AV node: Control center between the atrium and ventricle
The AV node is an area of dense, connective tissue-rich muscle fiber networks in the right atrium close to the border with the ventricle. It is the only conductive connection between the atrium and ventricle: the electrical impulses coming from the sinus node via the atrial muscles spread from the AV node via the His bundle and then further via the ventricular legs and the Purkinje fibers to the outermost heart muscle cells of the ventricles and trigger a contraction of the ventricles (systole).
The signals are transmitted to the AV node with a short time delay. This ensures that the atria and ventricles do not contract at the same time, but shortly after each other. This improves the blood filling of the ventricles: The atrial contraction pushes blood from the atria into the ventricles, which in turn contract shortly afterwards, forcing blood into the outgoing arteries.
AV time
The time that the electrical impulses need to pass from the sinus node via the atria and the AV node to the ventricles is called AV time (atrioventricular conduction time). On the ECG, it roughly corresponds to the PQ interval.
AV node as a secondary pacemaker
AV node as a frequency filter
The AV node is not only a pure control center between the atria and ventricles, but also a frequency filter. If the frequency of the atria is too high (as in atrial fibrillation), it does not allow all impulses to pass through to the ventricles, thereby protecting them.
Problems around the AV node
The so-called AV block is a form of cardiac arrhythmia in which the AV node is blocked to a greater or lesser extent. There are three degrees of severity:
In a 1st degree AV block, conduction of impulses between the atrium and ventricle is delayed, which usually causes no symptoms.
A 2nd degree AV block is a partial conduction block, which means that not all impulses are transmitted to the ventricle.
A 3rd degree AV block means a complete blockage of atrioventricular conduction: the excitation of the atrium does not spread to the ventricle. This develops its own rhythm as a substitute. Overall, the atria and ventricles then contract independently of each other. This is very dangerous. The symptoms range from loss of performance and dizziness to unconsciousness and cerebral seizures to irreversible brain damage and death.
Another health disorder of the AV node is AV node re-entry tachycardia: Here, in addition to the AV node, there is a second, functionally separate conduction pathway between the atrium and ventricle. The excitation can circulate via these two pathways between the atrium and ventricle. This reciprocal circular excitation (reentry) leads to seizure-like palpitations (tachycardia), which can last from seconds to days. AV nodal reentry tachycardia usually affects younger people with healthy hearts.