The electrocardiogram (ECG or EKG) is a diagnostic tool that measures and records the electrical activity of the heart in exquisite detail. Interpretation of these details allows diagnosis of a wide range of heart conditions. These conditions can vary from minor to life threatening.
The term electrocardiogram was introduced by Willem Einthoven in 1893 at a meeting of the Dutch Medical Society. In 1924, Einthoven received the Nobel Prize for his life's work in developing the ECG.
The ECG has evolved over the years.
- The standard 12-lead ECG that is used throughout the world was introduced in 1942.
- It is called a 12-lead ECG because it examines the electrical activity of the heart from 12 points of view.
- This is necessary because no single point (or even 2 or 3 points of view) provides a complete picture of what is going on.
- To fully understand how an ECG reveals useful information about the condition of your heart requires a basic understanding of the anatomy (that is, the structure) and physiology (that is, the function) of the heart.
Basic Anatomy of the Heart
The heart is a 4-chambered muscle whose function is to pump blood throughout the body.
- The heart is really 2 "half hearts," the right heart and the left heart, which beat simultaneously.
- Each of these 2 sides has 2 chambers: a smaller upper chamber called the atrium (together, the 2 are called atria), and a larger lower chamber called the ventricle.
- Thus, the 4 chambers of the heart are called the right atrium, right ventricle, left atrium, and left ventricle.
This sequence also represents the direction of blood flow through the heart.
- The right atrium receives blood that has completed a tour around the body and is depleted of oxygen and other nutrients. This blood returns via 2 large veins: the superior vena cava returning blood from the head, neck, arms, and upper portions of the chest, and the inferior vena cava returning blood from the remainder of the body.
- The right atrium pumps this blood into the right ventricle, which, a fraction of a second later, pumps the blood into the blood vessels of the lungs.
- The lungs serve 2 functions: to oxygenate the blood by exposing it to the air you breathe in (which is 20% oxygen), and to eliminate the carbon dioxide that has accumulated in the blood as a result of the body's many metabolic functions.
- Having passed through the lungs, the blood enters the left atrium, which pumps it into the left ventricle.
- The left ventricle then pumps the blood back into the circulatory system of blood vessels (arteries and veins). The blood leaves the left ventricle via the aorta, the largest artery in the body. Because the left ventricle has to exert enough pressure to keep the blood moving throughout all the blood vessels of the body, it is a powerful pump. It is the pressure generated by the left ventricle that gets measured when you have your blood pressure checked.
The heart, like all tissues in the body, requires oxygen to function. Indeed, it is the only muscle in the body that never rests. Thus, the heart has reserved for itself its own blood supply.
- This blood flows to the heart muscle through a group of arteries that begins less than one-half inch from where the aorta begins. These are known as the coronary arteries. These arteries deliver oxygen to both the heart muscle and the nerves of the heart.
- When something happens so that the flow of blood through a coronary artery gets interrupted, then the part of the heart muscle supplied by that artery begins to die. This is called coronary heart disease, or coronary artery disease. If this condition is not stopped, the heart itself starts to lose its strength to pump blood, a condition known as heart failure.
- When the interruption of coronary blood flow lasts only a few minutes, the symptoms are called angina, and there is no permanent damage to the heart. When the interruption lasts longer, that part of the heart muscle dies. This is referred to as a heart attack (myocardial infarction).
Nerves of the heart: The heart's function is so important to the body that it has its own electrical system to keep it running independently of the rest of the body's nervous system.
- Even in cases of severe brain damage, the heart often beats normally.
- An extensive network of nerves runs throughout all 4 chambers of the heart. Electrical impulses course through these nerves to trigger the chambers to contract with perfectly synchronized timing much like the distributor and spark plugs of a car make sure that an engine's pistons fire in the right sequence.
- The ECG records this electrical activity and depicts it as a series of graph-like tracings, or waves. The shapes and frequencies of these tracings reveal abnormalities in the heart's anatomy or function.
Before describing the ECG itself, let's take a look at the heart's electrical system.
