== LBBB vs RBBB ==
[[Image:LBTB_RBTB_en.png|thumb| A bundle branch block causes a delay in the depolarization of the right (RBBB) or left (LBBB) ventricle. In RBBB the QRS complex shows a second peak or R' in V1.]]
Check V1 when QRS > 0
When the "terminal force" of the QRS in V1 is below the baseline (i.e. QS wave), a LBBB is the most likely diagnosis.
When the "terminal force" of the QRS in V1 is above the baseline (i.e. RSR' wave), it's a RBBB.