Scenario: This 12-lead ECG waveform was obtained in a patient transferred to the intensive care unit from the general medicine floor because of acute shortness of breath. The patient has a history of chronic obstructive pulmonary disease, hypertension, coronary artery disease, and prior myocardial infarction (MI).

Interpretation: Normal sinus rhythm with first-degree atrioventricular block and left bundle branch block (BBB) with notching of the S wave in leads V3-V5, suggesting prior anterior MI.

The QRS complex duration is wide (>0.12 seconds or 3 small boxes) in every lead. Causes of a widened QRS complex include right or left BBB, pacemaker, hyperkalemia, ventricular preexcitation as is seen in Wolf-Parkinson-White pattern, and a ventricular rhythm. Because there is a P wave associated with every QRS complex, a ventricular rhythm can be ruled out. A Wolf-Parkinson-White pattern can also be ruled out because there is a P-R segment and...

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