Unfinished Wenckebach

Report:

Sinus rhythm 76/min

Wenckebach phenomenon

Atrial trigeminy, possible reentry beats of sinus origin, blocked

Frequent VEBs, uniform

Positive concordant precordial pattern

Left atrial abnormality (LAA)

Left anterior hemiblock (frontal axis –60o)

Late transition

Nonspecific ST/T changes; possible LVH

Comment:

There is a regular atrial trigeminy throughout. The PR interval of every second conducted sinus beat lengthens, indicating Wenckebach conduction. The next sinus beat is replaced by a retrograde P wave, premature with respect to the sinus rate. It is almost certainly an echo beat of sinus origin.

The VEBs introduce a complication in that there is no normal QRS complex between the second sinus beat (buried in the onset of the VEB) and its retrograde successor. However, the latter keeps its distance from the preceding sinus P wave – inferred as much as actually seen - as if the VEB never happened. The reentry circuit does not require ventricular participation. Below is another ECG, taken a few minutes earlier, with only a single VEB (Fig 95a). It shows more clearly where the buried sinus P wave resides.

The patient was given 0.5 mg atropine IV, abolishing the Wenckebach phenomenon and the VEBs. A surprising rhythm emerged (Fig 95b): sinus rhythm at approximately 98/min, alternating and fusing with the retrograde P waves throughout! The antegrade first degree block persisted for both, single or combined. Again, it is apparent that the QRS participation is not required for junctional reentry. But the retrograde beats are all conducted following atropine.

It is of course possible that the retrograde waves are not echo beats at all but independent low-atrial or junctional extrasystoles & rhythm. In that case, the latter would be showing astonishing synchronisation with the sinus rhythm.

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