What Is an Electrocardiogram (ECG)?

The electrocardiogram, also referred to as ECG, 12-lead ECG, or EKG, is a non-invasive diagnostic test that evaluates your heart's electrical system to assess for heart disease. It uses flat metal electrodes placed on your chest to detect the electrical charges generated by your heart as it beats, which are then graphed. Your doctor can analyze the patterns to get a better understanding of your heart rate and heart rhythm, identify some types of structural heart disease, and evaluate cardiac efficiency.

Purpose of Test

An ECG detects your heart's electrical rhythm and produces what's known as a tracing, which looks like squiggly lines. This tracing consists of representations of several waves that recur with each heartbeat, about 60 to 100 times per minute. The wave pattern should have a consistent shape. If your waves are not consistent, or if they do not appear as standard waves, this is indicative of heart disease.

There are a variety of characteristic changes that occur with different heart concerns, and your doctor can look at your ECG wave patterns to see if they are suggestive of certain types of heart disease.

Many doctors order an ECG as part of a yearly medical examination to screen for heart disease. This may apply to you if:

• You have had heart disease in the past
• You have a medical condition that predisposes you to heart disease, such as hypertension, diabetes, high cholesterol, or inflammatory disease
• You have other significant risk factors for cardiac disease

An ECG may also be recommended if you have signs or symptoms of heart disease, such as chest pain, shortness of breath, lightheadedness, dizziness, or fainting spells. Likewise, if you have signs of a TIA or stroke, such as vision changes, numbness, weakness, or communication problems, you are also likely to need an ECG because some types of heart disease can cause a stroke.

If you have heart disease, you may need periodic ECG testing to evaluate whether your disease is worsening and to monitor the treatment effects of your heart medications.

An ECG is also required prior to any type of heart surgery, including surgery for pacemaker placement. A pre-operative screening ECG is required prior to any surgical procedure that involves general anesthesia because heart disease increases the risk of adverse events from general anesthesia and because this helps your anesthesiologists as they plan your anesthetic medications and surgical monitoring.

Conditions

There are a number of conditions that can be detected when your doctor checks your pulse, such as tachycardia (rapid heart rate), bradycardia (slow heart rate), and arrhythmia (irregular heart rate). EKG wave patterns not only verify these alterations in heart rhythm, certain changes in the shape of the waves provides information about the specific type of heart disease and which region in the heart is affected.

Limitations

The ECG is one of the most commonly used tests in medicine because it can screen for a large variety of cardiac conditions, the machines are readily available in most medical facilities, the test is simple to perform, is safe, and relatively inexpensive.

That said, an ECG has its limitations:

• The ECG reveals the heart rate and rhythm only during the few seconds it takes to record the tracing. If an arrhythmia (heart rhythm irregularity) occurs only intermittently, an ECG might not pick it up, and ambulatory monitoring may be required.
• The ECG is often normal or nearly normal with many types of heart disease, such as coronary artery disease.
• Sometimes, abnormalities that appear on the ECG turn out to have no medical significance after a thorough evaluation is done.

Risks and Contraindications

ECG is a safe test that does not cause health complications. There are no medical conditions associated with an increased risk or adverse side effects from ECG.

Before the Test

If your doctor orders an ECG, you generally do not need to have any special tests or procedures to prepare for it. In fact, you can have it right in the doctor's office if there is available time, space, and equipment. Sometimes, depending on the reason for your ECG, your doctor may ask you to stop taking some of your medications for a day or two before the test.

Timing

If you are having an ECG as part of a doctor's visit, anticipate needing an extra 10 to 15 minutes for the test. If you are going to have a special visit for the ECG, you should expect it to take longer because of the registration and check-in process.

Location

Often, an ECG is done in the doctor's office, sometimes in the same exam room where you are seeing the doctor. Your doctor's clinic may have a separate space where you may need to go to have your test.

What to Wear

You will need to change into a hospital gown so that electrodes can be placed on your chest. You may be asked to remove large necklaces or chains if they dangle or get in the way, but you do not need to worry about electrical interference from metal jewelry.

Food and Drink

You can eat or drink whatever you want prior to your test. If your doctor is worried that you have an especially rapid heart rhythm, you may be asked to abstain from caffeine for six to 10 hours before the test.

