Electrocardiogram (ECG, EKG)

What Is an Electrocardiogram?

An electrocardiogram, also called an ECG (EKG in the USA), is a simple test that detects and records the electrical activity of the heart. It is used to detect and locate the source of heart problems.

Electrical signals in the heart trigger heartbeats. These signals start at the top of the heart in an area called the right atrium. The electrical signals travel from the top of the heart to the bottom. They cause the heart muscle to contract as they travel through the heart. As the heart contracts, it pumps blood out to the rest of the body.

An ECG shows how fast the heart is beating. It shows the heart's rhythm (steady or irregular) and where in the body the heartbeat is being recorded. It also records the strength and timing of the electrical signals as they pass through each part of the heart.

An ECG is sometimes called a 12-lead ECG (or 12-lead ECG) because the electrical activity of the heart is most often recorded from 12 different places on the body at the same time.

What ECG Reveals

Many heart problems change the electrical signature of the heart in distinct ways. ECG recordings of this electrical activity can help reveal a number of heart problems, including:

  • Heart attack
  • Lack of blood flow to the heart muscle
  • A heart that is beating irregularly, or too fast or too slow
  • A heart that does not pump forcefully enough

ECG recordings can help doctors diagnose a heart attack that is happening now or has happened in the past. This is especially true if doctors can compare a current ECG recording to an older one. ECG recordings can also reveal:

  • Heart muscle that is too thick or parts of the heart that are too big
  • Birth defects in the heart
  • Disease in the heart valves between the different heart chambers

An ECG also reveals whether the heartbeat starts at the top right part of the heart like it should. It shows how long it takes for the electrical signals to travel through the heart.

How the Heart Works

The heart is a muscle about the size of your fist. It works like a pump and beats 100,000 times a day.

The heart has two sides, separated by an inner wall called the septum. The right side of the heart receives the blood from the body and pumps it to the lungs to pick up oxygen. Then, oxygen-rich blood returns from the lungs to the left side of the heart, and the left side pumps it to the body.

The heart has four chambers and four valves connected to various blood vessels. Veins are the blood vessels that carry blood from the body to the heart. Arteries are the vessels that carry blood away from the heart to the body.

Heart Chambers

The heart has four chambers or "rooms"—two on the left side of the heart and two on the right.

  • The atria (AY-tree-uh) are the two upper chambers that collect blood as it comes into the heart.
  • The ventricles (VEN-trih-kuls) are the two lower chambers that pump blood out of the heart to the lungs or other parts of the body.

Heart Valves

Four valves control the flow of blood from the atria to the ventricles and from the ventricles into the two large arteries connected to the heart:

  • The tricuspid (tri-CUSS-pid) valve is in the right side of the heart, between the right atrium and the right ventricle.
  • The pulmonary (PULL-mun-ary) valve is in the right side of the heart, between the right ventricle and the entrance to the pulmonary artery that carries blood to the lungs.
  • The mitral (MI-trul) valve is in the left side of the heart, between the left atrium and the left ventricle.
  • The aortic (ay-OR-tik) valve is in the left side of the heart, between the left ventricle and the entrance to the aorta, the artery that carries blood to the body.

Valves are like doors that open and close. They open to allow blood to flow through to the next chamber or to one of the arteries, and then they shut to keep blood from flowing backwards.

When the heart's valves open and close, they make a "lub-DUB" sound that a doctor can hear using a stethoscope.

  • The first sound—the "lub"—is made by the mitral and tricuspid valves closing at the beginning of systole (SIS-toe-lee). Systole is when the ventricles contract, or squeeze, and pump blood out of the heart.
  • The second sound—the "DUB"—is made by the aortic and pulmonary valves closing at beginning of diastole (di-AS-toe-lee). Diastole is when the ventricles relax and fill with blood pumped into them by the atria.

Arteries

The arteries are major blood vessels connected to your heart.

  • The pulmonary artery carries blood pumped from the right side of the heart to the lungs to pick up a fresh supply of oxygen.
  • The aorta is the main artery that carries oxygen-rich blood pumped from the left side of the heart out to the body.
  • The coronary arteries are the other important arteries attached to the heart. They carry oxygen-rich blood from the aorta to the heart muscle, which must have its own blood supply to function.

Veins

The veins are major blood vessels connected to your heart.

  • The pulmonary veins carry oxygen-rich blood from the lungs to the left side of the heart so it can be pumped out to the body.
  • The vena cava are two large veins that carry oxygen-poor blood from the body back to the heart.

Why Is an Electrocardiogram Done?

