Awhile back I posted a quote from a wonderful website as a response to a question similar to this one. The original set of posts can be viewed at http://www.medhelp.org/posts/show/600583?personal_page_id=9026&post_id=post_3309923 Just
copy and past that address and you can see the whole discussion. The gist of it is listed below....
The following excerpt was taken from an article that was copied from this web address:
This has to be one of THE BEST descriptions of the causes of arrhythmia I have EVER SEEN. For all of you who are trying to understand what is going on in there, here it is!!!
Kudos to the author!!!!!
Your Heart's Clock Regulates Its Rhythm - Part XIV
Ed G. Lakatta, MD
December 6, 2002 (Reviewed: December 16, 2004)
The Heart's Clock and Electrical Pathways
To help understand skipped beats or arrhythmias let's begin by considering where a normal heartbeat originates and how it travels through the heart.
Your heart has an internal clock that's called a "pacemaker", which initiates an electrical discharge that transmits to every part of the heart muscle along a specific path of conduction fibers. This electricity travels at varying, but controlled speeds. When the heart's pacemaker and conduction system are working correctly there is a regularly-occurring and precise spread of electricity across your heart that causes a heartbeat. Follow Figure 1 as we look at this pathway.
The heart has four chambers, two on the top, called atria (2), and two on the bottom, called ventricles (5). The stimulus for a heartbeat starts in the wall of the right atrium where the heart's pacemaker, the sinoatrial, or SA node (1), is located.
The pacemaker generates an electrical current, and this wave of electricity spreads through the right and left atria (2). In response the muscles of the atria contract and force blood from these upper chambers into the lower ones, the ventricles.
As the moving current reaches a specific area between the atria and the ventricles, called the atrioventricular, or AV node (3), it is slowed down slightly. The AV node acts as a delay switch, giving the atria time to squeeze down and contract completely so that the ventricles can fill to capacity with blood before they in turn receive the moving electrical current.
Next, the current travels down a group of fibers that divides, one bundle going into the right ventricle (5) and the other going into the left ventricle (5). This group of fibers is called the bundle of His (4), pronounced, "hiss".
When the impulse reaches the ventricles it spreads across the ventricular muscle cells and initiates the orderly contraction of these lower chambers. By this action blood is ejected from the heart.
In summary, an electrical impulse initiates in the top part of your heart, usually the SA node, producing a spread of electricity across your heart and exciting the heart muscle's cells as it travels. This ultimately results in the contraction of the lower chambers, the ventricles, from which blood is pumped throughout your body.
The Pacemaker Can Lose Control!
The spread of electricity throughout the heart muscle is possible because all heart muscle cells, not just pacemaker cells, are excitable. Usually, the heart muscle cells keep time more slowly than normal pacemaker cells. This allows the pacemaker cells within the SA node to be in control of generating and pacing the heartbeat. But, if a particular group of cells in areas remote from the SA node become extra- or hyper-excitable they can take over as the pacemaker for one or several beats, or for a longer term.
You might think of the heart's cells as a schoolyard full of young boys playing soccer. Some are inherently quick and fast and are always on top of the game, while others usually play a little slower. Now let's say that to gain the attention of a pretty cheerleader some of the slower players begin showing off, speed up and start scoring. Suddenly, they are in control of the game. Likewise, cardiac fibers anywhere in the heart muscle can become hyper-excitable. This enables them to initiate a heartbeat and overtake the normal pacemaker (the SA node), producing an arrhythmia.
Common causes of such hyper-excitability of heart muscle cells include smoking, excessive alcohol consumption, stress (physical and emotional), thyroid disorders, and certain medications. Serious arrhythmias, however, are most often caused by underlying heart disease, commonly coronary heart disease, heart valve problems, and heart failure. (See Article #11 "The Inner Layer Of Your Blood Vessels Is A Battlefield", and Article #4 "Your Older Heart May Cause You To Feel Short Of Breath.")
Categorizing Arrhythmias: When and Where Does the Takeover of SA Node Pacemaking Occur?
Medical science categorizes arrhythmias in several different ways. One way is by the timing and rate of the initiating electrical discharge and the other by its location.
An initiating electrical impulse that occurs too early is called premature because it causes an early heartbeat or premature contraction to occur. A beat that occurs too late, or fails to occur, is often referred to as a blocked beat. If the rate at which initiating electrical impulses occurs is too fast it produces tachycardia (fast heart rate). If initiating electrical impulses occurs too slowly it causes bradycardia (slow heart rate).
Heart rhythms arising from excited cells outside of the SA node, in either the atria or ventricles, are referred to as ectopic. For example, premature atrial beats or contractions (PAC's), and atrial tachycardia originate in the atria. Premature ventricular beats or contractions (PVC's), and ventricular tachycardia originate in the ventricles. Premature beats can be from one focus (unifocal) or multifocal, depending on whether they always arise from the same location or from more than one location. Premature beats may be linked together, called couplets. Three in a row are called short runs of tachycardia. We all probably experience premature beats at some time or another, but they tend occur more often in persons diagnosed with heart disease, and some tend to be more serious than others.
You are almost an expert on arrhythmias at this point! You are familiar with the normal electrical pathway (called the depolarization pathway) shown in Figure 1, and you know that arrhythmias are named by when and where they originate in the heart. Imagine, that during your routine stress test your cardiologist says, "Your EKG looks great, just a couple of extra beats." You respond by asking, "Were they atrial or ventricular in origin, single beats or couplets, unifocal or multifocal, doc?"