Atrioventricular Node (AV Node)

What it is, why it is important

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The atrioventricular (AV) node is a key part of the heart's electrical system. It controls the passage of the heart’s electrical impulse from the atria to the ventricles.

What Is The AV Node?

The AV node is a tiny "button" of specialized cells (roughly 3 by 5 mm in diameter) located near the center of the heart, on the right side of the atrial septum at the junction of the atria and the ventricles.

Its job is to help coordinate the contraction of the atria and the ventricles in response to the heart’s electrical signal.

What Does the AV Node Do?

The AV node controls the passage of the heart’s electrical signal from the atria to the ventricles.

After an electrical impulse is generated by the sinus node, it spreads across the atria, causing the atria to beat. The AV node then "gathers" that electrical impulse and, after a brief delay, allows it to pass through to the ventricles.

Clinical Significance of the AV Node

A normal AV node is important to the efficient functioning of the heart. The brief delay in the electrical impulse caused by the AV node optimizes cardiac function. That delay permits the atria to finish beating, so that the ventricles completely fill with blood, before the ventricles themselves begin to beat.

Furthermore, in stark contrast to other parts of the heart’s electrical system, the more frequently the AV node is stimulated by electrical impulses, the slower it conducts electricity.

This feature - which is called “decremental conduction” - becomes very important with (for instance) atrial fibrillation, where the AV node is bombarded by hundreds of electrical impulses per minute. The decremental conduction prevents most of those impulses from reaching the ventricles, and keeps the heart rate from becoming dangerously elevated.

The AV node is richly supplied by the vagus nerve. An increase in vagus nerve tone (such as can be provoked with a Valsalva maneuver) can transiently block the transmission of the electrical impulse through the AV node. This maneuver is helpful in stopping many kinds of supraventricular tachycardia (SVT).

Some people are born with two distinct electrical pathways through the AV node, which can make them prone to an arrhythmia called AV nodal reentrant tachycardia, or AVNRT. You can read here about AVNRT.

Disease of the AV node can cause either a delay, or a partial or complete block, in the transmission of electrical impulses from the atria to the ventricles - a condition known as "heart block."

A delay in conduction through the AV node is seen on the ECG as an increased “PR interval.” (The PR interval measures the time between atrial contraction and ventricular contraction.) A prolonged PR interval, produced by a delay in AV nodal conduction, is called “first degree AV block.” First degree AV block rarely is medically significant.

If AV nodal conduction becomes slow enough, heart block may ensue. With “second degree AV block” some impulses are blocked from reaching the ventricles. With “third degree AV block” all of the impulses are blocked.

Occasionally AV nodal block is significant enough to produce severe bradycardia, and insertion of a pacemaker may be required. However, heart block caused by AV nodal dysfunction is usually related to drugs (such as beta blockers or calcium channel blockers), or by medical problems that increase vagal verve tone (such as nausea and vomiting), and can almost always be treated without having to resort to a pacemaker.

However, one cause of AV nodal block may require a permanent pacemaker, namely, myocardial infarction (heart attack). The AV node receives its blood supply from the AV nodal artery, which in 90% of people is a branch of the right coronary artery. So AV nodal block due to a heart attack is most commonly seen with right coronary artery heart attacks. Fortunately, even in these cases the AV node usually recovers sufficiently within a few days to make a pacemaker unnecessary.

Sources:

Epstein AE, DiMarco JP, Ellenbogen KA, et al. ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Revise the ACC/AHA/NASPE 2002 Guideline Update for Implantation of Cardiac Pacemakers and Antiarrhythmia Devices): developed in collaboration with the American Association for Thoracic Surgery and Society of Thoracic Surgeons. Circulation 2008; 117:e350.

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