Diagnosis: Normal Sinus Rhythm w/ LBBB (Left bundle branch block). Left atrial enlargement

Discussion: This is classic LBBB, with QRS widening, ST seg elevation, no Q waves in V6 or V1, possible RSR' in V6. In the normal heart, the septum depolarizes from left to right during the early portion of the QRS and thus can give a Q wave in V6 and lead I (since normal septal depolarization places a negative vector component in leads I and V6). In LBBB the septum depolarizes from right to left such that the electrical current vector has a positive component in the directions of V6 and lead I thus eliminating the Q waves in those two leads. The QRS widening is due to delayed LV depolarizing. Since the LV depolarization usually makes up the end portion of the QRS complex, delay of LV depolarization is much more likely to produce a widening of the QRS than is RBBB.

The ST segment elevation often seen in LBBB is due to abnomal repolarization. In order for the LV to depolarize in a pt w/ LBBB, current at some point must traverse myocardium from the Right bundle to the left sided conduction system. That pathway may be irregular and when it depolarizes/repolarizes causes ST-T changes. Thus it is difficult to make a diagnosis of acute injury in a pt w/ LBBB, although it can be argued that the ST seg elevation of acute injury is usually more "humped".

Left atrial enlargement is also seen, normally LA depolarization causes a negative deflection in the P wave in lead V1. A diphasic P wave in lead V1 is normal. If the negative portion of the P wave in lead V1 is deep enough (more than one little box deep, 0.04sec) LAE can be called. Also in LAE you can see (as you do here) two humps in lead II. Normally both LA and RA depolarizations cause positive deflections in lead II. Double humps in lead II are suggestive but not diagnostic of LAE.