A patient in their 40s with no medical history presented to the emergency department with chest pain of a 4-hour duration. The pain was substernal, intensified by lying supine, and relieved by leaning forward. On admission, the patient’s vital signs included a temperature of 37.3 °C, blood pressure of 147/93 mm Hg, pulse rate of 103 beats/min, and respiratory rate of 20 breaths/min. The results of the rest of the physical examination were normal. The troponin I and N-terminal pro–brain natriuretic peptide levels were normal. A transthoracic echocardiogram demonstrated normal ventricular ejection fraction, chamber size, and motion and the absence of pericardial effusion. The 12-lead electrocardiogram (ECG) at admission is shown in the Figure, A.