What is the sinus node of the heart?

Author Ольга Кияница

2018-07-01

The main driver of the rhythm of the heart, sinus node, has an interesting history of discovery and a number of amazing features in the structure and functioning. From the coordination of the work of this part of the heart depends the overall activity of the whole organ, therefore, with dysfunction of the sinus node, treatment is always carried out, otherwise there is a risk of fatal outcome.

The sinoatrial node (often abbreviated as ACS, also called the sinus node, the first-order driver) is a normal natural pacemaker and is responsible for starting the heart cycle (heartbeat). It spontaneously generates an electrical impulse, which, after passing through the whole heart, causes it to contract. Although electrical pulses are generated spontaneously, the rate of arrival of the pulses (and hence the heart rate) is controlled by the nervous system innervating the sinoatrial node.

The sinoatrial node is located in the wall of the myocardium near the place where the mouth of the hollow veins (sinus venarum) connects to the right atrium (upper chamber);hence, the name given to education is the corresponding - sinusoidal node. [1 - Elsevier, Dorland's Illustrated Medical Dictionary, Elsevier]

The value of the sinus node in the work of the heart is paramount, since with the weakness of the ACS, various diseases occur, sometimes contributing to the development of sudden cardiac arrest and death. In some cases, the disease does not manifest itself, but in others, specific diagnostics and appropriate treatment are needed.

Video: SA NODE

Opening

On a hot summer day in 1906, Martin Flack, a medical student, studied the microscopic sections of the mole heart, while his mentor Arthur Keith and his wife rode a bicycle along the beautiful cherry orchards near their cottage in Kent, England. Upon her return, Flack excitedly showed Keith "the wonderful structure that he found in the right ear of the mole's auricle, exactly where the upper vena cava enters this chamber." Kate quickly realized that this structure is very similar to the atrioventricular node described by Sunao Tavara earlier this year. Further anatomical studies confirmed the same structure in the hearts of other mammals, which they called a "sinusoidal node" (sino-auricular node). Finally, a long-awaited heart rate generator was discovered.

Since 1909, using a galvanometer with two strings, Thomas Lewis simultaneously recorded data from two sections from the surface of the dog's heart, making precise comparisons of the arrival of the excitation wave at different points. Lewis identified the sinus node as a heart pacemaker with two innovative approaches.

  • First, he stimulated a superior hollow vein (SVC), coronary sinus and left eye and showed that only the curves near the sinus node were identical to the normal rhythm.
  • Secondly, it was known that the point at which compression begins begins to become electrically negative with respect to the inactive points of the muscles. As a result, the electrode near the ACS invariably had a primary negativity, indicating: "The nodal region of SA is the place where the excitation wave starts."

The cooling and heating of the sinus node to study the reaction of the heart rhythm was carried out by G Ganter and others, which also pointed to the location and primary function of the sinusoidal node. When Einthoven was awarded the Nobel Prize in 1924, he generously mentioned Thomas Lewis, saying: "I doubt that without his valuable contribution, I would have the privilege of standing in front of you today." [2 - Silverman, ME; Hollman, A. (1 October 2007). "Discovery of the sinus node by Keith and Flack: on the centennial of their 1907 publication]

Location and structure

The sinoatrial node consists of a group of specialized cells located in the right atrial wall, only transversely to the mouth of the hollow veins at the junction, where the upper vena cava enters the right atrium. The SA node is located in the myocardium. This deep formation rests on cardiac myocytes belonging to the right atrium, and its surface part is covered with fat tissue.

This elongated structure, which extends from 1 to 2 cm to the right of the ear edge, is the crest of the right atrial appendage, and extends vertically to the upper end of the end groove. The SA node fibers are specialized cardiomyocytes that vaguely resemble normal, contractile cardiac myocytes. They have some contractile threads, but they do not shrink as tightly. In addition, the fibers of the CA node are noticeably thinner, sinuous, and stained less intensively than cardiac myocytes.

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Innervation

The sinus node is richly innervated by the parasympathetic nervous system (the tenth cranial nerve (the vagus nerve)) and the fibers of the sympathetic nervous system (spinal nerves of the thoracic region at the level of the stallions 1-4).This unique anatomical location makes the CA node susceptible to apparently paired and opposing vegetative influences. In a state of rest, the work of the node mainly depends on the vagus nerve or its "tone."

