What Is An Electrocardiogram?
An electrocardiogram is a graphic recording of your heart's electrical activity. Simply put, heartbeats are the result of this electrical activity in the cells of the heart. These electrical impulses cause the muscles of the heart to contract and relax in a regular rhythm, creating the pumping action that moves the blood through the body's circulatory system. The ECG records the patterns of that electrical activity. 'Electrocardiogram' is often abbreviated 'cardiogram,' ECG, or EKG (Greek for heart is Kardio, hence the 'K' which is sometimes used instead of a 'C').
What Is An Electrocardiogram?
Normally, electrical impulses move unimpeded throughout the heart and play an important role in causing the heart to beat. An electrocardiogram can provide considerable information about the health and functioning of your heart. Your doctor can learn about your heart rhythm, the chambers of your heart, the functioning of your heart muscle, whether the heart is enlarged or thickened, and whether you had a heart attack in the past. Abnormalities of the electrical conducting system can also be detected by an ECG.
Why Does My Doctor Want to Perform This Test?
The reasons for ordering an ECG vary. If you are a new patient, or if you have never had this test, an ECG may be performed as part of a complete history and physical examination. The ECG reading will give the doctor a baseline picture of your heart's activity. Future ECGs will be compared to determine if changes have taken place, or if you have had an intervening heart attack.
A doctor may recommend an ECG for several reasons:
Are There Any Risks Associated with an Electrocardiogram?
An electrocardiogram is a safe, painless test. There are no known risks associated with the ECG, and you will not have to sign a consent form.
What Preparations Should I Make before the Test?
Before having an ECG, you should let your doctor know if you are taking any medications or if you are allergic to adhesive tape. At the commencement of the test, you will be asked to undress from the waist up and expose your lower legs. There are no restrictions on food, liquid or medications prior to the test. Do not apply lotions, oils, or powder to the chest.
What Happens during the Test?
After you have removed your garments, a nurse or technician will clean areas of your chest, back and limbs where the electrodes will be placed. (Men with much chest hair may need to have small areas shaved; the hair will grow back.) Then the nurse or technician will place the electrodes on your body and attach the leads to the ECG machine. The electrodes are usually thin, gel-backed, self-sticking rectangles, about an inch long. The gel may feel a little cool at first. Then you will lie down while the technician enters information into the ECG computer. Next the technician will 'run' the ECG. You will be asked to lie still for less than a minute while your heart's electrical activity is recorded. You will not feel anything.
What Happens after the Test?
The results of your ECG are immediately available. The physician may discuss them with you at that time or may wait for laboratory or other tests to return first.
What Will My Physician Do with This Information?
Based on the ECG, your history and physical examination, your doctor will determine if additional testing is necessary. She may prescribe medication for you or decide that no further action is required.
Are There Different Types of Electrocardiograms?
There are three major types of ECG's:
An ambulatory or Holter ECG is done using a portable recording device that is worn for at least 24 hours. A person is free to move about normally while the monitor is attached. This type of ECG may be recommended for people with intermittent symptoms that may not occur during a resting ECG, such as dizziness, fainting, or bursts of rapid heart beats. It is usually done to see if an irregular heartbeat is the cause of the symptoms.
A cardiac stress test is used to determine what happens to a person's heart during exercise. During the test, a person rides a stationary bicycle or walks on a treadmill as the activity of the heart is recorded. The test takes from 15 to 30 minutes to complete.
The benefit of an ECG is that it helps your doctor better understand how your heart is working or how medications or a pacemaker affect your heart.
The Electrocardiogram - A More Technical Look
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. 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 attached with either rubber straps, suction cups, or sticky pads (to hold them onto the skin). Good electrical contact is necessary to record the tiny signals generated by the heart, so a special cream is sometimes used to help maintain this contact.
When it's finished, the ECG looks like a bunch of squiggles, but there is a tremendous amount of information on that 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.
Figure 1: Electrical Signal from a Single Heartbeat
Figure 2: Cardiac Electrical Anatomy
Figure 1 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 electrical depolarization of the left and right atria, the upper, smaller chambers of the heart. It is normally triggered in the Sino-Atrial (SA) Node (See Figure 2). 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 Atrio-Ventricular (AV) node.
The next large "squiggle" is called the "QRS Complex." The QRS Complex is the tall, spikey signal produced by the electrical depolarization of the ventricles, which are the lower, main pumping chambers of the heart. Following the QRS is another hump called the "T-Wave," which represents the electrical resetting (or repolarization) 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.
Figure 3: A Rhythm Strip, or single lead recording
Figure 4: A Full 12-lead ECG
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 the metals 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 SA 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 SA 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. We are able to do this now, much better than we could in the past, thanks to the advanced scientific data uncovered by dedicated researchers in the field.
Additional Information on Electrocardiograms
To learn more about electrocardiograms and how they relate to your personal medical needs, talk to your healthcare provider, or visit the sites below.
History of Electrocardiography
Simple explanations of Cardiovascular diseases and tests