Heart Function and the ECG
The heart normally beats between 60 and 100 times per minute, with many normal variations. For example, athletes at rest have slower heart rates than most people. This rate is set by a small collection of specialized heart cells called the sinoatrial (SA) or sinus node.
- It has "automaticity," meaning it discharges all by itself without control from the brain.
- Two events occur with each discharge: (1) both atria contract, and (2) an electrical impulse travels through the atria to reach another area of the heart called the atrioventricular (AV) node, which lies in the wall between the 2 ventricles.
- The AV node serves as a relay point to further propagate the electrical impulse.
- From the AV node, an electrical wave travels to both ventricles, causing them to contract and pump blood.
- The normal delay between the contraction of the atria and of the ventricles is 0.12 to 0.20 seconds. This delay is perfectly timed to account for the physical passage of the blood from the atrium to the ventricle. Intervals shorter or longer than this range indicate possible problems.
The ECG records the electrical activity that results when the heart muscle cells in the atria and ventricles contract.
- Atrial contractions (both right and left) show up as the P wave.
- Ventricular contractions (both right and left) show as a series of 3 waves, Q-R-S, known as the QRS complex.
- The third and last common wave in an ECG is the T wave. This reflects the electrical activity produced when the ventricles are recharging for the next contraction (repolarizing).
- Interestingly, the letters P, Q, R, S, and T are not abbreviations for any actual words but were chosen many years ago for their position in the middle of the alphabet.
- The electrical activity results in P, QRS, and T waves that have a myriad of sizes and shapes. When viewed from multiple anatomic-electric perspectives (that is, leads), these waves can show a wide range of abnormalities of both the electrical conduction system and the muscle tissue of the heart's 4 pumping chambers.
What You Can Expect During an ECG
Few procedures in medicine are easier than an ECG.
- You will lie down quietly on a bed or stretcher.
- A technician (or sometimes a nurse, doctor, or other medical professional) will place 6 small adhesive electrode pads across your chest from your lower breast bone (sternum) to an area below your left armpit. Other pads will be placed on each of your arms and legs. Insulated wires will connect each of these 10 pads to the ECG machine.
- Once these wires, called "leads," are attached, the ECG records a few heartbeats on a single sheet of graph paper.
Each heartbeat produces a set of P-QRS-T waves.
- This set of waves, in turn, is recorded and analyzed from each of 12 points of view.
- Six of these points of view are the locations of the 6 pads placed across your chest. These are called V1, V2, V3, V4, V5, and V6 (pronounced Vee One, Vee Two, and so on).
- The other points of view represent combinations of the pads placed on the arms and legs. These are called I, II, III , aVR, aVL, and aVF.
- The interpretation of the waves produced by each of these 12 views provides valuable information about the functioning of your heart.
In some circumstances, medical illnesses elsewhere in the body or various drugs (especially in overdose situations) affect an otherwise healthy heart in ways revealed by diagnostic or suggestive changes in to the ECG changes.
In addition to the 12-lead ECG, an additional "rhythm strip" may be taken. This represents only one point of view but is a good way to see important changes that may be occurring over longer periods of time.
- These may be changes that are hard to interpret or are not even detected in the handful of heartbeats recorded in the standard 12-lead ECG.
- This is especially useful when the heart is beating slower or faster than normal.
Some people with heart rhythm disorders (arrhythmias) or coronary heart disease have symptoms that come and go.
- These symptoms may include brief chest pain or angina, palpitations, dizziness, or weakness.
- If you are not having symptoms when you see your health care provider, your ECG result may be perfectly normal.
- This is a common occurrence, and it is frustrating because your health care provider cannot properly diagnose or treat your problem until it has been documented on ECG.
If this happens to you, your health care provider will probably recommend ambulatory ECG.
- This is a good way to "catch" and document any temporary or intermittent abnormalities such as irregular heartbeats.