  • Stimulation through the vagus nerves (parasympathetic fibers) causes a decrease in the speed of the CA node (which in turn decreases the heart rate). Thus, the parasympathetic nervous system through the action of the vagus nerve has a negative inotropic effect on the heart.
  • Stimulation through sympathetic fibers causes an increase in the speed of the CA node (this increases the heart rate and the force of contractions). Sympathetic fibers can increase the contraction force, because in addition to innervation of the sinus and atrioventricular nodes they directly affect the atria and ventricles.

Thus, the violation of innervation can lead to the development of various disorders of cardiac activity. In particular, the heart rate may increase or decrease and clinical signs may appear.

Blood supply

The CA node receives blood supply from the artery of the CA node. Studies of anatomical dissection showed that this diet can be a branch of the right coronary artery in the majority (about 60-70%) of cases, and the left coronary artery branch supplies the CA node approximately in 20-30% of cases.

In more rare cases, blood supply to both the right and left coronary arteries or two branches of the right coronary artery can be noted.

Functionality

  • The main driver of rhythm

Although some of the cardiac cells have the ability to generate electrical impulses (or action potentials) that cause cardiac contraction, the sinoatrial node usually initiates a heart rhythm simply because it generates pulses faster and stronger than other regions with a pulse generation potential. Cardiomyocytes, like all muscle cells, have refractory periods after contraction, during which additional contractions can not be caused. At such times, their action potential is redefined by the sinoatrial or atrioventricular nodes.

In the absence of external neuronal and hormonal control, cells in the sinoatrial node located in the upper right corner of the heart will naturally discharge (create action potentials) more than 100 beats per minute. Since the sinoatrial node is responsible for the rest of the electrical activity of the heart, it is sometimes called the primary pacemaker.

Clinical significance

Dysfunction of the sinus node is expressed in an irregular heartbeat, caused by abnormal electrical signals from the heart. When the sinus node of the heart is defective, the heart rhythm becomes abnormal - usually too slow. Sometimes there are pauses in its effects or combinations, and very rarely the rhythm is faster than usual.

Occlusion of arterial blood supply to the sinus node (most often due to myocardial infarction or progressive coronary artery disease) can cause ischemia and cell death in the CA node. This often violates the pacemaker activity of ACS and leads to a syndrome of weakness of the sinus node.

If the CA node does not work or the pulse generated in it is blocked before it passes down the electrically conductive system, a group of cells further down the heart play the role of second-order rhythm drivers. This center is usually represented by cells inside the atrioventricular node (AV node), which is the area between the atria and ventricles, inside the atrial septum.

If the AV node also fails, the Purkinje fibers can sometimes act as a pacemaker by default. If the Purkinje fiber cells do not control the heart rhythm, it is most often because they generate action potentials with a lower frequency than the AV or SA nodes.

Sinus node dysfunction

The node node dysfunction refers to a number of conditions that cause a physiological incompatibility of the atrial parameters. Symptoms may be minimal or include weakness, intolerance to effort, heart palpitations and fainting. The diagnosis is based on the ECG. Symptomatic patients require a pacemaker.

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Dysfunction of the sinus node includes

  • Life-threatening sinus bradycardia
  • Alternating bradycardia and atrial tachyarrhythmias (bradycardia syndrome and tachycardia)
  • Sinoatrial blockade or temporary stop of ACS operation
  • Output blockade ACS

Dysfunction of the sinus node occurs predominantly in the elderly, especially in the presence of other cardiac disorders or diabetes.

The stopping of the sinus node is a temporary cessation of activity of the sinus node, observed on the ECG in the form of disappearance of P-waves for a few seconds.

Pause usually causes evacuation activity in lower pacemakers (for example, atrial or connecting pacemakers), keeping the heart rate and function, but prolonged pauses cause dizziness and fainting.

At the exit blockade of the CA node, its cells are depolarized, but the transfer of impulses to the myocardium of the atria is impaired.

  • With the blockade of the ACS of the 1st degree, the pulse slows down slightly, but the ECG remains normal.
  • With blockade ACS of the 2nd degree of the first type, the impulse conductivity slows down to a complete blockade.On ECG disturbances are visible as intervals PP, which gradually decrease until the P-wave does not disappear at all.Instead, there is a pause and grouped blows. The pulse delay duration is less than 2 PP cycles.
  • In case of blockade of ACS of 2nd degree II type, the pulse conductivity is blocked without preceding deceleration, as a result a pause is created which is a multiple of the PP interval and is manifested on the ECG by grouped heartbeats.
  • In the case of blockade of the ACS of the 3rd degree, the conductivity of the pulses is completely blocked; P-waves are absent, which leads to complete failure of the sinus node.

