Alternating RBBB & LBBB?

Report:

Sinus rhythm

Second degree 2:1 AV block

Alternating RBBB & LBBB

Discordant PR intervals

Left axis deviation -35o (in RBBB)

Non-specific ST/T changes (during RBBB)

Or:

Conducted RBBB with 2:1 and advanced 2o AVB

Ventricular escape beats

Escape-capture bigeminy

Comment:

This is similar to the preceding Case 11.

The alternation holds true for most of the trace, but there are periods of consecutive RBBB complexes. Each BBB has its own fixed PR interval. The blocked P waves represent a bilateral BBB. This is one of its several possible electrocardiographic presentations.

An alternative explanation is that the LBBB complexes are not conducted (with first degree AV block), but are in fact escape beats from the distal right bundle. The LBBB beats all terminate longer pauses than do the RBBB beats. The fixed PR interval would then be fortuitous, being part of a fixed QRS-to-QRS escape interval. This cannot be discounted unless one observed a period of sustained LBBB conduction as well. This is in fact more likely: an escape-capture bigeminy whenever the AVB extends beyond 2:1 ratio.