This section of the article deals with basic ideas and concepts regarding the heart rhythm and the electrical signals that control the beating heart. Understanding the electrocardiogram is essential to appreciate the differences between different arrhythmias. If you want to actually learn how to read an ECG, you can find an thorough online course using video tutorials designed for beginners and experts alike at ECGAcademy.com.
Understanding The Rhythm
The heart must maintain a steady rhythm in order to pump properly. Excessively slow heart rhythms cause the cardiac output to be reduced, and this can cause symptoms of fatigue, weakness, lightheadedness, fainting, shortness of breath, and fluid accumulation (i.e. “congestive heart failure”). On the other hand, if the heart beats too quickly it doesn’t get enough time in between beats to fill up with blood. As a result, only a small amount of blood gets squirted out with each beat, and the cardiac output drops significantly. In order to understand this, imagine trying to put out a large fire with a shot glass. You can fill this tiny glass very quickly and splash the fire with lots of little drops, but it’s much more efficient to fill a large bucket and douse that fire with only a couple of trips to the faucet. Rapid heart rhythms decrease the efficiency of the heart’s pumping action, and this can cause the same symptoms as a slow heart rhythm, namely, weakness, shortness of breath, lightheadedness and fainting. In addition, rapid heart rhythms can cause a symptom known as “palpitations.”
The word “palpitation” refers to any abnormal or inappropriate feeling in the chest caused by the heart’s beating. It’s a generic term, and people can even feel palpitations when their heart is beating normally (like the feeling of pounding in your chest that you get after you run up two flights of stairs). Some palpitations can feel like a fluttering, a racing, or a pounding in the chest. Others may feel like a heavy or hollow feeling in the chest, or a feeling of a “skipped” or “missed” beat. For some people they can even be perceived as painful. Sometimes palpitations make you feel like you can’t catch your breath, or they make you feel like you have to cough. Palpitations can be brief – only a fraction of a second, like when the heart “skips a beat.” Then again, palpitations can last for several minutes or even hours. Sometimes, brief flutter (skipped beat) type palpitations may occur over and over every few seconds. This can cause an irregular pulse that can last for hours and can be very annoying.
You are probably wondering where all these abnormal rhythms come from and why the heart sometimes beats too fast or too slow. Arrhythmias result from the electrical signal of the heart getting fouled up by a “wiring” problem. Sometimes the signal gets blocked by a “loose connection” and the heart doesn’t beat when it is supposed to (because the muscle is not getting the signal). This can cause slow heart rhythms. Sometimes there is a “short circuit” in the wiring which causes the electrical signal to take an abnormal route or to get stuck traveling around and around in a circular pathway. This problem can force the heart to beat rapidly. Sometimes, an abnormal signal will arise from somewhere in the heart and cause the electrical system of the heart to fire prematurely. This is what causes a premature beat. There are many different kinds of rhythm problems caused by many different electrical disturbances. Some cause fast beats and some cause slow ones. Each one of these problems may require different treatment, and some may not require treatment at all. One medication may be good for one kind of rhythm problem and bad for another. Therefore, it is important for your doctor to determine the exact type of rhythm problem that you may have.
The electrical signals that flow through the heart can be picked up from the skin, amplified, and printed on paper for your doctor to examine. This recording is called an “electrocardiogram” [pronounced EE-LECK-TRO-CAR-DEE-OH-GRAM], The electrocardiogram is sometimes referred to as an “ECG,” “EKG,” or “Cardiogram.” By examining the ECG, your doctor can determine if an arrhythmia is present. The ECG can also give your doctor information about the overall health of your heart whether it may be enlarged or whether it may be damaged from an old heart attack.
The ECG is generally recorded using 10 wires or electrodes that a technician attaches to your arms, legs, and across your chest. The electrodes are usually attached with sticky pads (to hold them onto the skin), and good electrical contact is necessary to record the tiny signals generated by the heart. When it’s finished, the ECG looks like a bunch of squiggles, but there is a tremendous amount of information on that piece of paper. The ECG machine automatically records the signals from a number of different combinations of the electrodes. Each of these combinations is called a “lead” [pronounced LEED], and you may hear physicians referring to a “12-lead,” which is a standard ECG containing recordings from 12 different combinations of electrodes. For example, Lead I records the electrical signals between your left arm and your right arm, and lead II records signals between your right arm and your left leg. The “precordial” or chest leads (referred to as V1 to V6) are applied across your chest. Having twelve leads allows the doctor to examine different anatomical areas of the heart almost like a three-dimensional picture of the electrical activity.
