Figure 1-18 If the heart were composed of just muscle tissue, the electrical impulse would travel unimpeded from the atria to the ventricles, causing simultaneous contraction of the atria and ventricles.
© Jones & Bartlett Learning.
The Electrical Conduction System
The cardiac impulse is essentially bioelectrical energy. From our own experience in life, we know that electricity travels everywhere it can, as fast as it can. In the heart, one way that bioelectrical energy can travel is by direct cell-to-cell transmission of the impulse (Figure 1-18). This form of impulse transmission is slow and would lead to unsynchronized mechanical contraction of the heart. In this section, we begin to look at a group of specialized cells in the heart that make up a specialized system known as the electrical conduction system.
Figure 1-18 If the heart were composed of just muscle tissue, the electrical impulse would travel unimpeded from the atria to the ventricles, causing simultaneous contraction of the atria and ventricles.
© Jones & Bartlett Learning.
We have discussed the need for controlled contraction of the atria in order to superfill the ventricles. Let’s begin our look at the electrical conduction system by asking a simple question: If transmission occurred throughout the atria by any means available (cell-to-cell transmission, specialized conduction system, etc.), how can you trigger the atria and not trigger the ventricles at the same time (Figure 1-18)?
If the electrical impulse traveled freely from the atria to the ventricles, then both the atria and the ventricles would contract almost simultaneously. This simultaneous stimulation would not allow the main priming function of the ventricles to occur. In order to prime the ventricles, the atria must contract just before the ventricles begin to contract. Sequential contraction has to happen because once the ventricles contract, the increased force of the ventricular contraction shuts the AV valves. If the valves are closed, no blood can go through them. The blood pumped by the atria would not overfill the ventricles, but would instead be forced back into the circulatory system.
Nature, in its infinite wisdom, overcame this problem with some elegant solutions. First, it made the AV septum a wall of nonconductive tissue between the atria and the ventricles (Figure 1-19). This septum acts as a firewall between the atria and the ventricles, completely stopping the conduction of the impulse before it reached the ventricles. This alone was not a great solution. Now, how does the impulse reach the ventricles? It can’t. Enter elegant solution 2, the AV node.
Figure 1-19 The AV septum represents a nonconductive wall between the atria and the ventricles. If the AV septum did not have any communication between the atria and the ventricles, the impulse would never reach the ventricles.
© Jones & Bartlett Learning.
The only electrical communication between the atria and the ventricles is a small amount of gatekeeping tissue known as the AV node (Figure 1-20). The AV node slows the conduction of the impulse from the atria to the ventricles just enough to allow the atria to finish their mechanical contraction.
Figure 1-20 The AV node represents the only communication between the atria and the ventricles. It functions as the gatekeeper for the electrical impulse.
© Jones & Bartlett Learning.
Think of the AV node as a guarded gate at an apartment complex. When you first drive up to the guard gate, the gate is down and you need to pull your car to a stop. The guard asks you a bunch of questions, calls the person in the apartment and asks if they are ready to receive you, and then opens the gate. Only after the gate opens can you drive through without causing your car some serious damage. The guard, the gatekeeper, has effectively slowed your arrival to maximize the effectiveness of the owner of the apartment. This is exactly what the AV node does in the heart. The atrial impulse reaches the AV node and wants to pass. The AV node slows down the conduction until the ventricles are ready. When the ventricles are ready, the gate is opened completely and the impulse travels through to stimulate the ventricles. This slowing-down function of the AV node is known as the physiologic block (Figure 1-21).
Figure 1-21 Just as a bullet travels faster through the air than through water, the electrical impulse travels faster through the specialized conduction system and the myocardium than through the AV node. The AV node slows down the conduction of the impulse, causing the physiologic block.
© Jones & Bartlett Learning.
