In 1893, Wilhelm His Jr. discovered the physical link that electrically bridges the atria and ventricles of the heart. This specialized muscle bundle connecting the atrial and ventricular chambers of the heart is now known as the bundle of His. After electrical impulse is sent from the sinoatrial (SA) node to the atrioventricular (AV) node, the bundle of His quickly transmits the impulse to the left and right bundle branches and into the ventricles, resulting in a synchronized contraction of the ventricles. Over the years, diseases affecting the bundle of His have been discovered along with diagnostic and therapeutic interventions.
The AV conduction system propagates electrical impulses from the atria to the ventricles 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 of 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 and Golgi apparatus, and a sporadic presence of two separate nuclei.
The pacemaker (P) cells are oval in shape and contain “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.
Although much is known about the embryological development of the heart as a whole, studies performed on the embryological development of the bundle of His are limited. The heart is derived from the lateral plate mesoderm during the third week of development.  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 which involves inserting an electrode catheter into the femoral vein, through the inferior vena cava, and 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 dissociation in the electrical conduction between the atria and ventricles (third-degree heart block). Conduction delays are a direct cause of arrhythmias, and should therefore be clinically evaluated. Although the twelve-lead electrocardiogram is the gold standard for the evaluation of cardiac arrhythmias, other modalities such as the His bundle electrocardiogram may be more useful in narrowing the source of the conduction defect. 
The His bundle electrocardiogram is useful in detecting the following conduction delays and clinical findings :
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.
The bundle of His directly contributes to the pathophysiology of cardiovascular disease. Therefore, specific tools have been developed to reduce the risk of disease by manipulating the bundle of His.
His-bundle pacing (HBP) is a method in which 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. Additionally, 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.
In addition to detecting conduction delays, the His bundle electrocardiogram can also be utilized to examine the effect of drugs on the cardiac conduction system.
The following findings have been reported :
Findings from the His bundle electrocardiogram studies suggest that the P-H interval is the most susceptible region of the conduction system to therapeutic drugs.
|||von Knorre GH, The 125th anniversary of the His bundle discovery. Herzschrittmachertherapie [PubMed PMID: 29344737]|
|||James TN,Sherf L, Fine structure of the His bundle. Circulation. 1971 Jul [PubMed PMID: 5561420]|
|||van Weerd JH,Christoffels VM, The formation and function of the cardiac conduction system. Development (Cambridge, England). 2016 Jan 15 [PubMed PMID: 26786210]|
|||Scherlag BJ,Lazzara R, Functional aspects of His bundle physiology and pathophysiology: Clinical implications. Journal of electrocardiology. 2017 Jan - Feb [PubMed PMID: 27539164]|
|||Moorman A,Webb S,Brown NA,Lamers W,Anderson RH, Development of the heart: (1) formation of the cardiac chambers and arterial trunks. Heart (British Cardiac Society). 2003 Jul; [PubMed PMID: 12807866]|
|||Blom NA,Gittenberger-de Groot AC,DeRuiter MC,Poelmann RE,Mentink MM,Ottenkamp J, Development of the cardiac conduction tissue in human embryos using HNK-1 antigen expression: possible relevance for understanding of abnormal atrial automaticity. Circulation. 1999 Feb 16 [PubMed PMID: 9989966]|
|||Haft JI, The His bundle electrogram. Circulation. 1973 Apr [PubMed PMID: 4121317]|
|||Wang G,Liu R,Chang Q,Xu Z,Zhang Y,Pan D, Electrocardiogram: his bundle potentials can be recorded noninvasively beat by beat on surface electrocardiogram. BMC cardiovascular disorders. 2017 Mar 15 [PubMed PMID: 28298182]|
|||Correction to: Heart Disease and Stroke Statistics-2017 Update: A Report From the American Heart Association. Circulation. 2017 Mar 7 [PubMed PMID: 28264899]|
|||Sharma PS,Vijayaraman P, His Bundle Pacing Or Biventricular Pacing For Cardiac Resynchronization Therapy In Heart Failure: Discovering New Methods For An Old Problem. Journal of atrial fibrillation. 2016 Dec [PubMed PMID: 29250259]|
|||Deshmukh P,Casavant DA,Romanyshyn M,Anderson K, Permanent, direct His-bundle pacing: a novel approach to cardiac pacing in patients with normal His-Purkinje activation. Circulation. 2000 Feb 29 [PubMed PMID: 10694526]|
|||Ellenbogen KA,Vijayaraman P, His Bundle Pacing: A New Promise in Heart Failure Therapy? JACC. Clinical electrophysiology. 2015 Dec [PubMed PMID: 29759413]|
|||Rosen KM, The contribution of His bundle recording to the understanding of cardiac conduction in man. Circulation. 1971 Jun [PubMed PMID: 5578869]|