More than one way

44-year-old with transient left bundle branch (LBBB) morphology during tachycardia

What is the diagnosis ?

First, there is a left bundle branch block pattern, so the impulse conducts only through the right bundle branch.

We can follow the tachycardia.

At the level of the RV signal, activation is directed first toward
CS 1–2 / distal, with a VA interval of 150 ms, probably due to the left bundle branch block
The signal then rapidly returns to the His region

A ventricular premature beat shortens the AA interval, with conduction through the slow pathway, allowing the left bundle to recover, resulting in a narrow QRS.

Recording of a ventricular signal
Followed by rapid VA conduction of 75 ms toward CS 1–2 / distal
With an AV interval that is prolonged due to conduction over the slow pathway