Dig the Break!

Report:

Sinus rhythm 96/min

Second degree AV block, 2:1[!xe "AV block:concealed:dig the break" \b!]

SVEB[!xe "SVEBs:unmasking 2o AV block" \b!]

Left bundle branch block

Comment:

The blocked alternate P waves are almost completely hidden by the descending limb of the T wave; there is just a soupçon of a notch as it reaches the baseline. All is revealed, however, in the break provided by the SVEB! Dig the break! says Marriott31.

If the P waves are mapped backward, it can be seen that the premature complex cannot be a conducted sinus beat - it is a SVEB. The strip below (Fig 48a) shows, at the beginning of the upper strip, a conducted sinus beat: it has to be later than the SVEB in the original strip for its P wave to be conducted.

It also shows Möbitz 2 mechanism of the AV block (unless the very first QRS in the upper strip, whose P wave is partly cut off, is a SVEB). This makes it even more likely that the combination of LBBB and second degree AV block represents a true bilateral bundle branch block (BBBB). The AV block is, then, 2:1 block of the conducting (right) bundle branch.