Right and left bundle branch block on the 12-lead

12-lead tracings of complete right and left bundle branch block, each drawn from the heart's own late activation, with an article on reading it. Right: the rSR′ in V1 and the broad S in I and V6. Left: the QS in V1, the broad notched R in I and V6, the discordant ST and T, and why the usual signs of infarction cannot be read.