Cost and Health Insurance

Generally, an ECG is covered by most health insurance plans, but there are always exceptions. If you are insured and concerned that your plan may not cover the test, or if you have a plan with minimal coverage, you might want to check your benefits in advance. As with many procedures, your plan may also require you to pay a copay, and you should be able to find out by calling the number on your insurance card.

If you are paying for this test out of pocket, you can anticipate it costing around $50.

What to Bring

When you go for your ECG, you should bring your test order form (if applicable), your health insurance card, a form of identification, and a method of payment.

During the Test

Your test will be performed by a doctor, a nurse, or a technician.

Pre-Test

You will be asked to change into a hospital gown and lay down on an examination table.

Once in position, a total of ten electrodes are attached with a sticky, but easy-to-remove adhesive. One electrode is placed on each arm and leg, and six on the chest. 

Throughout the Test

Each electrode is a flat, coin-shaped plate with wires attached to the ECG machine, which looks like a computer. The electrodes detect electrical activity produced by the heart and transmit this information to the machine, where it is processed, and saved electronically or printed out as an ECG tracing.

Readings will be taken for about five minutes. During this time, you will be asked to remain still, as movement can disrupt the pattern. There is no pain or discomfort associated with this test.

Post-Test

After the test, the electrodes are removed. If there is any sticky material remaining, it can be easily wiped off with an alcohol pad. You may experience some pulling of hair underneath the nodes, but generally, the technicians are very careful with taking them off.

You should not expect any side effects after an ECG, and there are no limitations on your activity.

Rarely, the adhesive can cause allergic reactions or rashes, which may not be obvious until about 24 hours after the test. If you experience a rash in the area of the electrodes, call your doctor.

Interpreting the Results

The electrical signals generated from the electrodes are processed to obtain the heart’s electrical activity from 12 different angles, each of which shows a separate tracing. By examining any abnormalities on the ECG and which leads they are stemming from, your doctor can get important clues about the status of the heart. 

The Tracing

A tracing consists of repeated waves that have a standard shape. The waves have sections named the P wave, QRS complex, ST segment, and T wave. There is also a PR interval between the P wave and the QRS complex, and a QT interval between the QRS complex and the T wave.

Different conditions are associated with changes in the height, width, and length of these waves, and the intervals between them. A shortened QT interval, for example, can be a sign of elevated blood calcium levels.

Your ECG report may have a description of the wave pattern but is unlikely to describe your heart condition in detail because your doctor needs to take your medical history into account when determining whether or not you have (or could have) a heart condition. Learning to read an ECG and recognize these patterns takes months of training and practice.

Your doctor will be able to explain your results to you. Among many things, an ECG can indicate:

• Heart arrhythmias, such as premature ventricular complexes or atrial fibrillation
• Whether you have conduction abnormalities, which result from issues regarding how the electrical impulse spreads across the heart (such as with a bundle branch block)
• Signs of an ongoing or a prior myocardial infarction (heart attack)
• Whether you have signs of severe coronary artery disease (CAD), such as stable angina or unstable angina
• If your heart muscle has become abnormally thickened, as in hypertrophic cardiomyopathy
• Signs of congenital electrical abnormalities, such as Brugada syndrome
• Electrolyte imbalances, particularly elevated or decreased levels of potassium, calcium, or magnesium
• Congenital (from birth) heart defects
• Infections involving the heart, such as pericarditis, which is an infection of the protective tissue surrounding the heart

Follow-Up

While the ECG can make a clear diagnosis of some cardiac conditions, such as a cardiac arrhythmia, it is more often used as a screening test. Therefore, abnormalities seen on the ECG often need to be followed by a more definitive test in order to make a firm diagnosis.

For instance, If the ECG indicates possible coronary artery disease, a stress test or cardiac catheterization might be needed. If ventricular hypertrophy is seen, an echocardiogram is often needed to check for diseases such as aortic stenosis or other structural abnormalities.

A Word From Verywell

If you have an ECG as a routine test or because you have symptoms, it is a good idea to let your doctors know about your previous ECGs and save your tracings so that you can show them to your doctors when you have subsequent tests. This can allow your doctors to compare and look for changes over time. Remember that this test is very common and your doctor recommending it isn't a confirmation that something is wrong with you.