Blocked P Wave Long After a SVEB

Report:

Sinus rhythm 79/min

First degree AV block

PR 0.24”

SVEB (Atrial premature beat)

Scond degree AV block, unspecified

Nonspecific ST/T changes

Post-ectopic ST/T changes, postponed

Comment:

Is it significant that the unexpectedly blocked P wave is in the cycle after the one containing the SVEB? Yes: it illustrates basic electrophysiology.The refractoriness of any part of the conduction system is proportional to the length of the preceding cycle. The commonest expression of this is long-short cycle sequence resulting in RBBB, as in Ashman’s phenomenon in AF. The one here is rare: AV block pushed from first degree to complete block due to the pause engendered by the SVEB. The SVEB is atrial in shape and long P’R interval.

Or, a coincidence? AV block out of thin air? Well, below (Fig 151a, 151b) are four more instances of the same phenomenon. This patient had a first degree AV block to start with, a rather sinister finding in aortic endocarditis. It is associated with ring abscesses spreading to involve the AV node97.

Further two pages, from a 77 year old woman, show SVEBs with equally peculiar consequences: immediate block of the next sinus P wave, with two escape beats, perhaps from the distal left bubdle branch (they look like RBBB, whereas the conducted supraventricular beats are all LBBB). There are a few examples in this Library of VEBs doing this, but this is the only one I know of involving SVEBs. The explanation is rather vague and speculative: reentry within the junctional region making it refractory for the next sinus P wave. The PR intervals are somewhat variable throughout the recordings; multiple explanations could be adduced for that (but not here). Pseudoblock is the most likely.