Left ventricular versus biventricular for cardiac resynchronization therapy: comparable but not equal.

The primary mechanism of benefit associated with cardiac resynchronization therapy (CRT) is attributed to improvement in left ventricular (LV) function resulting from restoration of LV contractile synchrony.1,2 The vast majority of implanted CRT-capable devices are programmed to provide the therapy by simultaneous pacing of the right ventricle and LV (biventricular stimulation). This mode of CRT delivery has been the mode best tested in large-scale clinical trials that have demonstrated improvement in functional, anatomic, and event-driven outcomes.3–6 The reasons for the more thorough evaluation of biventricular stimulation compared with LV stimulation alone are largely practical in nature. Early studies of CRT systems were designed to demonstrate the safety and efficacy of CRT. Long-term transvenous epicardial LV stimulation was not an established therapy, and LV lead performance was unknown. Biventricular stimulation allowed backup pacing and sensing if the LV lead failed. Additionally, all bradycardia pacing and defibrillating lead timing and therapy delivery are determined by right ventricular lead–based sensing. These considerations necessitated the presence of a right ventricular lead in long-term studies of CRT. There was also the early short-term observation that short-term stimulation results in similar mechanical synchrony with LV or biventricular stimulation, but electric dispersion appears to be increased with LV stimulation. This finding raised concern that LV stimulation alone could create a more favorable milieu for the occurrence of ventricular proarrhythmia.1,7