Scenario: These electrocardiograms (ECGs) were obtained during a symptom-limited, exercise treadmill test (ETT) of a 41-year-old white male for an annual, military-duty clearance check-up. After the ETT, the patient’s abnormal QRS interval returned to the normal baseline observed before the test. Other than a history of hypothyroidism, the patient denies personal or familial history of cardiovascular diseases.

Top strip: normal sinus rhythm at 80/min. Bottoms strip: normal sinus tachycardia at 100/min with exercise-induced left bundle branch block (EX-LBBB).

The criteria for a complete LBBB include a QRS duration longer than 120 ms; broad notched R waves in leads I, aVL, V5, and V6; and ST and T waves opposite in direction to the QRS. After the ETT, the abnormal QRS interval returned to the previously observed normal baseline, thus it is categorized as exercise induced. Only 0.5% of patients...

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