Etiology

Dysfunction of the sinus node can develop when the electrical system of the heart is damaged due to organic or functional disorders. The causes of sinus node dysfunction include:

  • Aging . Over time, age-related heart wear can weaken the operation of the sinus node and make it function incorrectly. Age-related damage to the heart muscle is the most common cause of sinus node dysfunction.
  • Medicinal preparations . Some medications to treat high blood pressure, coronary artery disease, arrhythmias and other heart diseases can cause or worsen the function of the sinus node. These drugs include beta-blockers, calcium channel blockers and antiarrhythmics. Nevertheless, it is extremely important to take cardiac medications and, in most cases, they do not cause problems when performing medical recommendations.
  • Surgery on the heart Surgical interventions involving the upper chambers of the heart can lead to the formation of scar tissue that blocks electrical signals from the sinus node. Postoperative scars on the heart are usually the cause of sinus node dysfunction in children with congenital heart disease.
  • Idiopathic fibrosis of the CA node , which can be accompanied by degeneration below the located elements of the conducting system.

Other causes are drugs, excessive vagal tone and various ischemic, inflammatory and infiltrative disorders.

Symptoms and signs

Often dysfunction of the sinus node does not cause symptoms. Only when the condition becomes serious, problems arise. Even the signs of the disease can be vague or caused by other pathologies.

Symptoms of sinus node dysfunction include:

  • Fainting or a premature condition caused by the fact that the brain does not receive enough blood from the heart.Dizziness may also occur.
  • Pain in the chest (by the type of angina pectoris) occurs when the heart lacks oxygen and nutrients.
  • Fatigue caused by a heart failure that does not pump blood efficiently. When the blood flow is reduced, vital organs receive less blood. This can leave the muscles without enough nutrition and oxygen, causing weakness or lack of energy.
  • Shortness of breath , occurs mainly when adherence to the dysfunction of the CA heart failure or pulmonary edema.
  • Bad sleep caused by an abnormal heart rhythm. Sleep apnea, in which a person experiences pauses during breathing, can contribute to the dysfunction of the sinus node due to a decrease in the supply of oxygen to the heart.
  • Impaired heartbeat , changes more often in the direction of its rapidity (tachycardia). Sometimes it is felt that the rhythm is wrong or on the contrary there is a knock in the chest.

Diagnostics

After the medical collection of medical history and physical examination, tests are used to diagnose sinus node dysfunction. Most often they are:

  • Standard electrocardiogram (ECG). Widely used to detect irregular heart rhythms. Before the test, electrodes are placed on the chest, arms and legs to provide a versatile measurement of the heart. Through wires, the electrodes are attached to the apparatus, which measures the electrical activity of the heart and converts the pulses into lines that look like a series of teeth. These lines, called waves, show a certain part of the heart rhythm. During ECG analysis, the doctor examines the size and shape of the waves and the amount of time between them.
  • Holter monitoring . The device constantly records the heartbeat within 24-48 hours. Three electrodes attached to the chest are connected to a device that the patient wears in his pocket or puts on a belt / shoulder strap. In addition, the patient keeps a diary of their actions and symptoms while wearing the monitor. This allows doctors to determine what happened at the time of the rhythm disturbance.
  • Event monitor . This method registers the heartbeat only when the symptoms of the disease are experienced. An event monitor can be used in place of a Holter monitor if symptoms are less common in the patient than once a day.Some event monitors have wires that connect them to electrodes attached to the chest. The device automatically starts recording when it detects an irregular heartbeat, or the patient starts recording when symptoms occur.
  • Load test on the treadmill e. This testing can be performed to determine the appropriate response to training, represented as a change in the heart rate.

Forecast

The prognosis for sinus node dysfunction is ambiguous.

In the absence of treatment, the mortality rate is about 2% / year, mainly as a result of the progression of the underlying disease, which is often a structural lesion of the heart.

Every year, approximately 5% of patients develop atrial fibrillation with the occurrence of complications such as heart failure and stroke.

Treatment

Severe dysfunction of the sinus node is most often eliminated by implantation of an electrocardiostimulator. The risk of atrial fibrillation is significantly reduced when a physiological (atrial or atrial and ventricular) pacemaker is used, and not only a ventricular pacemaker.

New dual-chamber pacemakers that minimize ventricular stimulation can further reduce the risk of atrial fibrillation.

Antiarrhythmic drugs are used to prevent paroxysmal tachyarrhythmias, especially after the pacemaker is installed.

Theophylline and hydralazine are drugs that increase the heart rate in healthy young patients with bradycardia without fainting in the anamnesis.

Video: Live Fine! Weakness of the sinus node


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