1. Introduction
The advantageous effect of cardiac resynchronization therapy (CRT) are well known in heart failure (HF) patients with reduced left ventricular ejection fraction (LVEF) and prolonged QRS duration.1-3 Current global guidelines suggest CRT for class I or IIA indications in patients with LVEF ≤35%, and symptomatic HF despite receiving optimal drug therapy.4-6 However, an LVEF cut-off of ≤35% was determined by the patient enrollment criteria in major CRT trials, and of note, was adopted from cut-off values in prior major implantable cardiac defibrillator trials, rather than from a prospective risk-benefit analyses of CRT for all LVEF ranges.
Although resynchronization therapy for HF patients with LVEF ≥35% is controversial and has not been clearly established, HF with LVEF >35% shows disease features similar to LVEF <35%, and treatment patterns are similar.7,8 Additionally, there are reports that long-term clinical outcomes were poor when HF with mildly reduced ejection fraction (HFmrEF) was accompanied by left bundle branch block (LBBB) versus without LBBB.9 Moreover, in a retrospective analysis of the PROSPECT trial database, CRT demonstrated significant clinical benefit among patients with an LVEF >35%.10 The BLOCK-HF trial proved that CRT provided clinical benefit over right ventricular pacing in patients with LVEF ≤50% and atrioventricular block who require ventricular pacing, and almost 70% (483/691) of the study population had an LVEF of 36–50%.11
Conduction system pacing that directly activates the specialized conduction system was developed, and left bundle branch area pacing (LBBAP)—which overcomes the limitations of the previously used His bundle pacing (HBP)—is emerging and widely used.12-16
Furthermore, the effectiveness and safety of LBBAP in patients with HF has also been reported.17-19 To date, LBBAP has been used as an alternative to CRT in indicated patients, as well as a first option for patients indicated for CRT or pacemaker implantation.17-19 LBBAP is more simple, convenient, and cheaper than biventricular CRT; therefore, there is increasing clinical interest in adopting wider LVEF ranges for LBBAP-CRT among patients with HF and a long QRS duration, especially in patients with an LVEF of 36–50%. Therefore, this study aimed to conduct a systematic review of the literature and meta‐analysis on the impact of the LBBAP strategy in patients with HFmrEF with an LVEF between 35% and 50%.