex.

One is left with speculation: the atrial-paced P wave finds its way through the AV node while the more pedestrian ventricular-paced QRS travels through the myocardium and the supraventricular impulse captures the rest of the ventricle through the fast His-Purkinje network. The resulting QRS is, then, a fusion beat, a hybrid complex, analogous to WPW conduction of sinus rhythm.

Alternatively, the ventricular-paced impulse gets to the His-Purkinje system after the initial slow travel through the myocardium and captures the rest of the ventricle through a normal activation front, while the atrial impulse is still negotiating the AV node.

The answer is not forthcoming. The patient’s ICD did not work well and he was sent to Sydney for further EPS. He died there, suddenly. I still wonder if his putative dual pathway had something to do with his predicament.