Tachycardia with Complete & Incomplete LBBB

Report:

Atrial flutter with 2:1 AV block.

Alternating complete & incomplete LBBB.

Primary T wave changes.

Comment:

This is another form of bidirectional tachycardia. It is also an example of double 2:1 block: one in the AV node (for atrial flutter) and another for the left bundle branch.

The ventricular rate is regular at 156/min but the complexes alternate between complete and incomplete LBBB morphology. The complete LBBB has the same morphology as in sinus rhythm (below). The differential diagnosis should include fusion VEBs in bigeminy. Flutter waves are best seen in L2 strip. This rhythm is very sensitive to electrical shocks and low-current discharge should be chosen for “starters”. Bets were won by how few Watt/seconds (Joules) would suffice.

A bundle branch block is called complete by convention when the QRS exceeds 0.12", although one can never be sure just how "complete" it is: the QRS can always widen further. In fact, one does not know with any certainty that the "block" is an actual block, let alone whether it is, anatomically, at a bundle branch level: all we observe is the asynchronous activation of the ventricles. The complete LBBB in this example has LAD (found in 50% of LBBBs) while the incomplete one has normal frontal plane axis.

The primary T wave changes (T unexpectedly upright in V5-6 or lead 1) are better seen during slower sinus rhythm (Fig 85a). They imply myocardial disease of any ætiology additional to the conduction problem.