Double, Perhaps Triple Tachycardia

Report:

Bidirectional tachycardia 158-162/min[! XE "Bidirectional tachycardia" \t "See Ventricular tachycardia, bidirectional" !]

Suggestive of digoxin toxicity

Dissociated atrial tachycardia with block

Comment:

This is the classical form of bidirectional tachycardia, almost always due to digitalis toxicity. This patient’s digoxin level was 3.4 nmol/L (therapeutic range: 1.0 – 2.6) while serum potassium was 3.2 mEq/L (reference range 3.5 – 5.0). The scenario is also typical: an elderly patient continues to take (or is given) the prescribed dose of digoxin in the face of intercurrent illness which impairs renal function.

The characteristic configuration involves a positive QRS in V1 resembling RBBB with what looks like alternating hemiblocks in the limb leads. In this case, the alternating RBBB-like morphology is perhaps better appreciated in lead V6, with almost uniform R waves but alternating S wave width. After the initial few cycles, all the complexes show retrograde conduction – the sharp positive deflections in lead V1.

An alternative diagnosis would be a partially or entirely dissociated atrial tachycardia with block – another hallmark of digoxin toxicity. In that case, the small or negative complexes in V1 would represent fusion beats – an entirely credible morphology between the predominantly positive QRS in that lead and LBBB which this patient showed in junctional rhythm 5 minutes later (below, Fig 6a) and, later still, in restored sinus rhythm (not shown). The trace below showed an accelerated junctional rhythm 78 – 84/min (also characteristic of digoxin toxicity) completely dissociated from a completely blocked atrial tachycardia 150/min and interrupted by two RBBB-like complexes just before the end of the recording. Those two complexes resemble the complexes alternating in the original bidirectional tachycardia. The original trace can be viewed as double, even triple6, tachycardia! Its components are the bidirectional VT, the accelerated junctional rhythm and the (completely blocked, or at least dissociated) atrial tachycardia. Full hand for digoxin.