Atrioventricular node – the causes of

Author Ольга Кияница

2018-07-02

Heart rhythm disturbances can be associated with changes in the driver's heart activity. In such cases, the atrioventricular rhythm, which is also called the atrioventricular nodal rhythm, is often determined. Such a pathology poses a threat to human life, as the heart rate is markedly reduced.

The atrioventricular rhythm (AVR) is described as an abnormal heart rhythm resulting from impulses coming from the region of the atrioventricular node, a "connection" between the atria and ventricles.

Under normal conditions, the sinus node of the heart determines the speed with which the organ is beating - in other words, it is the "pacemaker" or the main driver of the heart. With it, the heart rate is maintained at the desired level - in an adult, 60-90 beats per minute.

The electrical activity of the sinus rhythm arises in the sinoatrial node and depolarizes the atria. Then the signal passes through the atrium, the atrioventricular node, along the bundle of His, and then moves along the Purkinje fibers, eventually reaching and depolarizing the ventricles. This sinus rhythm is extremely important, because it ensures that the atria are necessarily contracted in front of the ventricles.

Video: How the heart works

Description of the atrioventricular node

The definition of the "atrioventricular node" is from the Latin: atrium is the entrance, ventriculus is the small stomach.
Synonyms: AV node (AV node), Ashot-Tavar node, atrioventricular node.

In 1906, Sunao Tavara (1873-1952), a young Japanese anatomist, working under the guidance of Ludwig Aschoff in Marburg, Germany, published the "Pulse Transfer System in the Heart of Mammals," which describes his three-year histological search. Tavara discovered a "complex nodal" (atrioventricular (AV) node) site at the proximal end of the divergence of the conducting fibers. He concluded that this was the beginning of an electrically conductive system that propagated from the AV node through a bundle of His, divided into a right and a left branch and ending with Purkinje fibers. Tavara was the first to realize that Purkinje's fibers contain a tissue that quickly delivers impulses to the top of the ventricle, so that their compression extends from the top of the heart to its base.

The atrioventricular node, or AV node, is the secondary center of the cardiac conduction system (second-order node) involved in complex regulation of heart rate.

  • Anatomy

The AV node is a muscular structure in the Koch triangle, located in the right atrial region near the atrial septum. At the macroscopic level, it is difficult to stand out. Located at the junction with the ventricles and regulates the transfer of impulses through a non-conductive cardiac skeleton into the lower chambers of the heart. The node is located on top of the bundle of His. Probably, it is controlled by sympathetic and parasympathetic systems, which participate in complex regulation of the heart rhythm. Usually it is supplied with blood from the atrioventricular nodal branch that extends from the right coronary artery.

Thus, the atrioventricular node is the only electrical connection between the atria and the lower heart chambers located below.

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  • Histology

The atrioventricular node consists of specific cardiac cells (cardiomyocytes), which are partially associated with the connective tissue of the cardiac skeleton. Unlike the working muscles of the heart, the nodal cells are partly devoid of myofibrils and mitochondria.

  • Physiology

The AV node is a secondary center for heart stimulation. Usually he receives depolarization, distributed through the working muscles of the atria, and redirects them to the ventricles after a second delay. The time period for this transition is called the time of atrioventricular conduction (AV-time) corresponding to the ECG interval PQ. Delayed excitation in the AV node is necessary to ensure coordinated and orderly contraction of the atria and ventricles. The systole of the atria is carried out a little earlier than the ventricular systole, which is necessary for the full filling of the lower parts of the heart.

In addition, the AV node acts as a frequency filter. In physiological conditions, it does not conduct signals more than a certain frequency because of the refractory period of depolarized cells of the heart muscle. As a result, even with atrial flutter, when the heart rate increases to 300 beats / min and higher, much less signals reach the ventricles, so they shrink within acceptable limits.

  • Pathophysiology

In the event of a malfunction of the sinus node, the AV node can take the function of a primary rhythm generator and monitor the heart rate from 40 to 60 / min.

