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) andphysiology (that is, the function) of the heart.
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) orsinus node.
Located in the right atrium, the sinus node is the heart’s “natural pacemaker.”
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 theatrioventricular (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 rangeindicate 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.
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).
If you are about to have surgery with general anesthesia, you will have an ECG to detect any latent (silent) cardiac conditions that might worsen with the stresses of surgery and anesthesia.
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