The His bundle is composed of special cardiac muscle cells and, together with the sinus node and the atrioventricular (AV) node, is part of the excitation conduction system of the heart muscles. The His bundle provides the only electrical connection from the atria to the ventricles and, in the event of failure of the sinus and AV node, serves as a backup pacemaker with a base rate of 20-30 beats per minute.
What is the His bundle?
The His bundle, approximately 5 to 8 mm long and named after the Swiss anatomist Wilhelm His, consists of special cardiac muscle fibers (smooth muscle) that have the ability to transmit an electrical potential or to generate an electrical potential themselves. The HIS bundle is an extension of the AV node and forms the electrical bridge between the right atrium and the two ventricles by penetrating the septum between the atrium and the two ventricles and then dividing into right and left ventricular legs. The electrical excitation potential is transmitted along the septum of the right and left ventricles (one right and two left tawara legs) to the apex of the heart, leading to contraction of the ventricular muscles via Purkinje fibers. The His bundle not only provides the electrical connection between the atria and the ventricles, but can also act as a replacement pacemaker should the first pacemaker, the sinus node, and the second pacemaker, the AV node, fail as “pacemakers.” However, the “ventricular replacement rhythm” of the His bundle of 20-30 beats per minute is too slow in the long run.
Anatomy and structure
The His bundle is anatomically closely associated with the AV node and can be considered an extension of the common conduction trunk, truncus fasciculi atrioventicularis. The His bundle, composed of cardiac muscle fibers, traverses the septum between the atria and ventricles and branches into one right and two left excitation leads, the so-called tawara legs. The transmission of the electrical “beat impulse” within the excitation conduction system occurs exclusively via the special muscle fiber cells of the excitation conduction system. Thus, the transmission of the electrical beat impulses is completely autonomous and independent of nerves. This also applies to the His bundle, whose cells are furthermore capable of initiating their own excitation with a frequency of approximately 20 to 30 beats per minute in the event of a total failure of excitation by sinus or AV nodes, quasi as a last back-up.
Function and Tasks
The most important function of the His bundle is to transmit the electrical stimulus to contract the papillary muscles and ventricular muscles in a time-staggered sequence. The electrical “beat” impulse normally generated in the sinus node in the right atrium causes the muscles of the two atria to contract. This opens the two leaflet valves (mitral valve and tricuspid valve) located between the atria and the ventricles, allowing blood to flow from the atria into the ventricles. The two pocket valves (pulmonary valve and aortic valve) are closed during this phase. Only a few milliseconds after contraction of the atria, the His bundle transmits the contraction stimulus to the papillary muscles in the ventricles, causing them to contract first and the tendon filaments holding the edges of the leaflet valves to tighten and close the leaflet valves. Immediately thereafter, the chambers contract (systole) and pump blood through the open leaflet valves into the pulmonary circulation (right ventricle) and the systemic circulation (left ventricle). For optimal functioning, the precise timing of stimulus transmission through the His bundle is enormously important. In addition to controlling the transmission of the beat stimulus, the His bundle also has an emergency function. If the first pacemaker, the sinoatrial node, should fail or if the cells of the right atrium cannot transmit the beat stimulus to the AV node, the AV node steps in as a frequency generator. If the AV node also fails as a pacemaker, the His bundle and the downstream parts of the conduction system act as the very last back-up option with a very slow beat of 20 to 30 beats per minute. The rhythm thus established is also known as ventricular rhythm.
Diseases
The most common complaint and dysfunction associated with the His bundle is His bundle block.In this case, the His bundle blocks the transmission of the electrical impulse to contract the ventricular muscles. His bundle block corresponds to a cardiac arrhythmia that can be caused by inflammation, by inadequate blood flow, or by a degenerative change in the His bundle tissue. Analogously, in the same context is the so-called thigh block. In this case, the site of stimulus block is below the His node in one of the tawara legs. Two or all three legs may also be affected. In the latter case, there is total thigh block. The most common reasons for the development of a thigh block are coronary artery disease, myocardial infarction, myocarditis, or dysfunction of the heart muscle (cardiomyopathy). In very rare cases, junctional ectopic tachycardia may occur in newborns and infants up to 6 months of age. This is a life-threatening cardiac arrhythmia with a ventricular rate of 150 to 350 beats per minute. The exact causes for the occurrence of this disease have not (yet) been adequately researched. Genetic influences probably play an important role, because the occurrence of the disease in certain families accumulates beyond the normal statistical level. Effects of open heart surgery are also discussed as a cause. Superficially, the fast rhythm is caused by increased excitability of the AV node and His bundle.
Typical and common cardiac diseases
- Heart attack
- Pericarditis
- Heart failure
- Atrial fibrillation
- Heart muscle inflammation
