The bundle of His, also known as the AV bundle or atrioventricular bundle, is a collection of heart muscle cells specialized for electrical conduction that transmits the electrical impulses from the AV node (located between the atria and the ventricles) to the point of the apex of the fascicular branches. The fascicular branches then lead to the Purkinje fibers which innervate the ventricles, causing the cardiac muscle of the ventricles to contract at a paced interval.

Eponym

These specialized muscle fibres in the heart were named after the Swiss cardiologist Wilhelm His, Jr., who discovered them in 1893.

Function

Cardiac muscle is very specialized, as it is the only type of muscle that has an internal rhythm; i.e., it is myogenic which means that it can naturally contract and relax without receiving electrical impulses from nerves. When cardiac muscle cells are placed next to another, they will beat in unison.

The fibers of the Bundle of His allow electrical conduction to occur more easily and quickly than typical cardiac muscle. They are an important part of the electrical conduction system of the heart as they transmit the impulse from the AV node (the ventricular pacemaker) to the rest of the heart. The intrinsic rate of the Bundle of His is between 40 - 60 bpm. The bundle of His branches into the three bundle branches: the right, left anterior and left posterior bundle branches that run along the interventricular septum. The bundles give rise to thin filaments known as Purkinje fibers. These fibers distribute the impulse to the ventricular muscle. Together, the bundle branches and Purkinje network comprise the ventricular conduction system. It takes about 0.03-0.04s for the impulse to travel from the bundle of His to the ventricular muscle.

Pathology

It is extremely important for these nodes to exist as they ensure the correct control and co-ordination of the heart and cardiac cycle. They ensure that cardiac contraction follows the correct sequence and takes place in a coordinated manner. If the cardiac muscle just contracted and relaxed randomly at their own natural rhythm the cycle would become disordered and the heart would become unable to carry on its function of being a pump.

Sometimes when the heart undergoes great damage to one part of the cardiac muscle or the person incurs an electric shock, the cardiac cycle can become uncoordinated and chaotic. Some parts of the heart will contract whilst others will relax so that instead of contracting and relaxing as a whole, the heart will flutter abnormally. This is called ventricular fibrillation and can be fatal if not treated within 10 minutes, a standard clinical heuristic being that survivability decreases 10% per minute.

Fibrillation can be detected by an electrocardiogram which measures the waves of excitation passing through the heart and plotting a graph of potential difference (voltage) against time. If the heart and cardiac cycle is functioning properly the electrocardiogram shows a regular, repeating pattern. However if there is fibrillation there will be no apparent pattern, and defibrillation is necessary. Defibrillation consists of delivering a therapeutic dose of electrical energy to the affected heart with a device called a defibrillator. This depolarizes a critical mass of the heart muscle, terminates the arrhythmia, and allows normal sinus rhythm to be reestablished by the body's natural pacemaker, in the sinoatrial node of the heart.

If the Bundle of His is blocked, it will result in dissociation between the activity of the atria and that of the ventricles, otherwise called a third degree heart block. The other cause of a third degree block would be a block of the right, left anterior, and left posterior bundle branches. A third degree block is a very serious medical condition that will most likely require an artificial pacemaker.

His Bundle Pacing

In 2000, Dr. Pramod Deshmukh, an electrophysiologist in Sayre, PA announced that he had successfully performed Direct His-Bundle pacing that produced synchronous ventricular depolarization and improved cardiac function relative to apical pacing.

References

Silverthorn, Dee U. Human Physiology: an integrated approach. Fourth Edition. University of Texas. Benjamin Cummings publishing, 2007.

External links

• - "Right atrioventricular bundle branch, anterior view"
• Scheinman MM, Saxon LA (2000). "Long-term His-bundle pacing and cardiac function". Circulation 101 (8): 836–7.
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• http://www.healthyheart.nhs.uk/heart_works/heart03.shtml