ECG Findings
• Wide QRS complex, at least 120 ms (three small blocks).
• T wave appears on the opposite side of the baseline from the QRS complex.
• The QRS precordial axis is normal or deviated to the left.
• QRS complex deflection is predominately downward in lead V1 and upward in lead V6.
Points to Remember
1. The signal exiting the AV node does not proceed through the left ventricular conduction system. It must propagate more slowly cell-to-cell through the myocardium, starting in the septum. Therefore, the QRS is wider and the bulk of the depolarization signal is deflected toward the far lateral aspect of the heart.
2. Acute myocardial infarction may produce a new onset LBBB on ECG. Therefore, patients with new onset LBBB with a clinical presentation consistent with acute coronary syndrome should be treated as having an acute ST segment elevation myocardial infarction (STEMI).
The QRS is wider than 120 ms (double arrow). The T-wave deflection is in the opposite direction from the QRS deflection (arrowhead).