Scenario: This 72-year-old male patient had a routine resting 12-lead ECG obtained during his annual physical. He has no significant medical history, maintains a daily exercise regimen, and works full time as an executive accountant for a local grocery store chain.

Sinus rhythm at 80/min with a left bundle branch block (LBBB)

A typical LBBB has characteristically wide QRS complexes with abnormal waveform morphology in leads V1 and V6. In LBBB, conduction is blocked at some point as it travels down the left bundle branch. The left ventricle will eventually contract as depolarization spreads from the right ventricle, but the delay in conduction results in a wide QRS complex.

The specific ECG criteria to diagnose LBBB include the following: (1) a cardiac rhythm of supraventricular origin, (2) a QRS duration greater than 120 msec, (3) a QS or rS complex in lead V1, and (4)...

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