Gradual Completion of LBBB

Report:

Sinus rhythm

SVEBs

Incomplete left bundle branch block

Complete left bundle branch block, rate-dependent

Comment:

The tall monophasic R complexes in 1 and V6, with slightly delayed peak (late intrinsicoid of yore) and no preceding septal q wave, are those of incomplete LBBB. The complete variety is plumper and squatter; by definition, its duration exceeds 0.12”. As expected, the complete LBBB beats terminate the short cycles.

This is not necessarily an all-or-nothing phenomenon. The three beats from the third to the fifth illustrate gradual QRS widening, as well as gradual attainment of left axis deviation. In this patient, the more there is of the LBBB, the more leftward is its axis.

Below (Fig 43a) is another trace, with somewhat narrower incomplete LBBB in two beats; both have positive QRS in Lead 2 rhythm strip. Interestingly, the second one terminates a short cycle: supernormal conduction or something to do with synchronised deficit in the right bundle branch. Indeed, RBBB rears its head at other times (Figs 43b, 43c)

The progression of LBBB suggests, superficially, a LBBB Wenckebach23, but the sinus rate acceleration negates this notion24.