As we all know, arrhythmias can be quite deadly. Oftentimes hemodynamic compromise occurs because arrhythmias can cause asynchronous, ineffective contraction of the heart. The asynchronous contraction decreases cardiac output, which, in turn, decreases blood pressure and tissue perfusion. No circulation plus no delivery of oxygen or nutrients to the cells equals no life. The sequential, orderly, and controlled means of transmitting the electrical impulse through the AV node and its physiologic block are critical to life.
The electrical conduction system of the heart is made up of specialized cells (Figure 1-22). Some of these are specialized for pacemaking functions and some for the transmission of the impulses that travel through them. We will break down the system in the following paragraphs and describe the functions of each of the parts in greater detail.
Figure 1-22 The electrical conduction system.
© Jones & Bartlett Learning.
DescriptionThe main function of the system is to create an electrical impulse and transmit it in an organized manner to the rest of the myocardium. This is an electrochemical process that creates electrical energy that is picked up by the electrodes when we perform an electrocardiogram. (More on this in Chapter 4, Vectors and the Basic Beat.)
The specialized conduction system is interwoven with the myocardial tissue itself, and is only distinguishable with certain stains under a microscope. So in looking at Figure 1-23, keep in mind that the system is actually in the heart walls. The atrial myocytes are innervated by direct contact from one cell to another; the first cell innervates the second, the second innervates the third, and so on. The internodal pathways transmit the impulse from the sinoatrial (SA) node to the AV node. The Purkinje system encircles the entire ventricles, just under the endocardium, and is the final component of the conduction system. The Purkinje cells innervate the myocardial cells themselves.
Figure 1-23 The electrical conduction system of the heart.
© Jones & Bartlett Learning.
DescriptionWhat is the pacemaker function of the heart, and why do we need it? The pacemaker dictates the rate at which the heart will cycle through its pumping action to circulate the blood. The pacemaker creates an organized beating of all of the cardiac cells, in a specialized sequence, to produce effective pumping action. It sets the pace that all of the other cells will follow. Let’s look at an analogy.
Imagine that each cell of the heart represents a single musician. When we have a few dozen of these musicians, we have an orchestra—the heart. Now, if each musician decides to play whenever he or she wants to, they would make an unrecognizable jumble of sound. The musicians need a beat or signal to cue them when to start to play, direct them when to come into the piece and when to leave, and coordinate their actions to create a beautiful melody. In music, that pacemaker is the underlying beat kept by the drummer or the conductor. In sections that are swift, the beat increases. In sections that are slow and soft, the beat decreases. The same thing happens in the heart; during exercise the pace speeds up, and during rest it slows.
As we have mentioned, there are specialized cells whose function is to create an electrical impulse and act as the heart’s pacemaker. The main area that fills this important function is the SA node, found in the muscle of the right atrium. This area responds to the needs of the body, controlling the beat based on information it receives from the nervous, circulatory, and endocrine systems. The main pacemaker paces at a rate of 60 to 100 beats per minute (BPM), with an average of 70.
One thing we know about the body is that everything has a backup. Every cell in the conduction system is capable of setting the pace (Figure 1-24). However, the intrinsic rate of each type of cell is slower than the cells that precede it. This means that the fastest pacer is the SA node, the next fastest is the AV node, and so on. The fastest pacer sets the pace because it causes all the ones that come after it to reset after each beat. In this way, the slower pacers will never fire. If the faster pacer doesn’t fire for some reason, the next fastest will be there as a backup to ensure function that is as close to normal as possible.
Figure 1-24 Intrinsic rates of pacing cells.
© Jones & Bartlett Learning.
DescriptionThe SA node, the heart’s main pacemaker, is found in the wall of the right atrium at its junction with the superior vena cava (Figure 1-25). Its blood supply comes from the right coronary artery in 59% of cases. In 38%, the blood supply originates from the left coronary artery, and in the last 3%, it arises from both.
Figure 1-25 SA node.
© Jones & Bartlett Learning.
