Skipped and Blocked SVEBs

Report:

Sinus tachycardia 100/min

Blocked atrial premature beats interrupting Wenckebach sequences

Over the top conduction (skipped P’ waves)

Post-ectopic sinoatrial depression

VEB

Left atrial abnormality (LAA)

Small voltage in frontal leads

Nonspecific ST/T changes

Comment:

Sinus P waves are conducted with PR intervals 0.24” to 0.48”, inscribing typical Wenckebach sequences, with slight aceleration of the distal chamber due to decremental increases in the conducted PR intervals. There are no “dropped beats” because atrial premature waves, P’s, are in front of the last QRS before the pause. There are two instances of this. Both P’R intervals are much shorter than any of the conducted PR intervals – they are in fact blocked and the ensuing QRS is caused by the preceding sinus P wave.

The pauses are thus created by marked SA suppression following premature P’ waves (overdrive supression) rather than Wenckebach 2o AV block. The third P’ (after the third last QRS complex) is blocked, but not skipped, since it occurs much earlier in the QRS cycle and there is no preceding sinus P wave to skip over it. This is is a common occurrence, Marriott’s commonest causes of pauses98. The situation with the first two P’s has not been seen before, by me at least. These P’s are subject to both insult (being skipped over) and injury99 (being blocked)!

There is always a differential diagnosis in medicine. In this case, this would be supernormal conduction of the atrial (positive P’) premature impulses instead of over the top conduction of sinus P waves. Most unlikely, but possible, perhaps via different pathway.

The VEB has the typical slow descent in V1. It has retrograde conduction, since its pause is longer than fully compensatory. Rhythm-wise it behaves like a late SVEB.

Fig 154a is a trace taken minutes later, very similar except for a couplet of P’s sandwiching the third last QRS; the second P’ is conducted, with the expected long P’R interval, producing the penultimate QRS. The last PR interval is long, refleting the proximity of its P wave to the penultimate QRS complex.