The atrioventricular (AV) node is a small structure within the heart’s electrical system. It coordinates each heartbeat by controlling the transmission of electrical signals from the upper chambers (atria) to the lower chambers (ventricles). This coordination is important for maintaining a regular heart rhythm and ensuring the heart effectively pumps blood throughout the body.
Location and Basic Anatomy
The AV node is located in the lower back portion of the interatrial septum, a wall of tissue that separates the heart’s two upper chambers. It is found near the opening of the coronary sinus and above the tricuspid valve. This compact structure measures approximately 5 millimeters long and 5 millimeters wide.
The AV node consists of specialized cardiac muscle cells, distinct from the surrounding atrial tissue. These cells include nodal cells, which conduct electrical impulses slowly, and transitional cells, which bridge the nodal cells and the atrial tissue.
The AV Node’s Role in Heartbeat Regulation
The heart’s electrical activity begins with the sinoatrial (SA) node, often called the heart’s natural pacemaker, located in the right atrium. The SA node generates electrical impulses that spread across the atria, causing them to contract and pump blood into the ventricles. These electrical signals then travel to the AV node.
The AV node functions as a gatekeeper, the only normal electrical connection between the atria and ventricles. Its function is to delay the electrical impulse from the atria before it passes to the ventricles. This delay, typically around 0.09 to 0.13 seconds, is important for effective heart function.
This delay allows the atria to fully contract and empty blood into the ventricles before the ventricles begin to contract. Without this pause, the atria and ventricles might contract almost simultaneously, leading to inefficient blood pumping. The delay also helps prevent excessively fast electrical signals from reaching the ventricles during rapid atrial rhythms, protecting the heart from potentially dangerous irregular beats.
When the AV Node Doesn’t Function Properly
When the AV node malfunctions, the heart’s rhythm can be affected. Problems can arise if electrical signals are delayed too much, blocked entirely, or conducted too quickly. These issues can impact the heart’s ability to efficiently pump blood.
One common problem is called heart block or atrioventricular block, where the electrical signal from the atria to the ventricles is impaired. In a first-degree heart block, the electrical signal is delayed more than usual as it passes through the AV node, but all signals still reach the ventricles. Second-degree heart block involves some electrical signals failing to reach the ventricles, leading to skipped beats. In third-degree heart block, there is a complete blockage of electrical signals between the atria and ventricles, causing the ventricles to beat independently at a much slower rate.
Conversely, the AV node can conduct impulses too rapidly, or abnormal electrical pathways can form within or around it. These conditions can lead to very fast heart rates, such as atrioventricular nodal reentrant tachycardia (AVNRT), where a rapid circuit of electrical activity forms within the AV node itself. Whether AV node conduction is too slow or too fast, these dysfunctions can result in symptoms like palpitations, dizziness, fatigue, or fainting, impacting the heart’s overall pumping efficiency.