- For this test, you are attached to an ECG recording device (sometimes called a Holter monitor) that records every heartbeat for periods of 24 hours (or longer, if necessary). An alternative method is to record the heartbeats only intermittently but for a longer period of time, days or weeks.
- Long-term monitoring significantly increases your chances of "catching" any abnormalities on the ECG, even if they last only a few minutes or seconds.
Reasons to Have an ECG
Heart problems can produce a wide array of symptoms.
- Without the benefit of an ECG, it may be impossible to tell whether these symptoms are being caused by a heart problem or just mimicking one.
- Therefore, unless your symptoms are explained by an illness, injury, or condition known to not affect the heart, an ECG will generally be done.
Common symptoms that frequently require an ECG include the following:
- Chest pain or discomfort
- Shortness of breath
- Palpitations (rapid or pounding heartbeats or increased awareness of heart beating)
- Abdominal pain
- Fainting (syncope)
ECG often reveals a problem that is not primarily cardiac in nature. Examples are overdoses of certain drugs (such as certain antidepressants, cocaine, or amphetamines) or electrolyte abnormalities (especially potassium).
People of any age who are in occupations that stress the heart (professional athletes or firefighters, for example) or involve public safety (commercial airline pilots, train conductors, and bus drivers) require ECGs as well.
Anyone aged 40 years or older should have an ECG done. This first ECG serves as a screening tool to detect any cardiac problems and as a baseline for comparison of future ECGs.
A complete list of who should obtain an ECG, called Guidelines for ECG, is published by the Joint Committee of the AHA/ACC (American Heart Association/American College of Cardiology).
Common Causes of ECG Tracings
If you are having symptoms, the ECG is just one test your health care provider will use in making an overall evaluation. Your ECG may be completely normal despite the presence of significant heart disease.
- Despite this, during the quiet resting conditions under which most ECGs are performed, coronary blood flow will be sufficient to give the heart muscle the oxygen it needs. The ECG, therefore, may be perfectly normal. In these circumstances, a normal ECG would give a false sense of security that all is well with the heart.
- If this person exercises on a treadmill (or shovels snow, chops wood, digs a hole for a fence, or performs similar activity), his heart will need to pump twice as much blood to deliver twice as much oxygen to the muscles of the arms and legs. With 70% blockage of the coronary arteries, the heart muscle likely will not get the blood flow it needs. This may manifest as fatigue, shortness of breath, or discomfort in the chest or arms.
- An unsuspecting person may attribute these symptoms to overexertion and will slow down or stop the exercise and never realize that is heart is producing warning signs. On a treadmill under continuous ECG monitoring, however, the ECG usually identifies the problem before a full-blown heart attack has occurred, it is hoped.
In cases of angina, when the blockage is temporary, the telltale ECG changes will be temporary as well. When a heart attack has occurred, on the other hand, a part of the actual heart muscle has died, and the ECG changes will be permanent.
- In a heart attack, affected portion(s) of the heart will be electrically silent, and, like a burned-out light bulb, no longer radiate energy.
- This shows up as changes in the voltages of the ECG, especially of the QRS complex.
For these reasons, it is especially important to go to a hospital emergency department in the early stages of a heart attack.
- Some people are eligible to receive a medication that rapidly dissolves the coronary artery blockage.
- This medication works only if it is given within 6 hours after the onset of symptoms.
- Other people are not eligible to receive this medication because of potential complications, such as bleeding.
Electrical problems within the heart may disrupt the heart's natural pacemaker.
- The extensive electrical network of nerves and nerve centers that coordinate the firing of the 4 chambers is made of living cells that require oxygen every bit as much as the heart muscle. These cells are subject to malfunction when starved of oxygen by blockage of a coronary artery. When this occurs, the heart may beat too fast, too slowly, or too irregularly to sustain its normal pumping function.
- For example, if the coronary artery supplying the sinus node is blocked, the sinus node may fail. If the failure is partial, the heart rate will slow down. If the failure is complete, then there will be no activation of the atria, no atrial contraction, and no signal to trigger the AV node. The heart will stop pumping. This is called cardiac arrest and usually causes death.
