fascicular blocks or bundle branch blocks). When normal transit through the His–Purkinje system is disrupted, stereotypic conduction block patterns are manifested on the 12-lead surface ECG (e.g. 1 The rapid and synchronous activation of the LV through the specialised His–Purkinje network is highly intricate and efficient, preserving normal physiological coupling between electrical excitation and mechanical contraction. recorded the total excitation of the human heart with 870 electrodes and confirmed the first 10 ms of left ventricular (LV) activation as trifascicular in nature. Left-sided EPS has the potential to refine patient selection in CRT trials and may be used to physiologically phenotype distinct conduction patterns beyond LBBB pattern. Future EPS are required to determine the impact of newer approaches to conduction system pacing, including intraseptal or left ventricular septal pacing. Patients with intact Purkinje activation and intraventricular conduction delay are less likely to benefit from HBP. During a novel extension of the classical EPS to involve left-sided recordings, we found that conduction block associated with the LBBB pattern is most often proximal, usually within the left-sided His fibres, and these patients are the most likely to demonstrate QRS correction with HBP for resynchronisation. The determination of LBBB pattern is conventionally made by assessment of surface 12-lead ECGs and can include patients with and without conduction block, as assessed by invasive electrophysiology study (EPS). The ability to normalize wide QRS patterns with His bundle pacing (HBP) has called this notion into question. One historical presumption of LBBB has been that the underlying pathophysiology involved diffuse disease throughout the distal conduction system. Left bundle branch block (LBBB) is associated with improved outcome after cardiac resynchronisation therapy (CRT).
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