Recently, I have been asked by several students at my home institution (UTHSC at San Antonio) to help them understand bundle branch blocks. This is different than some of my usual posts because it is meant to be more educational than evidence based. So here we go. The normal conduction system of the healthy heart is shown to the right. If there is a delay or block in the left or right bundle, depolarization will take longer to occur. Therefore we get a widened QRS (>0.12 sec or >3 small boxes).