**8.4 His bundle pacing and His-optimized cardiac resynchronization therapy for electrical resynchronization in heart failure**

In 1977, Narula et al. reported that the QRS complex may be normalized by pacing the distal His bundle in patients with LBBB [98]. Permanent pacing of the His bundle region to achieve ventricular resynchronizing has been described, with clear clinical advantages over traditional RV apical pacing [99–102]. Medtronic has announced US Food and Drug Administration (FDA) clearance and commercial launch for the SelectSite C304-HIS deflectable catheter system for use in procedures involving His bundle pacing (HBP). The physiologic benefit of permanent His bundle pacing (HBP) is theresult of synchronous electrical and mechanical activation with stimulation of both ventricles through the intrinsic His-Purkinje system. The anatomic site of the conduction disorder seen with BBB is frequently located proximally within the bundle of His, with longitudinal dissociation of the conducting fibers [103, 104]. Overall, it has been reported that approximately three quarters of BBB patients were found to respond with QRS narrowing using HB pacing [103]. Using epicardial electrocardiography (ECG), imaging Arnold et al, demonstrated that HB pacing was superior to biventricular pacing for restoring LV synchrony in selected patients with LBBB [105]. In the presence of distal BBB or the co-existence of IVCD, QRS may not normalize. In patients without complete LBBB correction, Vijayaramanet P et al demonstrated that His-optimized CRT (HOT-CRT) with synchronized LV pacing resulted in significant QRS duration narrowing [106, 107]. In patients with atrioventricular (AV) block in whom fusion with intrinsic His-Purkinje conduction cannot be achieved, HOT-CRT may provide the new therapeutic option. However, it is wise to remember that QRS duration reflects total ventricular activation time which is not always equivalent to a perfect marker of LV synchrony. HOT-CRT was found to be a novel approach to further optimize electrical resynchronization by combining the concept of fused adaptive LV pacing with HBP.
