5. The left bundle branch

The left bundle branch arises from the left side of the branching His bundle beneath the membranous septum and spreads over the left ventricular aspect of the muscular septum. It should not be represented as a thin cable that simply divides into two isolated fascicles. In many hearts its proximal origin is broad, and its specialized cardiomyocytes form a subendocardial network with anterior, posterior, and septal radiations connected to the Purkinje system. The shape and relative prominence of these radiations vary considerably between individuals [2,3,14–17].

Proximally, the left bundle may descend as a compact sheet or ribbon before widening. Some ramifications arise close to the lower border of the membranous septum; others emerge more distally along the smooth left septal surface. The anterior radiation generally courses toward the anterosuperior papillary region, the posterior radiation toward the posteroinferior septum and papillary region, and septal ramifications spread through the midseptal territory. These directional descriptions are useful, but the extensive interconnections contradict a rigid bifascicular model. In the atlas, the branch is best shown as a variable three-dimensional network [14–17].

Histologically, left-bundle cardiomyocytes may be separated from working ventricular myocardium by connective tissue, but the thickness and completeness of this insulation are not uniform. The branch itself also varies in thickness and depth. In some regions it is immediately subendocardial; elsewhere it is covered by endocardium, connective tissue, or a layer of working myocardium. These features influence whether a pacing lead records a discrete conduction potential and whether current first captures local septal myocardium, specialized tissue, or both. Similar fluoroscopic depths can therefore produce different electrical transitions [14,15,18].

The branch lies on the left side of the septum, while implantation usually begins from the right ventricular septal surface. The lead must cross right ventricular endocardium and a variable thickness of muscular septum before approaching the subendocardial left-bundle network. The target is consequently a volume, not a visible line. Septal thickness, obliquity of the lead, local trabeculation, and the position of proximal left-bundle radiations determine whether the helix reaches an effective site.

6. Left bundle branch area pacing

Left bundle branch area pacing encompasses direct capture of the left bundle or its ramifications and deep septal pacing in their vicinity. The usual right-sided entry region is below the membranous septum and distal to the His-bundle target, often guided by the tricuspid annulus, His recording, septal fluoroscopic orientation, and paced QRS pattern. Attitudinally, this is an anterosuperior to midseptal approach whose exact level varies with the anatomy of the aortic root, membranous septum, and muscular septal crest [14,18–20].

Lead advancement through the septum is monitored by impedance, electrograms, paced morphology, activation time, and imaging or contrast when used. A left-bundle potential can support proximity but is neither invariably present nor by itself proof of capture. More persuasive evidence includes output-dependent transition between non-selective and selective conduction-system capture, stable and short left ventricular activation time at different outputs, and characteristic changes during programmed stimulation. These criteria translate tissue recruitment; they should not be replaced by a single morphological sign [13,18–20].

Anatomy also defines complications. A lead that remains shallow may provide only right ventricular septal myocardial pacing. Excessive advancement may perforate into the left ventricular cavity or damage the left septal endocardium. Septal coronary arterial branches and veins are variably distributed and cannot be assumed absent from the trajectory. A septal haematoma, coronary injury, thromboembolic exposure of a perforating lead, or delayed loss of capture are uncommon but anatomically plausible. The safest procedure balances the desire for conduction-system capture with stable fixation and an adequate residual margin from the left ventricular cavity [18–20].

7. Choosing and documenting the anatomical target

His-bundle pacing and left bundle branch area pacing recruit different portions of the same axis. His pacing is the most proximal strategy and can preserve physiological activation when the distal system is intact, but its narrow and variably insulated target may yield higher thresholds or lower sensing amplitudes. Left bundle branch area pacing uses a broader distal network and often offers robust sensing and thresholds, but requires transseptal lead penetration and may activate the right ventricle partly through working myocardium. The choice is clinical as well as anatomical and depends on the level of conduction disease, septal anatomy, ventricular function, and the desired resynchronization effect [12,13,18–20].

Reports should distinguish the intended region, observed electrical criteria, and final evidence of capture. “His region,” “deep septal,” “left bundle branch area,” and proven selective or non-selective capture are not interchangeable labels. Fluoroscopic projection should be stated in attitudinal terms, and when computed tomography or echocardiography is available, lead depth and relation to the membranous and muscular septum should be documented. Precise language allows anatomy, electrical behaviour, and outcomes to be compared across studies.