A 58-year-old man followed for coronary artery disease presented with an ECG showing sinus rhythm at 88 beats per minute, a prolonged QRS duration of 145 milliseconds consistent with left bundle branch block, and no evidence of right or left ventricular hypertrophy or atrial enlargement. The left bundle branch block precludes diagnosing left ventricular hypertrophy and can mask underlying myocardial infarction.

1. Stepwise interpretation of ECG ID 168 Left bundle branch block

2. ID 175 – 58 year old man followed in the Cardiac Clinic because of coronary artery disease

3. There is no right ventricular hypertrophy or left ventricular hypertrophy ID 175 – 58 year old man followed in the Cardiac Clinic because of coronary artery disease

4. Can you see P waves? ID 175 – 58 year old man followed in the Cardiac Clinic because of coronary artery disease

5. Yes. There is sinus rhythm, 88/min. Each P is followed by a QRS complex, with a constant P-R interval (140 msec) ID 175 – 58 year old man followed in the Cardiac Clinic because of coronary artery disease

6. There are no signs of right or left atrial enlargement ID 175 – 58 year old man followed in the Cardiac Clinic because of coronary artery disease

7. The QRS duration is prolonged (145 msec.), with a pattern of Left Bundle Branch Block (LBBB) ID 175 – 58 year old man followed in the Cardiac Clinic because of coronary artery disease

8. The presence of LBBB precludes the diagnosis of left ventricular hypertrophy and usually masks an underlying myocardial infarction ID 175 – 58 year old man followed in the Cardiac Clinic because of coronary artery disease

9. In left bundle branch block the abnormal depolarization is usually associated with secondary abnormal repolarization (ST depression and negative T in the leads showing a broad R wave) ID 175 – 58 year old man followed in the Cardiac Clinic because of coronary artery disease

10. ID 380 – Final diagnosis: Normal Sinus rhythm, 88/min. Left bundle branch block.