The figure above is an example of a “squiggle” from one of these leads. The “squiggle” is actually the electrical signal that is produced during one single heart beat. Remember, however, it’s the electrical signal that causes the heart beat, so there is actually a split-second delay between the production of the signal and the contraction of the heart muscle. There is a small hump in the beginning of the signal. This hump, called the “P-Wave,” is the signal produced by the right and left atria. There is a flat area after the P-Wave which is part of what is called the PR Interval. During the PR interval the electrical signal is traveling through the AV node. The next large “squiggle” is called the “QRS Complex.” The QRS Complex is tall, spikey signal produced by the ventricles. Following the QRS is another hump called the “T-Wave,” which represents the electrical resetting of the ventricles in preparation for the next signal. When the heart beats continuously, the P-QRS-T waves repeat over and over as seen below.
You might ask just what causes the electrical signals what could actually be producing electrical currents large enough to be detected from the outside? Actually, each individual muscle cell (and nerve cell, for that matter) is capable of producing a tiny electrical signal through a complicated bio-electrical process known as “depolarization.” It turns out that the membrane that surrounds each muscle cell has a number of special protein pores that control the flow of charged atomic particles in and out of the cell. The particles are known as “ions” and examples of these are individual atoms of sodium, potassium, and calcium. Ions are charged with a tiny bit of positive or negative electrical energy, and it’s the movement of these ions in and out of the cell that creates electrical currents that we can measure. Most tissues in the heart are “sodium-dependent,” meaning that depolarization occurs because sodium rushes into the cell. Some cells in the heart, particularly in and around the sinus node and within the AV node are “calcium-dependent,” meaning that it’s movement of calcium ions that is primarily responsible for the cells firing. You may have heard of medications known as “Calcium-channel Blockers.” These drugs are designed to block the movement of calcium into these cells, and so they primarily affect the sinus node and the AV node. In fact, understanding the physiologic mechanisms at work in the heart is of paramount importance in prescribing the proper medications to treat cardiac arrhythmias, which is why EP docs make good use of the advanced scientific data uncovered by dedicated researchers in the field.
Arrhythmia Terminology and Classification
Arrhythmias are named according to three main features: 1) whether the heart rate is to fast or too slow; 2) which part of the heart the arrhythmia is coming from; and 3) what specific physiological derangement may be causing the arrhythmia. All the most common arrhythmias will be explained in Part 3 of this series.
1. Rate Terminology The technical name for an abnormally slow heart rhythm is “bradycardia” [pronounced BRAY-DEE-CAR-DEE-UH], while an abnormally fast heart rhythm is called “tachycardia” [pronounced TACK-EE-CAR-DEE-UH]. Most often, a “normal” heart rate is taken as between 60 to 100 beats per minute (bpm), so arrhythmias that cause the heart to beat less than 60 bpm are considered bradyarrhythmias and arrhythmias that cause the heart to accelerate over 100 bpm are sometimes referred to as tachyarrhythmias. Keep in mind that one has to consider the body’s needs, and a normal rhythm can exceed 100 bpm. For example, a rate of 120-150 bpm is expected if someone is running on a treadmill. This is not abnormal under the circumstances, so that heart rate is not necessarily an indication of a problem.
2. Anatomical Terminology Arrhythmias are generally classified into two broad groups, depending on which part of the heart gives rise to the rhythm disturbance. Ventricular arrhythmias, obviously, arise from the lower chambers of the heart. On the other hand, “supraventricular” arrhythmias arise from tissues above the ventricles (“supra” means “above”). Supraventricular tachycardia, for example, refers to any rapid beat that does not arise from the ventricles. This could include arrhythmias coming from the sinus node, the atrium, or even the AV node.
3. Electrophysiologic Mechanism Electrophysiologists classify arrhythmias into two mechanisms: Disorders of Impulse Conduction and Disorders of Impulse Generation. Impulse generation refers to the creation of an electrical signal in the heart, and disorders of impulse generation include arrhythmias that occur due to either an inadequate rate of “firing” of the electrical signals (resulting in bradycardia) or an excessive rate of generation of electrical signals, which would cause tachycardia. For example, if the sinus node (which is the natural pacemaker of the heart) has trouble generating electrical impulses, the heart will beat too slowly. This condition is called Sinus Bradycardia and is a disorder of impulse generation. Conversely, if another area in the atrium starts generating rapid, abnormal signals which “take over” as the predominant pacemaker, the tachycardia that results is another example of a disorder of impulse generation. On the other hand, disorders of impulse conduction include arrhythmias caused by abnormalities in the path of movement of the electrical signals through the various heart tissue. A “loose connection” in the AV node that results in failure of conduction of the signal from the atrium to the ventricles (known as AV Block) is one example. A common cause of tachycardia results if the electrical signal gets “stuck” traveling around and around in a circular pathway. This phenomenon of the signal chasing it’s own tail is referred to as “Reentry” and is another type of disorder of impulse conduction.