Acute Effects of Fusion Pacing Versus Standard CRT on Myocardial Function in Heart Failure Patients with LBBB

<b>Background/Objectives:</b> Although cardiac resynchronization therapy (CRT) plays an established role in the management of heart failure, a significant proportion of patients do not respond despite appropriate candidate selection. The optimization of CRT pacing is one strategy to enhance response. Fusion pacing algorithms aim to synchronize intrinsic right ventricular (RV) conduction with paced left ventricular (LV) activation, resulting in a more physiological ventricular depolarization pattern. This approach may improve electrical synchrony and enhance left ventricular contraction compared to conventional simultaneous biventricular pacing. The aim of this study was to compare the acute, beat-to-beat effects of standard biventricular pacing versus fusion pacing on myocardial function, using both conventional and speckle-tracking echocardiography in heart failure patients with left bundle branch block (LBBB). <b>Methods</b>: In total, 27 heart failure patients (21 men and 6 women) with reduced ejection fraction (EF < 35%), left bundle branch block (QRS > 150 ms), and newly implanted CRT-D systems (Abbott) underwent echocardiographic assessment immediately after device implantation. Echocardiographic parameters-including left atrial strain, left ventricular strain, TAPSE, mitral and tricuspid valve function, and cardiac output-were measured at 5 min intervals under three different pacing conditions: pacing off, simultaneous biventricular pacing, and fusion pacing using Abbott's SyncAV^® algorithm. <b>Results</b>: In our study, CRT led to a significant shortening of the QRS duration from 169 ± 19 ms at baseline to 131 ± 17 ms with standard biventricular pacing, and further to 118 ± 16 ms with fusion pacing (<i>p</i> < 0.05). Despite the electrical improvement, no significant changes were observed in global longitudinal strain (GLS: -9.15 vs. -9.39 vs. -9.13; <i>p</i> = NS), left ventricular stroke volume (67.5 mL vs. 68.4 mL vs. 68.5 mL; <i>p</i> = NS), or left atrial parameters including strain, area, and ejection fraction. However, fusion pacing was associated with more homogeneous segmental strain patterns, improved aortic valve closure time, and enhanced right ventricular function as reflected by tissue Doppler-derived S'. <b>Conclusions</b>: Immediate QRS narrowing observed in CRT patients-particularly with fusion pacing optimization-is associated with a more homogeneous pattern of left ventricular contractility and improvements in selected measures of mechanical synchrony. However, these acute electrical changes do not translate into immediate improvements in stroke volume, global LV strain, or left atrial function. Longer-term follow-up is needed to determine whether the electrical benefits of CRT, especially with fusion pacing, lead to meaningful hemodynamic improvements.