We have seen how the atrioventricular (AV) node functions as a gatekeeper between the atria and the ventricles. In this section, we are going to look at rhythms that originate primarily in the AV node. In order to do this, we need to look at the anatomy of the AV node and its surrounding structures in closer detail. Physiologically and arrhythmogenically (ability of the tissue to generate arrhythmias), the area around the AV node is just as important as the AV node itself. This distinction is so important that this area has been named the AV junction, and rhythms formed in this area are referred to as junctional rhythms.
The AV junction is actually not a homogenous structure, but instead is composed of three different zones along with one or two main approaches or tracts separated by nonconductive fibrous tissue. Each of the tracts and zones has its own particular histologic make-up and arrhythmogenic and conductive properties. The junctional area is located at the base of the right atria near the meeting place of the interatrial and interventricular septum. Figure 21-1 is a graphic representation of the AV junctional area and its approaches or tracts.
Figure 21-1 The AV junction is composed of one or two approaching tracts separated by fibrous, nonconductive tissue and three separate zones in the AV node proper. For simplicity, only one tract is presented here. We shall review the particular issues related to two tracts in Chapter 25, AV Nodal Reentry Tachycardia.
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All of us have either one or two main approaches or tracts leading to the AV nodal area proper. When there are two tracts, these are named the anterosuperior and the posteroinferior approaches. These approaches are separated from the rest of the myocardium, and each other, by fibrous and other nonconductive tissues. They functionally resemble specialized conduction tracts (most people use the term “tracts” even though this is technically incorrect, so for the sake of conformity, we will be doing the same).
Each of these two tracts has its own individual conductive properties. The anterosuperior tract conducts impulses very rapidly, and so, it is also known as the “fast” tract. The posteroinferior tract is slower in conducting the impulses and is, therefore, also known as the “slow” tract. Most clinicians use the terms “fast tract” and “slow tract” exclusively to keep things simple. As we shall see in Chapter 25, AV Nodal Reentry Tachycardia, the physiologic characteristics of the two tracts will be a critical factor in forming the rapid circus movements leading up to some pretty fast supraventricular tachyarrhythmias.
Now, let’s turn our attention to AV node proper and its surrounding areas. A good way to think about the zones is to envision the AV node like a Q-tip. As you first arrive at the AV node from the atria, you encounter a small area of tissue that surrounds the tip of the node known as the transitional cell zone (see Figure 21-1). This is equivalent to the cotton part that is always found at the end of the Q-tip. This zone is full of autonomic fibers and is very arrhythmogenic.
The center or core of the AV node is also called the compact zone. This area is composed of cells very similar in histologic appearance and function to those cells found in the sinoatrial (SA) node. As you can imagine, the compact zone is prone to function as a primary pacemaker. Whether autonomic fibers influence the compact zone directly is debatable, but the influence of autonomic innervation is definitely felt through the adjoining zones.
The final zone we will look at is the area extending from the compact zone to the bifurcation (splitting) of the His bundle. This area is insulated from the rest of the myocardium and penetrates the membranous septum. The main function of this zone is to conduct the impulse rapidly to the bundle branches.
In about 90% of people, the blood supply to the AV node is derived from the right coronary artery system. The remaining 10% usually have the circulation of the node originating in the circumflex artery. The dominance of the right coronary artery as the leading supplier of blood and nutrients to the AV junction is the main reason that myocardial infarctions involving the inferior wall are usually associated with AV nodal blocks and bradyarrhythmias. (The inferior wall of the heart is also primarily supplied by the right coronary artery system, and the inferior descending artery in particular.)