Concealed Extrasystoles & Pseudoblock

Report:

Multifocal atrial rhythm

VEBs, multiform

Fusion beat

Second degree AV block, probable pseudoblock

Comment:

This would have been a multifocal atrial tachycardia (MAT) if the rate was over 100/min. AV block is uncommon in MAT and should be even less common at slower rates. The blocked atrial impulses are not particularly early - hence not analogous to blocked SVEBs. When an unexpected block occurs in the presence of multiple extrasystoles one should always think of concealed extrasystoles causing it – a pseudoblock.

In this case, there are two basic types of extrasystoles, both in a variety of shapes: a RBBB type, and a LBBB type. Below, it can be seen that they tend to come in couplets. If the first (LBBB-type) one were to be both antegradely and retrogradely blocked from its site of origin, it could be manifest only as a blocked P wave. The RBBB-type extrasystole would still show up "as usual", but after a considerable pause from the last supraventricular QRS complex. If one "timed" the LBBB-type extrasystole (using the couplets shown below) in front of the RBBB-type one in the original strips, it would fall exactly where it would block a P wave.

The above disquisition remains, in the absence of a His-bundle recording, conjectural. A more secure example is shown elsewhere in this Library.