Scenario: Below are 2 rhythm strips taken from resting 12-lead electrocardiograms (ECGs) for a 73-year-old man who came to the emergency department (ED) with complaints of a productive cough, shortness of breath, and chest discomfort for the past week. Leads II, V1 and V6 are shown below and were recorded on ED admission (left) and on the day of discharge (right). His temperature on admission was 103.1°F. He has a history of hypertension, hyperlipidemia, and coronary bypass graft surgery. He was admitted to the medical telemetry unit with pneumonia and treated with supplemental oxygen and antibiotic therapy.

The ED admission ECG shows sinus tachycardia with LBBB. On the day of discharge, the ECG strip shows normal sinus rhythm. These ECGs demonstrate that this patient has a rate dependent LBBB (RD-LBBB). The ST-segment changes noted on ED admission are most likely secondary...

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