Warning : Use the following information at your own risk. While accuracy is one my goals, there is always the possibility that some of the information could be wrong. There could be typos. I could also be severely mistaken in some of my knowledge. This site is meant to help clarify certain concepts of ECG and at no point should any life-or-death decision be made based upon the information contained within. Remember, this is just some page on the internet. (If you do find errors, please notify me by feedback.)
Muscles cells are told to contract by electrical impulses. In addition to the ability to contract, muscle cells can conduct these electrical impulses to neighboring cells. In fact, some specialized muscle cells in the heart do nothing except conduction.
There are three types of muscle tissue in your body : skeletal (a.k.a. voluntary) muscle, smooth muscle, and cardiac muscle. Your biceps are an example of skeletal muscle. Another skeletal muscle is the diaphragm. The diaphragm is the big muscle that separates your chest cavity from your abdominal cavity. When it is time to breathe in, your brain sends a signal down a nerve which tells the diaphragm to contract. This in turn causes the chest to expand and air to rush in the lungs. The brain thus controls every breath you take (as well as every move you make). People who receive injury to their spinal cord above a certain level might damage this nerve and consequently lose the ability to breathe on their own.
Your heart is different than the diaphragm. It tells itself to beat; it is the master of its own destiny. It does not require the brain. That's not to say the brain cannot influence the heart; indeed, your brain gives your heart advice on when to speed up and when to slow down. In one of the Indiana Jones movies, the evil cult leader punches through a man's chest and rips out his still beating heart. While I imagine the punching and ripping-the-heart-out aspects of the film are impossible, the fact that the heart still beats is not pure Hollywood fiction.
What tells the heart to beat? Pacemakers do. (When I use the word pacemaker, you should assume that I am referring to the heart's natural pacemakers. I will call those man-made devices artificial pacemakers.) What are pacemakers exactly? They are special muscle cells in the heart that fire themselves after a certain amount of time has elapsed. This property is called automaticity. The normal pacemaker is the sinus pacemaker, which usually fires between 60 and 100 times a minute.
The pacemaker generates the electrical impulse and sends it to its neighboring heart muscle cells. These cells then, in turn, conduct to neighboring cells.
The normal contractile cells of the heart will do two things :
Other heart muscle cells are specialized for conduction only.
While all these conduction cells are muscle, it might be helpful to break them down according to function and location :
When talking about the general region of the AV node and the bundle of His, we refer to it as the AV junction.
An ECG says nothing about the contraction of the heart cells. In fact, it is possible for heart cells to lose their contraction ability while maintaining the conduction. Thus, a person may show a perfectly normal ECG without having a heart beat. Even if the heart is beating, there is no guarantee that there is any blood to pump. You can only measure the pulse by mechanical means (e.g. pressing your finger against the artery in the patient's neck or wrist). Don't ever forget this.
Once upon a time (1800's and before), scientist were not that familiar with the underlying electrical system that control the heart. The main thing that was measured was the pulse. The terms bradycardia and tachycardia were given to signify a slow and fast pulse, respectively. There is somewhat of a danger in using these words with ECG interpretation in that we are seeming to imply the presence of a pulse when we describe an electrical pattern. If I use the term "beats per minute", be sure to adapt it to whatever context. It would probably be more accurate to say "ventricular depolarizations per minute," but I will not.
When a heart cell contracts, it does not occur the same instance it is signaled to do so. While a fraction of a second may seem "instantaneous" to us, it is not to the heart.
Depolarization and repolarization
Depolarization is what happens when the heart cell receives an electrical impulse. When you see the word depolarize, you should think trigger.
Repolarization is what happens when the heart cell is recharging itself. When you see the word repolarize, you should think recharge.
When a child refuses to obey his parents or teacher, we say that he is refractory. We also use the term to describe a heart cell that will not depolarize when an electrical impulse is sent to it. When this occurs, it is usually due to the fact that the cell has not had a chance to adequately repolarize yet. There is a period after the cell has been depolarized that absolutely nothing will cause it to depolarize again. We describe this span of time the absolute refractory period. Following this is a span of time in which certain strong impulses may depolarize the cell but the response will not be as strong as a normal depolarization. This time span is called the relative refractory period.
One analogy that is often used to demonstrate this concept is that of a toilet. One cannot continue to flush a toilet and expect a reaction. One must wait for the water level in the tank to go back up. Immediately after the flush, nothing you can do will make it flush again. This is the toilet's absolute refractory period. After a little while, the toilet may flush, but the response is somewhat weaker than a normal flush. This occurs during the toilet's relative refractory period.
The ECG is a record of certain depolarizations and repolarizations only. It does not directly reflect the mechanical actions of the heart.
What can go wrong
You know that the term dysrhythmia (arrhythmia) is used to indicate abnormal electrical activity in the heart. For basic ECG interpretation, there are a number of these dysrhythmias you should be able to recognize. The easiest of them is asystole. It is the flat-line that is so popular on television and essentially means that the ECG machine measures no electrical activity in the current lead.
Dysrhythmia can be used to describe the electrical pattern that causes a heart to beat too fast or slow. It is also used to describe when the impulse does not originate in the place it should, regardless of the heart rate. If some part of the conduction path affects the conduction, it might produce a change on the ECG that will qualify the rhythm as a dysrhythmia.
You may be wondering : "What causes arrhythmias?" There are many things that are known or suspected to cause arrhythmias.
