Definition
The sinus node (also: sinuatrial node, SA node) is the primary electrical pacemaker of the heart and is largely responsible for heart rate and excitation.
Function of the sinus node
The heart is a muscle that pumps on its own, which means it is not dependent on nerves like most muscles. This is because the heart has so-called clocks or pacemakers. These are cells that discharge themselves spontaneously, practically as if they had been excited by an incoming signal from a nerve.
The most important of these pacemaker centers is the sinus node. It is usually located at the junction of the superior vena cava with the right atrium, at the outermost layer of the heart muscle (epicardium), and various abnormalities have been described. It is not an actually palpable node, but simply a rather spindle-shaped assembly of cells and measures about 0.5 cm in average.
It is supplied with blood through a branch of the right coronary artery. In a healthy person, the sinus node operates at a frequency of about 60 to 80 beats per minute at rest. The excitation then spreads from the sinus node through the entire working musculature of the atria and then reaches the next component of the stimulus conduction system, namely the atrioventricular node (AV node), which lies exactly between the atria and chambers.
After the excitation is delayed here so that the atria and ventricles beat separately, it is transmitted via His-bundle, Tawara leg and Purkinje fibers until it finally reaches the working muscles of the ventricle, where it causes the ventricles to contract and the blood to be expelled from the heart. From the outside, the sinus node can be influenced by the antagonists of the autonomic nervous system, the sympathetic and parasympathetic nerves. If the sympathetic nervous system is more active, the sinus node accelerates its discharges, while the parasympathetic nervous system predominates, the frequency becomes lower.
There are various disorders affecting the sinus node, which are summarized under the term of pathological sinus node “sick sinus syndrome” (SSS). These include simple changes in frequency: if it is too fast, we speak of tachycardia; if it is too slow, we have bradycardia. The worst variant of the sick sinus syndrome is sinus arrest, i.e. a complete failure of the sinus node, which results in a suspension of the heart function and thus an acute cardiac arrest.
Normally, however, after a short pause, a secondary pacemaker is activated, i.e. usually the AV node, which can perform the same function as the sinus node, but operates at a lower frequency of usually 40 to 60 beats per minute (the His bundle also has pacemaker characteristics, but the frequency is even lower here). However, this frequency is sufficient for a healthy person and therefore a sinus arrest is rarely life-threatening. Nowadays this disease can be well controlled with the help of an artificial pacemaker.