An electrocardiogram (ECG) is done to evaluate signs and symptoms that could indicate heart problems. Some of the signs and symptoms that might be evaluated with an ECG include:

  • Chest pain
  • Heart pounding, racing, or fluttering, or the sense that your heart is beating unevenly
  • Difficulty breathing
  • Feeling tired and weak (fatigue)
  • Unusual heart sounds when the doctor listens to your heartbeat

When an adult—usually someone who is older than 40 or 50 years of age—has a routine health exam, the doctor may order an ECG to screen for early heart disease that has no symptoms. The doctor is more likely to look for early heart disease if the person has a family history of heart disease in a mother, father, brother, or sister—especially if the heart disease developed early in those family members' lives.

Doctors also use ECGs to check how well heart treatments, such as drugs or medical devices, are working.

What Happens During an Electrocardiogram?

An electrocardiogram (ECG) is painless and harmless. A technician first attaches 12 soft patches called electrodes to the skin of the chest, arms, and legs. These electrodes are about the size of a quarter. To help an electrode stick to the skin, the technician may have to shave a patch of hair where the electrode will be attached.

After the electrodes are placed on the skin, the patient lies still on a table for a few minutes while the electrodes detect the electrical signals of the heart. A machine then records these signals on graph paper or displays them on a screen.

The entire test takes about 10 minutes. After the test, the electrodes are removed from the skin and discarded.

Special ECGs

The common ECG described above, called a resting 12-lead ECG, records only a few minutes of heart signals at a time. It will show a heart problem only if the problem is present during the few minutes that the test is being run. Many heart problems are present all the time and will be found by a resting 12-lead ECG. But some heart problems, like those related to irregular heartbeat, can come and go. They may be present for only a few minutes out of the day or only while exercising.

Special types of ECGs are used to help diagnose those kinds of problems. Three of these special ECGs are:

  • Exercise stress test
  • Holter monitor
  • Event monitor

Exercise Stress Test

Some heart problems are easier to diagnose when the heart is working hard and beating fast. During stress testing, a patient exercises, or is given medicine, to make the heart work harder and beat fast while heart tests are performed. During exercise stress testing, blood pressure and ECG readings are monitored while the patient runs on a treadmill or pedals a bicycle.

In addition to an ECG, other heart tests, such as nuclear heart scanning or echocardiography, can also be done at the same time. During nuclear heart scanning, radioactive dye is injected into the bloodstream, and a special camera shows the flow of blood to the heart muscle. Echocardiography uses sound waves to show blood flow through the chambers and valves of the heart and to show the strength of the heart muscle. If a person is unable to exercise, a medicine can be injected into the bloodstream to make the heart work harder and beat fast. Nuclear heart scanning or echocardiography is then done.

Two newer tests that are being done with stress testing are magnetic resonance imaging (MRI) and positron emission tomography (PET) scanning of the heart. MRI shows detailed images of the structures and beating of the heart. PET scanning shows blood flow to the heart muscle and areas of damaged heart muscle.

Holter Monitor (Also Called an Ambulatory ECG)

This test records the electrical signals of the heart for a full 24-hour period. The patient wears electrodes on the chest that are connected by wires to a small, portable recorder. The recorder can be clipped to a belt, kept in a pocket, or hung around the neck. During the 24 hours, the patient engages in usual daily activities and keeps a notebook, recording any symptoms he or she has and the time they occurred. The patient then returns both the recorder and the notebook to the doctor to read the results. The doctor can see how the heart was beating at the time the patient had symptoms. The purpose of a Holter monitor ECG is to record heart signals during typical daily activities and while sleeping, and to find heart problems that may occur for only a few minutes out of the day.

Event Monitor

In this test, the patient wears a small, portable ECG device, but the device only records electrical signals when the patient presses a button on it, and it only records a few minutes at a time. These recordings can then be sent by telephone hookup to the patient's doctor. This test is used to diagnose symptoms that may happen for only a short time each day or only once every few days. The patient can wear an event monitor for days or weeks, if necessary.

Key Points

  • An electrocardiogram (ECG) is a test that detects and records the electrical activity of the heart.
  • ECGs are used to evaluate signs and symptoms that could indicate heart problems.
  • Many heart problems change the electrical "signature" of the heart in distinct ways. ECG recordings of this electrical signature can help reveal these heart problems.
  • During an ECG, electrical signals in the heart are detected by electrodes placed on the skin. A machine records them on graph paper or displays them on a screen.
  • An ECG is painless and harmless, and usually takes about 10 minutes to complete.
  • Special ECG tests are done to detect certain kinds of heart symptoms, such as those that are present for only a few minutes out of the day, or that happen only while a person is exercising.

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