Excessive temporary lag in impulse conductivity through the AV node or conduction failure can lead to AV blockade, which is divided into three stages of clinical severity. Sometimes accelerated transmission of electrical impulses is determined, then there is tachycardia and rapid pulse, mostly present in the context of Wolff-Parkinson-White syndrome.

Diseases associated with the atrioventricular node

Blockade of the atrioventricular node

The most common atrioventricular disorder is AV blockade . This is a cardiac arrhythmia that develops due to delayed or interrupted transmission of signals through the atrioventricular node. Often the blockade remains unnoticed and in such cases it is usually determined that the blockade corresponds to the first degree. However, a heavy AV blockade causes the heart to beat very slowly. This phenomenon causes a so-called bradycardia, which sometimes turns into a temporary cardiac arrest with all the ensuing consequences. To treat such conditions, a pacemaker is usually used, which stabilizes the impaired transmission of impulses. With such a serious violation of the work of the AV node, they say about the AV blockade of the third degree.

The blockade of the AV node can be diagnosed by ECG, where it is expressed in an extended PQ interval, depending on the degree of severity. Congenital AV-blockades are extremely rare, but can be defined as part of another congenital heart disease. Most AV blockades are acquired. They usually arise as a result of degenerative changes in the heart. For example, inflammation or infection of the heart muscle can contribute to the formation of a blockade. Patients with this disease are first treated medically to eliminate arrhythmia. If the condition worsens, patients with AV block 2 and 3 blockade are usually implanted with a pacemaker, as drug therapy is considered unreliable in severe symptoms.

Video: AV blockade and its degree on ECG (atrioventricular blockade)

Accelerated conduction between the atria and ventricles

The opposite of AV blockade is the accelerated conduction between the atria and ventricles. This phenomenon is often determined against the background of Wolff-Parkinson-White syndrome. With this cardiac arrhythmia, one or more additional routes of conduction, which communicate ventricles and atria, bypassing the AV node, are usually determined. Accelerated transmission is usually manifested by a significant increase in the pulse, also with it can be determined tachycardia, that is, rapid heartbeat.

In most cases, tachycardia can be regulated by the patient himself. For example, the heartbeat and rhythm of the heart stop a little when the air retention on exhalation. In addition, the doctor usually appoints patients with tachycardia appropriate drugs like Adzhmaline. Unlike delayed transmission of excitation of the sinus node, surgical implantation of the pacemaker with accelerated conduction and tachycardia in most cases is not performed.

Atrioventricular nodal reentering tachycardia

It can occur suddenly and is accompanied by an increase in normal cardiac contractions between the pathological ones.Symptoms can last from several minutes to several days, and some patients do not notice any signs at all.

Re-tentric tachycardia becomes a problem when it occurs frequently and persists for a long time, especially against the background of other heart injuries.

The main signs of the disease are:

  • Fluttering in the chest
  • Frequent heartbeat
  • Inconsistent Breath
  • Dizziness
  • Sweating
  • Heaviness in the neck
  • Fainting or a condition

The most common types of reentry of tachycardia:

  • Atrioventricular nodal reentry tachycardia (AVNRT). It occurs in men and women of any age, although it is more common in young women.
  • Atrioventricular recurrent tachycardia (AVRT) . It is the second type of tachycardia reentry. Most often it is diagnosed in young people.

In the treatment of clinically pronounced pathology, conservative treatment is first used. It is very important to take any antiarrhythmic drugs exactly as prescribed by the doctor to minimize complications. If drugs do not help, catheter ablation is used. During this procedure, the doctor sends one or more catheters through the blood vessels to the heart.Electrodes at the tips of the catheter can act on the tissue with heat, extremely low temperatures or radiofrequency energy. This allows to damage a small area of the myocardium and create an electrical block along the path that caused the arrhythmia.

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Sometimes, the treatment of the reentry of tachycardia is based on the use of a small implantable device called a pacemaker, which generates electrical impulses that stimulate the heart to beat in a normal rhythm. The pacemaker is located under the skin around the collarbone during a minor surgical intervention. The insulated wire is routed from the device to the heart, where it is fixed for a constant time.

