Sinus node

definition

The sinus node (also: sinoatrial node, SA node) is the heart's primary electrical pacemaker and is largely responsible for the heart rate and stimulation.

Function of the sinus node

The heart is a musclethat pumps independently, that is, unlike most muscles, it does not depend on it annoy is excited. That is because there is so-called heart on the heart Clock or Pacemaker gives. These are cells that spontaneously discharge, practically as if they had been excited by an incoming signal through a nerve.

The most important of these pacemaker centers is the sinus node. This is usually at the mouth of the superior vena cava (Superior vena cava) in the right atrium, on the outermost layer of the Heart muscle (Epicardium), whereby various deviations have already been described. It is not actually a palpable knot, but simply a more spindle-shaped aggregation of cells and on average roughly 0.5 cm large. Through a branch of the right coronary artery he is supplied with blood. In healthy people, the sinus node works with a frequency of about 60 to 80 beats per minute. The excitation then spreads from the sinus node over the entire working muscles of the atria and then reaches the next component of the conduction system, namely the atrioventricular node (AV node), which lies exactly between the atria and ventricles. After the excitation is delayed here so that the atria and ventricles beat separately, it becomes over Bundle of his, Tawara thigh and Purkinje fibers until it finally reaches the working muscles of the chamber, where it causes the chambers to contract and the blood to be expelled from the heart.

Figure sinus node

1. Sinus node -
   Nodus sinuatrialis
2. AV node -
   Nodus atrioventricularis
3. Trunk of the excitation conduction
   systems -
   Atrioventricular fasciculus
4. Right thigh -
   Crus dextrum
5. Left leg -
   Crus sinistrum
6. Rear thigh branch -
   R. cruris sinistri posterior
7. Front thigh branch -
   R. cruris sinistri anterior
8. Purkinje fibers -
   Subendocardiales
9. Right atrial -
   Atrium dextrum
10. Right ventricle -
    Ventriculus dexter

You can find an overview of all Dr-Gumpert images at: medical illustrations

From the outside, the sinus node can be affected by the opponents of the autonomic nervous system, Sympathetic and Parasympathetic nervous system, to be influenced. If the sympathetic nervous system is more active, the sinus node accelerates its discharges; if the parasympathetic nervous system predominates, the frequency becomes lower.

There are various disorders that can cause the Sinus node concern, which one under the term of the pathological sinus node "Sick Sinus Syndrome" (SSS) summarizes. These include, on the one hand, the simple changes in the frequency: if this is too fast, one speaks of TachycardiaIf, on the other hand, it is too slow, there is one Bradycardia in front. The worst variant of sick sinus syndrome is that Sinus arrest, i.e. a complete failure of the sinus node, which causes the heart to stop functioning and thus a acute cardiac arrest has the consequence. Usually, however, a secondary pacemaker steps in after a short break, usually the AV node, which can fulfill the same function as the sinus node, only with a lower frequency than usual 40 to 60 beats per minute works (also that Bundle of his has pacemaker properties, but the frequency is even lower here). However, this frequency is sufficient for a healthy person and therefore sinus arrest is rarely life-threatening. Nowadays this disorder can be dealt with using a artificial pacemaker be well controlled.

Sinus node defect

If the sinus node fails as the primary pacemaker and stimulus center of the heart, a secondary clock has to step in for it (sick sinus syndrome). This becomes the atrioventricular node (AV node) and can take over the function of the sinus node to a certain extent. It generates a rhythm with a lower frequency, so the heart does not beat 60-70 times per minute as usual, but only about 40 times. In the case of certain diseases (e.g. coronary heart disease), the sinus node remains functional, but generates the excitations at a greater distance so that the heart rate becomes slower (so-called. Sinus bradycardia).

Sick sinus syndrome

The term Sick sinus syndrome summarizes several cardiac arrhythmiasresulting from a defective sinus node. Men and women over the age of 50 are affected roughly equally often. The cause is often a scarring change in the tissue in the heartwhere the specialized excitation cells of the sinus node are located. In the first place is the high blood pressure (arterial hypertension), which leads to a pressure load on the atria and thus to overstretching and damage to the tissue in the area of ​​the sinus node. Also one Myocarditis (inflammation of the heart muscle) or one coronary heart desease can be causative. Also other heart diseases like Valvular heart disease are possible triggers.
Likewise, a Overdosage of certain medications such as beta blockers cause sick sinus syndrome. Sick sinus syndrome can also develop in children who have to undergo an operation due to a congenital heart defect.

There is no general consensus on the exact definition of the term sick sinus syndrome; clinically, the term refers to one Arrhythmia that occurs together with pronounced low and high pulse rates (Tachycardia-bradycardia syndrome). Sinus bradycardias without any other identifiable cause or sino-atrial block are also summarized under the term. One makes itself symptomatic Bradycardia (too slow heartbeat) than Dizziness, syncope (fainting), or hearing and vision problems noticeable, whereas a Tachycardia (too fast heartbeat) than Palpitations, chest tightness (angina pectoris) or shortness of breath (Dyspnoea).

The most important tool in diagnostics is a Long-term ECG and stress ECGwith which the electrical activity of the heart can be shown. Depending on the exact clinical picture, the therapy can be medicated (with so-called antiarrhythmics, Medicines for arrhythmias) or it must be a Pacemaker can be used, which replaces the function of the defective sinus node. In Germany, almost every third pacemaker is used in a patient with sick sinus syndrome.