Slow Bidirectional Tachycardia

Report:

Bidirectional tachycardia 104/min

Retrograde VA conduction

Junctional escape beats

AV dissociation

Sinus rhythm 95 – 98/min

Overdrive (post-ectopic) SA suppression

Comment:

The rhythm is obviously bigeminal, with narrow rSR’ complexes followed by plump R complexes; both leave retrograde P waves in their wake. It is tempting to postulate reentry involving those retrograde P waves, but the cycle-length sequence and the QRS morphology does not support it. Any reentry present would be taking place down in the ventricles, involving the two bigeminal QRS complexes. These complexes both resemble – however remotely – RBBB; both are clearly different from the sinus rS complexes. If frontal leads were available, the bigeminal complexes would be likely to show alternating right and left axis deviation typical of bidirectional ventricular tachycardia. Its reentry has been shown to involve the two fascicles of the left bundle branch141. The strips show its typical appearance in V1 – except that most cases are faster.

At the end of the top strip, bigeminy ceases and a junctional escape complex appears (the last in the top strip); a dissociated sinus P wave may be contained in it. The next escape beat has a more definite sinus P wave at its onset. Sinus rhythm takes over from then on, showing the rSR’ beats for what they really were: fascicular ventricular ectopics. (They come after longer cycles, during bigeminy, than ordinary sinus beats during sinus rhythm – there is no reason for them to be RSR’-aberrant.) The lower strips show another run of bidirectional tachycardia, this time with both its onset and termination.

Most cases of this rare rhythm are due to digoxin toxicity. This one is an exception.

Another episode is shown in Fig 205b.