Cystic tumor of the atrioventricular node

Primary cardiac tumors are rare diseases, defined with a frequency of 0.0017% to 0.03% of the total.

The cystic tumor of the atrioventricular (AV) node, also known as the AV node's mesothelioma, is a benign congenital tumor that is located in the Koch triangle in the AV-node region of the interatrial septum of the heart.

The AV-nodal region is formed during embryonic fusion, in which the tissue is transformed into a similar tumor. Studies have shown that 10% of people with a cystic tumor of the AV node also have defects in the development of the midline along the central axis of the vertical body (1.9), which indicates a genetic defect associated with the migration of embryological tissues and with adverse heredity. It was suggested that, unlike the present tumor, this is most likely the result of dilatation of cystic spaces, rather than cell replication. In addition there are no mitoses of this tumor

Since implantation of the pacemaker does not prevent sudden death in patients with this tumor, surgical intervention is most often performed. There are different ways to excise the area of the lesion, however, due to the rarity of the AV cystic tumor in combination with the complexity of the lesion diagnosis, surgical resection methods and therapeutic concepts are not fully standardized. For example, the question remains whether the lesion should be completely or partially removed from the base of the interatrial septum.

The prognosis for a cystic tumor of the AV node is favorable for early diagnosis with subsequent surgical and complete surgical removal of pathological formation. Cases of sudden cardiac death have shown that this tumor is associated with fatal cardiac dysrhythmia and partial / complete blockade of the heart. Thus, in spite of the fact that the tumor is benign, most patients get the final diagnosis after autopsy. In addition, it should be pointed out that the size of the tumor, apparently, is not associated with symptoms of lethal arrhythmia or sudden death.

Diagnosis of pathologies of the atrioventricular node

To diagnose the pathology of the AV node, the doctor takes into account the symptoms, the medical history and conducts a physical examination. He can also ask about the presence of risk factors, which often provoke the development of the disease. It can be another pathology of the heart or a problem with the thyroid gland. If necessary, tests are performed to study the heart. Most often used:

  • Electrocardiogram (ECG) . During the ECG, the sensors (electrodes) that will determine the electrical activity of the heart attach to the chest and sometimes to the extremities. The ECG measures the time and duration of each electrical heartbeat phase.

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  • Holter monitoring . This is a portable ECG device that can be worn for a day or more to record heart activity during everyday human activities.
  • Monitoring of events . For sporadic episodes of the disease, you need to keep the portable ECG device within reach, in case of an attack, attach it to your body and press the button of the recording device. This will allow the doctor to check the heart rate during the episode of rhythm disturbance.
  • Echocardiography . During the use of this non-invasive study, the hand device (transducer) is placed on the chest.Through the use of sound waves, images are created, according to which the size, structure and movements of the heart are studied.
  • Implantable recorder . The device detects irregular heart rhythms, for which it is implanted under the skin in the chest area.

If the doctor does not determine the signs of arrhythmia during these studies, other diagnostic methods may be involved:

  • Stress test . In some people, the abnormal rhythm is caused or progressed under the influence of stress or physical exertion. During the stress test, it is suggested to do an exercise on a treadmill or on a stationary bicycle, while the heart activity will be normal. If the doctors during the evaluation determined the probability of arrhythmia due to coronary artery disease, but the patient is experiencing difficulty with training, then a medication that stimulates cardiac activity in such a way that it is similar to physical exercises can be used.
  • Test on an inclined table. A doctor can recommend this test if the patient has fainting conditions. Heart rhythm and blood pressure are measured when a person is lying on a table. Then the table bends down, as if he is getting up.The doctor observes how the heart and the nervous system react to the change in the location of the body.
  • Electrophysiological testing and mapping . In this study, the doctor conducts thin tubes (catheters) through blood vessels to various places of the heart. Being in the cardiac chambers, the electrodes can display the propagation of electrical impulses along the conduction system of the heart.

If necessary, a cardiologist can use electrodes to stimulate the heart to beat at such a rate to cause or stop arrhythmia.This allows us to determine the localization of arrhythmic activity and what causes it.

Video: Conductive Heart System (MSS)


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