An 86-year-old woman with a known history of coronary artery disease and a prior percutaneous coronary intervention presented to the emergency department with several episodes of self-resolving chest discomfort occurring at rest. She denied any shortness of breath, palpitations, and diaphoresis. Her home medications included a β-blocker. Her blood pressure was 140/85 mm Hg, and she had an irregular heart rate of 125/min. Figure 1 shows the initial electrocardiogram (ECG) obtained in the emergency department.