A New Tachyarrhythmia in a 60-Year-Old Woman

A 60-year-old woman with a history of hypertrophic obstructive cardiomyopathy, paroxysmal atrial fibrillation, and metastatic rectal cancer presented to the emergency department with new-onset respiratory distress. Her home medications included, among others, metoprolol and digoxin, both of which had recent dose increases after a hospitalization complicated by atrial fibrillation with rapid ventricular response. In the emergency department, the patient was normotensive and tachypneic with hypoxia that resolved with supplemental oxygen. Her laboratory tests were significant for elevated troponin T (0.13 ng/mL, normal<0.01), N-terminal pro b-type natriuretic peptide (40 001 pg/mL, normal<300), and creatinine (1.42 mg/dL, baseline=0.7). Her electrolytes, hemogram, and thyroid-stimulating hormone were normal. The admission ECG is shown in Figure Given frequent rhythm changes noted on telemetry after intravenous diuresis and continuation of her home medications, a repeat ECG was ordered (Figure What is the rhythm during each recorded ECG; what is the most likely diagnosis; and what is the next best step in management?