Scenario:A 59-year-old woman had brief episodes of rapid atrial fibrillation and bradycardia several months ago. It was not recurrent or sustained, so she was not treated with invasive or pharmacological interventions but followed up with occasional clinic visits. She was instructed to continue taking her beta blocker, which she had done for 10 years without side effects. She has a history of hypertension and a significant family history; her mother and grandmother had hypertension and heart failure, and both died of cerebral vascular accidents. The patient is now in the emergency department with complaints of chest “discomfort” and shortness of breath. She had the rhythm below (lead II), which was only captured because the patient’s daughter, a nurse, pushed the record button on the bedside monitor. A regular feature of the American Journal of Critical Care, the ECG Puzzler addresses electrocardiogram (ECG) interpretation for clinical practice. To send an eLetter or to contribute to an online discussion about this article, visit www.ajcconline.org and click “Respond to This Article” on either the full-text or PDF view of the article. We welcome letters regarding this feature.