Right and Left Bundle Branch Conduction

Right and Left Bundle Branch Conduction is the parallel transmission of the ventricular activation signal along two anatomically and functionally distinct pathways descending from the bundle of His, one supplying the right ventricle and one the left, whose coordinated, near-simultaneous conduction ensures that both ventricles, despite their separation and differing mass, begin and largely complete depolarization in close temporal synchrony.

Anatomical Organization

The Right Bundle Branch

The right bundle branch continues as a relatively thin, discrete, and comparatively long structure from the distal His bundle, coursing along the right side of the interventricular septum, often within the septal myocardium itself, before reaching the right ventricular apex and moderator band region, where it arborizes into the right ventricular Purkinje network.

The Left Bundle Branch

The left bundle branch emerges as a broader, sheet-like structure that fans out over the left side of the interventricular septum considerably earlier and more diffusely than its right-sided counterpart, and is conventionally described as dividing into an anterior fascicle, supplying the anterosuperior left ventricular wall, and a posterior fascicle, supplying the posteroinferior wall, though the precise fascicular anatomy varies among individuals and some descriptions recognize a distinct septal component as well.

Functional Characteristics of Bundle Branch Conduction

Rapid Conduction Velocity

Both bundle branches are composed of specialized Purkinje-type conducting cells with abundant fast sodium channel expression and large cell diameter, properties that confer conduction velocities substantially faster than ordinary ventricular working myocardium, allowing the activation wavefront to traverse the considerable distance to the ventricular apices and free walls far more rapidly than direct cell-to-cell myocardial conduction alone could achieve.

Structural Basis for Near-Simultaneous Activation

Because both bundle branches originate from the same His bundle at essentially the same point in time and conduct at comparably rapid velocities, the right and left ventricular Purkinje networks, and through them the respective ventricular working myocardium, begin their depolarization within a few milliseconds of one another, producing the coordinated biventricular activation essential for synchronized, hemodynamically effective contraction.

Distribution to the Working Myocardium

Purkinje Network Arborization

Each bundle branch terminates in an extensively arborizing Purkinje fiber network that ramifies subendocardially throughout its respective ventricle, delivering the rapidly conducted signal to numerous discrete points of contact with the working ventricular myocardium, from which activation then spreads via ordinary, slower myocardial cell-to-cell conduction outward from endocardium toward epicardium.

The Purkinje-Myocardial Junction

At the numerous points where Purkinje fibers connect to working myocardium, there is a characteristic, brief additional conduction delay attributable to the relatively small number and caliber of connecting fibers relative to the larger mass of myocardium they must excite, a physiological source-sink mismatch that, while normally overcome without difficulty, becomes clinically relevant when excitability is reduced by disease.

Consequences of Bundle Branch Dysfunction

Right Bundle Branch Block

Interruption of conduction through the right bundle branch delays right ventricular activation, which then proceeds via slower, cell-to-cell myocardial conduction spreading from the normally activated left ventricle across the interventricular septum, producing a characteristic, prolonged and distinctively shaped terminal QRS deflection on the surface electrocardiogram.

Left Bundle Branch Block

Interruption of the left bundle branch produces the analogous but electrocardiographically distinct pattern of delayed left ventricular activation via septal spread from the right ventricle, and, because the left ventricle bears the greater share of systemic mechanical work, left bundle branch block carries greater potential for producing clinically significant mechanical dyssynchrony than right bundle branch block of comparable electrical severity.

Fascicular Block

Because the left bundle branch's fascicular anatomy provides some redundancy, isolated block of only the anterior or only the posterior fascicle produces a more limited, characteristic axis deviation on the electrocardiogram rather than the full QRS widening seen with complete left bundle branch block, illustrating the graded relationship between the anatomical extent of conduction system disruption and its electrocardiographic and mechanical consequences.

Clinical and Therapeutic Relevance

Diagnostic Localization

Because right and left bundle branch conduction produce distinct, recognizable electrocardiographic patterns when disrupted, surface electrocardiographic analysis allows clinicians to localize conduction disease to a specific branch or fascicle, informing both prognosis and the selection of appropriate further evaluation or intervention.

Cardiac Resynchronization Therapy

In cases of severe left bundle branch block producing clinically significant mechanical dyssynchrony and associated heart failure, cardiac resynchronization therapy—biventricular pacing designed to restore near-simultaneous activation of both ventricles despite the underlying conduction system disease—directly targets restoration of the coordinated bundle branch conduction pattern described throughout this article.