Unusual Wenckebach Sequences

Report:

Sinus arrhythmia

Second degree AV block, Möbitz 1

Bradycardia-dependent LBBB (4th beat in the top strip)

? Respiratory artefact

? Bradycardia-dependent incomplete LBBB

? Junctional escape beats with non-phasic aberrant conduction

Comment:

The QRSs starting each Wenckebach sequence are the largest in the trace. One possibility, that they are in fact escape beats, is opposed, but not excluded, by observing the constant PR interval associated with all of them, despite variable preceding RR intervals.

While there is an obvious respiratory gradation in the QRS amplitudes, the pattern observed would require rather irregular breathing. The LBBB morphology of the 4th beat in the top strip suggests, too, that the other large QRSs have a degree of incomplete LBBB, presumably induced by the preceding long cycles.

The respiratory sinus arrhythmia has its longest sinus cycles precisely during the long pauses, favouring respiratory contribution as the mechanism of QRS size variation (except for the 4th beat in the top strip).

It is possible that all three mechanisms are present. A trace taken at another time (Fig 3a below) shows obviously dissociated junctional escape beats terminating Wenckebach pauses, as well as respiratory effects; if one postulates non-phasic aberrancy as the basis for the QRS size difference, then the first two beats in each sequence are non-conducted junctional escapes.

The 4th beat in the top strip could also be a fusion beat; it is unlikely since no other ectopic beats are present.

Fig 3a. The Wenckebach progression is imperceptible in terms of PR prolongation before the dropped beat here1; the initial beats in each sequence are obviously dissociated from the preceding P waves, while the following ones (in each large-QRS pair) have a conductable PR interval while their QRS size suggests dissociated, junctional origin. Fusion from two supraventricular foci is extremely rare and there is nothing in the second large QRS to suggest it by way of size intermediate between the obviously junctional and onviously sinus beats. Examples of non-phasic aberration in junctional beats are discussed in the Ventricular Conduction volume.

Fig 4. Lead V1. Sleeping 50 year old man on quinidine and digoxin, scheduled for DC cardioversion in the morning.