Wilhelm His Jr. described the physical link electrically bridging the atria and ventricles of the heart in 1893. The specialized muscle bundle connecting the atrial and ventricular chambers of the heart is now known as the bundle of His. The bundle of His quickly transmits the electrical impulse from the atrioventricular (AV) node to the left and right bundle branches and into the ventricles. Over the years, diseases affecting the bundle of His have been discovered along with diagnostics and therapeutic interventions.
The AV conduction system propagates electrical impulses to maintain proper and efficient heart function. Malfunction of this system can lead to disruption of the heart’s normal sinus rhythm, such as by inducing arrhythmias that may require critical treatment or invasive surgical procedures.
The bundle of His comprises a complex organization of cells, predominantly consisting of Purkinje-type cells, slender transitional cells, broad transitional cells, and varying amounts of pacemaker cells. The Purkinje-type cells of the bundle of His are composed of randomly oriented myofibrils, an abundance of mitochondria, Golgi apparatus, and a sporadic presence of two separate nuclei.
The pacemaker (P) cells are oval, and they include “sparse and randomly oriented myofibrils,” simple membranes, and trivial amounts of sarcoplasmic reticulum.
The bundle of His is an elongated segment connecting the AV Node and the left and right bundle branches of the septal crest. It is approximately 1.8 cm long in an adult heart and is primarily located deep within the dense connective tissue.
The bundle of His is characterized by longitudinal collagen partitioning histology, distinguishing it from the AV node.
The intercalated discs in the bundle of His differ from the myocardial cells such that the discs in the bundle of His are oriented obliquely compared to the perpendicular alignment seen in the myocardium cross-sectional view. The bundle of His appears to have more “tongue-and-groove joints” compared to the “jagged perpendicular line” in the myocardium. Also, more tight junctions are found in the Purkinje cells of the bundle of His than in the intercalated discs of the myocardium.
Studies performed on the embryological development of the bundle of His are limited. The heart is derived from the lateral plate mesoderm, and as the heart continues to develop the newly formed cardiomyocytes begin to depolarize creating a slow electrical impulse. In addition, studies show that the specialized conduction tissue in the atrioventricular canal is insulated by fibroblasts and fibrous tissue that stem from the migratory, multipotent neural crest cells. The fibrous tissue, called the annulus fibrosus, develops from the movement of the epicardial mesenchyme into the myocardial space and separates the developing atria and ventricles. Further research suggests the bundle of His emerges from an interventricular ring during development.
Because the bundle of His is found deep within fibrous tissue, its precise function has been difficult to study. It is hypothesized that the “longitudinal partitioning of strands of the His bundle” and the abundant nexuses within the bundle of His aid the rapid conduction of the electrical signal from the AV node. Also, the encompassing collagen may potentially prevent the lateral spread of the propagated impulse.
The standard bundle of His ECG is an invasive electrophysiology study involving inserting an electrode catheter into the femoral vein, through the inferior vena cava, and into the into the heart. It is advanced into the right ventricle and pulled back until the continuous monitor notes a spike between the P wave and the QRS complex, signifying the depolarization of the bundle of His. Newer studies have suggested that noninvasive surface electrocardiograms may be sufficient to record bundle of His potentials.
Because the bundle of His transmits the electrical impulse from the AV node to the bundle branches, malfunction of the bundle of His can lead to heart blocks ranging from AV conduction delays (first-degree heart block) to a complete disconnect in the electrical conduction between the atria and ventricles (third-degree heart block) causing arrhythmias. His bundle electrocardiograms may reveal important clinical findings in relation to heart blocks of the cardiac conduction system.
There are 92.1 million Americans currently affected by some form of cardiovascular disease, and 43.9% are expected to have a type of cardiovascular disease by 2030.
His-bundle pacing (HBP) is a method where leads are placed directly on the AV Node. This stimulates the bundle of His, generating conduction synchronization through the ventricles. As ventricles return to their normal pump function, the risk of cardiomyopathy decreases.
Studies show permanent direct His bundle pacing (DHBP) may be utilized in patients with chronic atrial fibrillation and dilated cardiomyopathy. In addition, studies support the use of permanent HBP as an alternative to biventricular pacing for patients who have a low success rate with left ventricular lead placement.
The bundle of His ECG has also been utilized to examine the effects of drugs, including vagolytic, sympathomimetic, sympatholytic, and antiarrhythmic.
Vagolytic drugs, such as atropine, demonstrate a shortening between atrial depolarization and His bundle depolarization interval (P-H interval); however, they do not affect the interval between His bundle and the ventricular activation (H-Q interval).
Isoproterenol, a sympathomimetic drug, works similarly to vagolytic drugs. A shortened P-H interval is observed, and there is no effect on the H-Q interval.
Propranolol, a sympatholytic drug, increases P-H interval without an effect on H-Q interval.
Antiarrhythmics drugs, such as lidocaine, either shorten or prolong the P-H interval with a variable effect on the H-Q interval; some patients demonstrate a modest prolongation of the intraventricular conduction.
In summary, the P-H interval is the most susceptible region of the conduction system for intervention by use of therapeutic drugs.
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