A 55-year-old man with a 6-month history of heart failure is evaluated during a follow-up appointment. At diagnosis, his left ventricular ejection fraction was 15%, and he had moderate mitral and tricuspid regurgitation. Cardiac catheterization at that time revealed normal coronary arteries. He was started on the appropriate medications and is now back to working at a desk job. He has dyspnea walking up a flight of stairs. Medical history is also significant for hypertension. Medications are enalapril, carvedilol (25 mg twice daily), furosemide, and spironolactone.

On physical examination, blood pressure is 100/65 mm Hg, and pulse rate is 56/min. Weight is 72 kg (159 lb). On cardiovascular examination, there is a grade 3/6 holosystolic murmur at the apex radiating to axilla and no S3 gallop. The remainder of the examination is normal.

Serum electrolyte levels and kidney function tests are normal. Recent echocardiogram shows a left ventricular ejection fraction of 20% and moderate mitral regurgitation. Electrocardiogram demonstrates normal sinus rhythm with a QRS width of 100 ms.

Which of the following is the most appropriate management?