How would you interpret the three successive lead MCL-1 rhythm strips shown in the figure below?

This is a fascinating tracing. P waves march out through the entire tracing (red arrows). There is some conduction. That said, there are two different QRS complexes, and the PR interval is not the same in front of all conducting beats.

Start with what you know. Focus on the middle and lower tracings (Panels B and C). Beats 1, 2, 12, 13, and 14 are all preceded by a similar-looking P wave with a constant PR interval. This tells us that these beats clearly are conducted.

There is no 12-lead ECG on this patient, which means that our assessment of QRS morphology is limited to this single right-sided MCL-1 monitoring lead. That said, the QRS complex for all beats on this tracing looks to be widened. The predominantly negative rS configuration of beats 1, 2, 12, 13, and 14 is consistent with left bundle branch block (LBBB).

Beats 3, 4, 5, 6, 7, 8, 9, 10, and 11 also appear to be conducted as the PR interval preceding these beats looks to be constant. However, QRS morphology of these beats suggests a change to right bundle branch block (RBBB) conduction. If confirmed on a 12-lead, this would mean there is alternating bundle branch block (ABBB).

There is also 2:1 AV block in some parts of this tracing. Interestingly, second-degree AV block with 2:1 AV conduction occurs in association with the QRS complexes manifesting LBBB (i.e., beats 1, 2, 12, 13, and 14). In contrast, 1:1 AV conduction occurs in association with the QRS complexes manifesting RBBB (i.e., beats 3, 4, 5, 6, 7, 8, 9, 10, and 11).

There are some additional confounding findings on this tracing. These relate to a highly unusual pattern of variation in the PR interval in which the PR interval preceding LBBB beats is different from the PR interval preceding RBBB beats. In short, we cannot explain all findings on this tracing. That said, what is apparent is that there is second-degree AV block with significant bradycardia and intermittent 2:1 AV conduction. Also, there is a pattern of ABBB, which almost always indicates severe His-Purkinje disease. A pacemaker almost certainly will be needed.

Additional information about this case can be found at:

ECG Figure