A 14-year-old girl with osteosarcoma of the distal femur has completed 3 cycles of neoadjuvant chemotherapy (doxorubicin, cisplatin, methotrexate). Repeat MRI shows 40% reduction in tumor size. Chest CT is negative for metastases. She is scheduled for wide surgical resection in 2 weeks. During the preoperative assessment, she complains of dyspnea on exertion and has a resting heart rate of 110 bpm. Echocardiography reveals a left ventricular ejection fraction (LVEF) of 42% (baseline 55%). What is the most appropriate next step?