Analyzing Insights of Super-Response in Cardiac Resynchronization Therapy with Fusion Pacing.

: Cardiac resynchronization therapy (CRT) with fusion pacing ("LV only"), also known as fusion-CRT (f-CRT), represents a feasible alternative to cardiac resynchronization therapy (CRT) with biventricular pacing (BiVP), not only in cases of BiVP failure, but also as a primary therapy option due to its potential benefits over traditional CRT. Fusion pacing may be particularly beneficial in selected patients and understanding the structural and functional differences between responders could guide future optimization strategies. This study provides a descriptive comparison between super-responders (SRs) and non-super-responders (NSRs) undergoing fusion-CRT. : Patients with RA/LV-only pacing systems or biventricular CRT systems operating predominantly in LV-only pacing mode due to intrinsic RV conduction were included. A follow-up protocol was conducted for all patients at 6 months and then annually. Data from the most recent follow-up were used for statistical analysis. Super-responders (SRs) were those with substantial reverse remodeling, quantified by a ≥30% reduction in LVESV and a stable LVEF of ≥45% at follow-up. Although SRs were defined based on these reverse remodeling criteria, separate analyses of additional echocardiographic parameters (e.g., left atrial dimensions) were performed to independently assess the broader impact of fusion-CRT on cardiac structure and function. : Among 71 patients, 55 were non-super-responders (NSRs) and 16 were super-responders (SRs), with a mean follow-up of 43.2 months. SRs were predominantly female and had smaller left ventricular (LV) dimensions: LVEDd (6.30 cm vs. 6.80 cm, = 0.02), LVEDV (185 mL vs. 240 mL, = 0.03), LVESV (132.5 mL vs. 175 mL, = 0.03), and a higher LVEF ( = 0.03). The follow-up LVEF was positively correlated with changes in LVESV (ρ = 0.557, < 0.001), but not with NYHA class changes (ρ = 0.184, = 0.125). Larger baseline LV and left atrial (LA) volumes were associated with a reduced follow-up LVEF (LVESV: ρ = -0.426, < 0.001; LVEDV: ρ = -0.394, < 0.001; LAv: ρ = -0.374, = 0.001). Both groups showed improvement in the NYHA class ( < 0.001, = 0.007). MR improved significantly in SRs ( = 0.02) and worsened slightly in NSRs ( = 0.13), while TR worsened significantly in the NSRs group ( = 0.03). : Our findings highlight key differences in clinical and echocardiographic parameters between SRs and NSRs following fusion-CRT. These observations may contribute to a better understanding of response patterns and inform future prospective studies aiming to optimize patient selection and timing of therapy.