A Tale of Two Nodes
In dealing with the heart, there are two nodes you must be familiar with : the SA node and the AV node. But first, what does node mean? It is a knot, a knob, a swelling, etc. You may have heard a lump referred to as a nodule; this word means "little node." The two nodes I have mentioned were named because they appear as knots (or swellings) in the tissue. We, however, will not be concerned with their appearance but rather with their function.
Our heart's normal pacemaker is located in the sinus node. The sinus node is also called the sinoatrial (SA) node. It is located in the right atrium and is responsible for producing the signal that tells our heart to beat. This marks the beginning of the electrical journey through the heart. (This journey usually repeats itself about 70 times a minute.) A rhythm that originates from the pacemaker in the sinus node is called a sinus rhythm.
From the sinus node, the electrical signal travels along the wall (through the muscle) of the atria. Electricity can freely flow from one atrium to the other. It can also flow from one ventricle to the other. It cannot, however, pass freely between the atria and ventricles; the two sets of chambers are electrically insulated. There is a way across, however. The pathway that connects the atria and the ventricles is called the atrioventricular (AV) node. Think of it as a tunnel. The AV node is essentially the "Mines of Moria" of your heart's electrical system. (If your heart's electrical system were middle earth, the sinus node would be the hobbit village). In a normal heart, it is the only way to reach the ventricles. Passage through the node is considerably slower than through other parts of the conduction system.
After it emerges from the AV node, the electrical pathway is known as the bundle of His (pronounced "hiss"- in figure 3-1, it is the thick yellow line leaving the AV node). The bundle soon forks into the left bundle branch and the right bundle branch. (I guess this fork would represent the last location in "Fellowship of the Ring". Both the fellowship and the analogy break up at this point.) The electrical paths travel down to the tip of the heart and then curve back up the sides. Connecting the main electrical pathway to the various portions of the muscle are microscopic electrical pathways (not shown on diagram) known as Purkinje fibers. Purkinje is pronounced pur-KIN-jee (in English, that is. I'm not quite sure how Mr. Purkinje pronounced it.) The electrical signal travels through these fibers, eventually causing contraction of the muscle.
The itinerary of the electrical impulse (subject to change) :
Memorize this now :
Make sure that you can follow this path in figure 3-1.
Imagine one of those loud "machine gun" firecrackers that consists of two columns of miniature-dynamite-looking explosives. The fuse runs down between the two columns. When the fuse is lit, the burning part of the fuse travels from the tip of the fuse down between the two columns, lighting each individual firecracker as it passes it. The firecrackers closest to the side the fuse was lit explode first, followed by the next row, and so on. The fuse is much like the electrical system in your heart. It is "lit" at the pacemaker, and travels a certain path, almost down through the middle of the heart. As the pulse travels, it causes the adjacent muscle to contract, which causes blood to be ejected from the heart. This creates the pulse. It should do this between 60 and 100 times a minute, or more if you are exerting yourself. While the firecrackers can only be triggered once, your muscle recharges after every beat and can be triggered over and over. As the "lit fuse" of electricity travels through the heart, the depolarization/repolarization can be detected through wires connected to the skin. This is where the ECG comes in.
(Note : the normal range of 60-100 beats per minute refers to ADULT HUMANS. What is normal in adults is NOT NECESSARILY normal in infants, children, or pets. Infants, for example, generally have a much faster normal heart rate than adults.)
Pacemakers are groups of cells that fire "on their own." They are not triggered by some outside stimulus. This ability to fire on their own is called automaticity.
Sites where pacemakers normally exist :
Occasionally, something will cause a group of cells in the heart that are not considered pacemakers to assume the role. For example, something may happen that aggravate cells in the atrium, causing a group of them to start firing off signals to depolarize. This is called abnormal automaticity.
Anytime pacemaker cells are affected such that they are likely to fire at a faster rate, we call it enhanced automaticity. This is also used in cases (as mentioned above) when cells that are not pacemakers begin to act as such.
Automatic versus reentrant
Certain types of arrhythmias are caused by the sinus pacemaker faltering or by its conduction being unable to reach its target. Other types, however, involve a normal sinus pacemaker but have some other part of the conduction system misbehaving. They often produce tachycardias; these types of dysrhythmias are generally grouped into one of two categories : automatic and reentrant.
Automatic in this case does not what it usually does in everyday life. Instead, it is the adjective form of automaticity, i.e. the special quality expressed by pacemaker cells. Arrhythmias that arise from the actions of a misbehaved pacemaker are often called automatic.
Reentry refers to arrhythmias that result from the same original impulse that refuses to "die off." Instead, it does loops around the heart (or certain portions of the heart), often causing the heart rate to become extremely fast. Other situations may involve such a loop or series of loops that result in what appears to be random electrical activity.
Premature complexes occur when an ectopic pacemaker decides to "butt in" on the underlying rhythm. Because they do so before the next normal complex was scheduled to arrive, we designate them premature and we call the set of waves that result a premature complex. We will discuss three types of these, premature atrial complexes (PAC), premature junctional complexes (PJC), and premature ventricular complexes (PVC). Each will be covered in the section corresponding to its region.
Occasionally, ectopic pacemakers are helpful. When the sinus node falters and its rate becomes too slow, one of the alternate pacemaker sites may take over. We call the rhythms that result from this sort of rescue escape rhythms. There are two types of escape : junctional escape and ventricular escape. It would actually be hard to call the ventricular escape a "rescue"; it is usually only slightly better than no activity at all. We will cover these rhythms in later sections.
What is detectable
Not all of the electrical activity in the heart is detectable by an ECG. In fact, only the large regions show up.
What we can see :
What we cannot see :