A 38-year-old woman with a 3-month history of progressive fatigue, palpitations, and syncope on exertion is found to have a heart rate of 110 bpm at rest and a blood pressure of 95/60 mmHg. Electrocardiography shows sinus tachycardia with low voltage complexes. Chest X-ray reveals an enlarged cardiac silhouette. Which investigation would be most appropriate to assess the degree of ventricular dysfunction and determine the etiology of the cardiomyopathy?