Bidirectional AIVR
Report:
Alternating (bidirectional in some leads) accelerated idioventricular rhythm
Comment:
At first glance, the trace suggests RBBB and ventricular bigeminy. It is quite regular and no definite atrial activity can be discerned, except for the wrinkle after the QRS in lead 3. It could be junctional (or main-stem) rhythm with VEBs in bigeminy, but the regularity of the ventricular cycles is somewhat against it: there are no premature beats.
At a faster rate, this could pass for an atypical form of bidirectional VT. The latter is, according to some theories, only half ventricular in origin. Most of those have alternating cycle length as well, anyway.
The QS complexes in the inferior leads may suggest inferior infarction, which the patient in fact had (below, Fig 220a), later to become inferolateral infarction.
In this setting (of reperfusion and inferior MI) the most likely diagnosis is AIVR, bidirectional from two foci or two conducting pathways . An idiofocal phenomenon (pace Schamroth). Does it matter? It does: idiofocal rhythms are tolerated, while bigeminal (or even frequent) VEBs would still, in most units, be treated in the wake of acute infarction.
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