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.

Fig 18a. VEBs, some in dimorphic couplets suspected of causing the pseudoblock in Fig 18.

Fig 19. 73 year old woman on Holter monitor following an episode of SVT.