These simple questions were asked once upon a time when cardiologists were not known as interventionists.Now many of them have neither time nor interest to ask such questions .! An article which came in Circulation 1974 addresses this question lucidly.
It is known, HV interval represents infra hisian conduction . And LBBB is just that ! Then , why it is not prolonging it ?
The answer is , it does prolong the HV interval by 20 -40 ms , but , it is not manifested in surface ECG . A 20 ms increment in PR interval (Say 160 to 180 ms ) is not a big issue generally.
* Then the concept of incomplete and complete LBBB is always there to confront us !
Many believe , the intraventricular conduction delay in LBBB may simply represent the “unmaking effect” of LBBB which re-routes the conduction in the slightly delayed , circuitous right bundle highway . But it is only a assumption.Many things can happen in a ischemic , degenerative, dysfunctional heart.
When does the HV interval prolong pathologically in LBBB ?
Acute LBBB (Note Left bundle has two fascicles , technically equivalent to bifasicular block ) A 40ms increment in HV interval (Hence in PR interval also) can be a dangerous delay in LBBB .