Scenario: This electrocardiogram (ECG) was obtained from an 83-year-old woman admitted to the hospital for new epigastric discomfort. Aside from persistent fatigue, she has no other recent changes in her health. The patient said that she previously consulted a cardiologist for some “skipped heart beats,” but has not seen a cardiologist in 2 years because she moved to Florida to retire.

Atrial and right ventricular paced rhythm at 69/min.

Although the universal measurements on the ECG are correct (eg, rate, duration, intervals), the computerized algorithm at the top of the ECG identified the rhythm as sinus rhythm with a left bundle branch block because the QRS duration was >0.12 seconds plus a broad notched or slurred R wave in leads I, aVL, V5, and V6, and an RS pattern in V5 and V6.

However, looking closely...

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