A man in his 20s presented with syncope and shortness of breath. Electrocardiography revealed monomorphic ventricular tachycardia. Echocardiography demonstrated severe biventricular dysfunction with normal ventricular size and thickness. Septal e' was 5 cm/s, mitral valve inflow deceleration time was 120 milliseconds, E/e' was 18, and lateral s' was 6 cm/s. An endomyocardial biopsy did not show any specific pathology. In this patient with severe heart failure, ventricular arrhythmias, and unrevealing biopsy, what should be the next step?