The ECG below was reported as sinus rhythm with intermittent bundle branch block.

The first complex is sinus rhythm with a narrow QRS. Then follows 10 broad QRS complexes; the first two and the last one have P waves preceding the broad QRS wave, but the PR intervals are shorter than the narrow sinus complexes. The cycle lengths of both the narrow and broad complexes are almost identical. The confusion resulted from the P waves preceding the broad QRS complexes, suggesting an intermittent bundle branch block. The broad QRS rhythm is an idioventricular rhythm (a ventricular run less than 60 bpm) in competition with sinus rhythm at an almost identical rate. Those broad QRS complexes with a preceding P wave are fusion beats; an amalgam of sinus QRS and a QRS from an ectopic ventricular focus. Neither of these QRS complexes are early enough to inhibit the other and both contribute to ventricular depolarization.The next illustration shows examples of idioventricular rhythm.

A, the first complex is also a fusion beat, albeit 95% sinus origin and the PR interval marginally shorter than the sinus beats at the end of the tracing. You can also see concealed P waves buried in the broad QRS (red arrows). The competition between the two pacemaker foci is dependent on sinus arrhythmia.B, the idioventricular rate is faster and this leads to a confusion in terminology. Some people may call this an accelerated idioventricular rhythm or even slow ventricular tachycardia. However, I prefer idioventricular up to 99 bpm. Again, there are several fusion beats with different contributions from each pacemaker as the sinus rate slows.

In the above ECG, the cycle lengths of the first five QRS complexes are near identical and these are junctional escape beats competing with last three sinus beats in which the cycle length now becomes slightly faster. This competition between two pacemaker foci, where the QRS complexes are identical is referred to as isorhythmic AV dissociation. Do the QRS complexes from the two foci have to be identical or could one be broader and originating from a focus within the ventricle? It is the same mechanism.Returning to our original ECG:The diagnosis of sinus rhythm with bundle branch block is clearly incorrect. There is a variable PR interval and fusion beats confirming two pacemaker foci.I call this an idioventricular rhythm with isorhythmic AV dissociation.Assoc Prof Harry Mond