Fortunately, the AV node has automaticity of its own. This means that in the absence of a normal incoming signal from the sinus node, the AV node will fire on its own, but at the slower rate of 35-60 times per minute.
- Depending on the condition of the rest of the heart (the coronary arteries and the valves, for example), this slower rate may or may not result in symptoms.
- Because a heart so affected loses its ability to speed up when needed, it is generally only a matter of time before the condition results in noticeable symptoms. This condition, known as sick sinus syndrome, is one of the more common reasons that people need an artificial pacemaker.
- Sometimes a body's natural pacemaker malfunctions despite an otherwise perfectly healthy heart. (This is the equivalent to a car engine that doesn't run well because of a spark plug problem.) This was the situation for Arne Larsson, a Swedish engineer who received the first artificial pacemaker in 1958. He died in December 2001, aged 86 years, of causes unrelated to his heart.
Sometimes the heart's 2 ventricles beat so rapidly that very little or no blood at all is pumped because there is not enough time between contractions for the ventricles to fill.
- This dangerous condition is known as ventricular tachycardia if the heartbeat is regular and ventricular fibrillation if the heartbeat is irregular.
- When this occurs, a well-placed electrical shock across the chest may be life saving.
- The shock, known as defibrillation, neutralizes all the abnormal electric circuits, thus giving the heart's pacemaker a chance to kick in at a normal rate.
Because the brain and heart cannot survive total loss of blood flow lasting much more than about 10 minutes, it is crucial that the shock be delivered within this time frame. A device called an AED (automatic external defibrillator) is increasingly being made available in public locations such as large office buildings, shopping malls, golf courses, and airplanes. For further information, see the American Heart Association's Questions and Answers about AEDs.
Interpreting the ECG Results
Interpretation of an ECG is no simple matter.
- There are hundreds of patterns to recognize.
- It may be impossible to tell how long an abnormality has been present.
- This issue becomes crucial in an emergency situation when a person has symptoms consistent with a heart problem and an abnormal ECG.
Some or all of the abnormalities may have been caused by an event long in the past and unrelated to the current situation.
- Some people are even born with ECG abnormalities.
- This can make it difficult to identify which problems require urgent treatment.
- This is the equivalent of examining a car that has been in both a recent car accident and accidents in the past. Which dents were caused by which accident?
For these reasons, if you have either a heart condition or an abnormal ECG, you would be wise to keep a recent copy of your ECG handy in your wallet, purse, or car glove compartment for immediate availability.
- This way you will be able to provide your baseline ECG to a very grateful emergency care provider in case you unexpectedly end up in an emergency department, especially in the middle of the night or far from home.
- A copy of your ECG can be shrunk to credit card size and laminated to be carried in your wallet. If you have an abnormal ECG or you travel a lot, ask your health care provider about doing this for you.
MultimediaMedia file 1: Rhythm strip showing a normal 12-lead ECG.
Media type: Rhythm Strip
Media file 2: Conducting system of the heart: SA means sinoatrial node. AV means atrioventricular node. RB and LB mean right and left bundle, respectively, and are the nerves that spread the electric impulse from the AV node into the ventricles.
Media type: Illustration
Media file 3: Basic P-QRS-T wave sequence: Strip shows a simple sequence where M equals 1.0 millivolts.
Media type: Photo
Media file 4: Location for placement of the 6 precordial leads across the chest around the heart (V1 to V6).
Media type: Illustration
Media file 5: A cross-section of the heart: The 6 precordial leads showing that P-QRS-T wave shapes vary with position around the heart.
Media type: Illustration
Media file 6: Rhythm strip of a person who was cardioverted out of ventricular tachycardia by an electric shock.
Media type: Rhythm Strip
Media file 7: A 12-lead electrocardiogram (ECG) of a person with chest pain. It shows heart attack (acute inferior wall myocardial infarction). Image courtesy of Vibhuti N Singh, MD, MPH, FACC.
Media type: Rhythm Strip