Palpitations in a Young Woman With Breast Cancer

A woman in her 20s presented to the emergency department with a 1-day history of palpitations and dyspnea. Her medical history was significant for breast cancer, for which she had received total mastectomy of the right breast, followed by 5 cycles of dual anti–human epidermal growth factor receptor 2 (ERBB2 [formerly HER2]) treatments with trastuzumab and pertuzumab before the admission. She had no history of cardiovascular diseases or family history of sudden death and hereditary arrhythmia syndromes. Electrocardiography (ECG) and echocardiography performed before initiating the anti-ERBB2 therapy yielded normal results. On arrival, her heart rate was 209/min, her blood pressure was 90/60 mm Hg, her respiratory rate was 31/min, and her oxygen saturation was 92% on room air. Investigations revealed a troponin I level of 0.017 ng/mL (reference range, <0.034 ng/mL) (conversion of troponin I to μg/L is 1:1), an N-terminal fragment of the prohormone brain natriuretic peptide (NT-proBNP) level of 3600 ng/L (reference range, <125 ng/L), and normal levels of serum electrolytes. A transthoracic echocardiogram revealed a reduced left ventricular ejection fraction (LVEF) of 46%. Her initial ECG is shown in the Figure, A.