There are three internodal pathways: anterior, middle, and posterior (Figure 1-26). Their main purpose is to transmit the pacing impulse from the SA node to the AV node. In addition, there is a small tract of specialized cells known as the Bachman bundle that transmits the impulses through the interatrial septum. All of these pathways are found in the walls of the right atrium and the interatrial septum.
Figure 1-26 Internodal pathways.
© Jones & Bartlett Learning.
The Atrioventricular (AV) Node
The AV node is located in the wall of the right atrium just next to the opening of the coronary sinus, the largest vein of the heart, and the septal leaflet of the tricuspid valve (Figure 1-27). It is responsible for slowing down conduction from the atria to the ventricles just long enough for atrial contraction to occur. This slowing allows the atria to “overfill” the ventricles and helps maintain the output of the heart at a maximum level. The AV node is always supplied by the right coronary artery.
Figure 1-27 AV node.
© Jones & Bartlett Learning.
The bundle of His, or His bundle, starts at the AV node and eventually gives rise to both the right and left bundle branches (Figure 1-28). It is found partially in the walls of the right atrium, and in the interventricular septum. The His bundle is the only route of communication between the atria and the ventricles.
Figure 1-28 Bundle of His.
© Jones & Bartlett Learning.
The left bundle begins at the end of the His bundle and travels through the interventricular septum (Figure 1-29). The left bundle gives rise to the fibers that will innervate the LV and the left face of the interventricular septum. It first connects to a small set of fibers that innervate the upper segment of the interventricular septum. This will be the first area to depolarize, meaning that the heart’s cells fire. The left bundle ends at the beginning of the left anterior fascicle (LAF) and left posterior fascicle (LPF).
Figure 1-29 Left bundle branch.
© Jones & Bartlett Learning.
The right bundle, which starts at the His bundle, gives rise to the fibers that will innervate the RV and the right face of the interventricular septum (Figure 1-30). It terminates in the Purkinje fibers associated with it.
Figure 1-30 Right bundle branch.
© Jones & Bartlett Learning.
The Left Anterior Fascicle (LAF)
The LAF, also known as the left anterior superior fascicle, travels through the left ventricle to the Purkinje cells that innervate the anterior and superior aspects of the left ventricle (Figure 1-31). It is a single-stranded fascicle, in comparison to the LPF.
Figure 1-31 Left anterior fascicle (LAF).
© Jones & Bartlett Learning.
The Left Posterior Fascicle (LPF)
The LPF is a fan-like structure leading to the Purkinje cells that will innervate the posterior and inferior aspects of the left ventricle (Figure 1-32). It is very difficult to block this fascicle because it is so widely distributed, rather than being just one strand.
Figure 1-32 Left posterior fascicle.
© Jones & Bartlett Learning.
The Purkinje system is made up of individual cells just beneath the endocardium (Figure 1-33). They are the cells that directly innervate the myocardial cells and initiate the ventricular depolarization cycle.
Figure 1-33 Purkinje system.
© Jones & Bartlett Learning.
When it comes to arrhythmias, the anatomy of the heart is easy. You need to know and understand the electrical conduction thoroughly and you need to know that there are four arrhythmogenic zones in the heart (Figure 1-34). The four zones are:
Figure 1-34 Four arrhythmogenic zones.
© Jones & Bartlett Learning.
The first three zones, the SA node, atria, and the AV node, can be classified together as supraventricular because they encompass everything above the ventricles.
Basically, all of the arrhythmias that we will review in this book have their source of origin in one of these four zones (Figure 1-35). We will review each of these rhythms in great detail in their individual chapters. For now, it is just important to understand the concept of the four zones. In Chapter 37, Wide-Complex Tachycardia: Putting It All Together, we will review this concept again with a focus on diagnosing the individual arrhythmias.
Figure 1-35 The cardiac rhythms based on the four arrhythmogenic zones.
© Jones & Bartlett Learning.
Description