An Overview of the Workings and Afflictions of the AV Node
Afflictions of the AV node are the second-most common cause of bradycardia, after SA nodal disorders. The AV node is a small, bulbous structure that conducts electrical signals between the atria and the ventricles of the heart. Conduction through the AV node is purposely slower than through regular myocardium. This is to ensure that all blood is pumped out of the atria before the ventricles have a chance to contract.
If a patient is experiencing the symptoms of bradycardia (lightheadedness, syncope) and an AV nodal conduction disorder is suspected to be the cause, then a pacemaker may be necessary to fix the problem. The indication for a pacemaker depends on two major characteristics of the AV nodal disorder (aka AV block): the site of the disorder and the degree of block. An electrophysiology (EP) study can help in defining both of these characteristics.
Determining the site of the AV nodal disorder is very important. If the site of AV block if proximal (close to) the AV node itself, then chances are no symptoms are occurring and the disorder is benign. This is because pacemakers located in the AV junction reliably take over the duties of the malfunctioning AV node. On the other hand, if the site of block is distal to the AV node, then the escape pacemakers that take over tend to be unreliable, unstable, and slow, therefore producing the symptoms of bradycardia. If block is distal to the AV node, then it is occurring in the His-Purkinje system. The His-Purkinje system consists of the His bundle + Purkinje fibers. The His-Purkinje system conducts rapidly so that electrical impulses exiting the AV node can quickly reach the ventricular myocardium and cause them to contract.
AV block can be first-degree, second-degree, or third-degree. These can be differentiated by observing wave patterns on a typical 12-lead surface electrocardiogram (ECG). A typical, simplified ECG is depicted below. P waves indicate depolarization of the atria while QRS complexes indicate depolarization of the ventricles. The PR interval is the time from the beginning of a P wave to the beginning of a subsequent QRS complex. In other words, the PR interval is an indication of the conduction time between the atria and the ventricles. Because the AV node is the major conducting element between the atria and the ventricles, the PR interval is directly related to AV node conduction time and therefore AV nodal health. The slower the conduction velocity of the AV node, the more likely that there is something wrong with it. T waves indicate repolarization of the ventricles.
In first-degree AV block, all impulses originating in the atria are conducted to the ventricles. This is manifested on an ECG by a 1:1 ratio of P waves to QRS complexes. In other words, for every atrial contraction, there is a subsequent ventricular contraction. The difference between first-degree AV block and normal sinus rhythm, then, is that longer-than-normal PR intervals are observed during first-degree AV block. In most cases, first-degree AV block is proximal to the AV node so it is asymptomatic. However, the EP study can confirm if first-degree AV block is truly proximal or distal to the AV node.
In second-degree AV block, some atrial impulses are conducted to the ventricles, and some are not. There is a special classification system — Mobitz classification — for second-degree AV block. If the PR intervals of conducted beats preceding a non-conducted beat gradually get longer, then the AV block is classified as second-degree Mobitz type I. Mobitz type I block is usually, but not always, localized to the AV node and is therefore proximal. The EP study can help determine this for sure. If the PR intervals of conducted beats preceding a non-conducted beat remain constant, then the AV block is classified as second-degree Mobitz type II. Mobitz type II block is always distal to the AV node.
The exception to the Mobitz classification system is 2:1 AV block. In 2:1 AV block, there are 2 atrial impulses (P waves) for every 1 ventricular impulse (QRS complex). In other words, every other impulse originating from the SA node is not conducted. It is therefore impossible to observe prolongation or constancy in PR intervals because there is only one PR interval preceding every dropped beat. The EP study can help determine the site of block when Mobitz classification is not applicable.
Lastly, third-degree AV block (aka complete heart block) is a type of atrioventricular (AV) dissociation. No atrial impulses are conducted to the ventricles via the AV node in complete heart block. AV dissociation occurs when the atria contract completely independently of the ventricles. This means that the ventricles have some other source of impulse generation than the SA node. A clear explanation + examples of AV dissociation are discussed in the video linked below.
Complete heart block is completely dependent on escape pacemakers, so determining the site of block is integral to deciding if a pacemaker is necessary or not. If the block is proximal, a pacemaker is most likely not needed. If the block is distal, symptoms will likely occur and a pacemaker is likely needed. The EP study can help with determining the site of third-degree AV block.
