What is a 12-Lead ECG?

Maria Overstreet

A 12-lead electrocardiogram (ECG) is a medical test that is recorded using 12 leads, or nodes, attached to the body. Electrocardiograms, sometimes referred to as ECGs or EKGs, capture the electrical activity of the heart and transfer it to graphed paper. The results can then be analyzed by medical professionals, such as paramedics and cardiologists.

An ECG may be performed to diagnose a heart murmur.
An ECG may be performed to diagnose a heart murmur.

An electrocardiogram captures the electrical charges emitted through the skin during every heartbeat. When the charge of a heart muscle cell depolarizes or reduces to zero, the cell contracts. Healthy hearts display an orderly wave of depolarization that starts in the sinoatrial node, moves through the atrium, spreads through the intrinsic conduction system and then passes through the ventricles. A 12-lead ECG detects and amplifies these voltage changes between two electrodes as wavy lines on paper or a monitor screen.

Paramedics may analyze the results of a 12-lead electrocardiogram.
Paramedics may analyze the results of a 12-lead electrocardiogram.

Electrodes are commonly placed in pairs, such as on the right leg and left leg. Each pair’s output is called the lead, and it captures heart information from different angles. Unlike 12-lead ECGs, the information recorded by 3- and 5-lead ECGs is rarely printed out and generally is used as a form of continuous monitoring via a screen during ambulance transport or hospitalization.

Twelve-lead ECGs are used to diagnose heart abnormalities.
Twelve-lead ECGs are used to diagnose heart abnormalities.

Although it is called a 12-lead ECG, it uses only 10 electrodes. Certain electrodes are part of two pairs and thus provide two leads. Electrodes typically are self-adhesive pads with a conducting gel in the center. The electrodes snap onto the cables connected to the electrocardiograph or heart monitor.

Electrode placement for a 12-lead ECG is standard, with leads placed on the left and right arm and left and right leg. Another pair of electrodes is placed between the fourth and fifth ribs on the left and right side of the sternum. A single electrode is positioned between this pair of electrodes on the fourth intercostal space.

An eighth electrode is placed between the fifth and sixth ribs at the mid-clavicular line, the imaginary reference line that extends down from the middle of the clavicle. The ninth electrode is positioned in line horizontally with the eighth electrode but in the anterior axillary line or the imaginary reference line running southward from the point where the collarbone and arm meet. A final electrode is placed on the same horizontal line as the eighth and ninth electrodes but oriented with the midaxillary line, the imaginary reference point straight down from the patient’s armpit.

Twelve-lead ECGs are used to diagnose heart murmurs and heart attacks. Symptoms that often call for the use of a 12-lead ECG include fainting or collapse, seizures or chest pain. Hospitalized patients or those undergoing surgery also might be monitored with an electrocardiogram. Medical professionals can analyze the electrocardiogram printout to diagnose coronary ischemia, ventricular hypertrophy, hypokalemia, hypercalcemia, hypocalcemia and even specific genetic abnormalities.

Medical professionals can analyze ECG printouts to diagnose a variety of conditions.
Medical professionals can analyze ECG printouts to diagnose a variety of conditions.

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Discussion Comments


Several years ago, I had a full blown panic attack, and thought I was having a heart attack. I went to the ER and they did a 12-lead ECG on me. It seemed to take forever to get all the leads attached, and then the ECG just took about two minutes. It was a much longer time getting all the leads on!

Seems like there ought to be a way to do an ECG with a finger clamp like they use to get someone's oxygen level. That would make much more sense, but I suppose that's not possible, at least not yet.


When I had thyroid surgery, I was under general anesthesia and was put on a 12-lead EKG. The nurse in the recovery room took the leads off before I left recovery. Well, I thought he did, anyway.

About two weeks after surgery, I was showering and felt something itchy on my back. I couldn't quite reach it, but it felt like foam. I got out of the shower, puzzled, and asked the hubs to take a look. He did and said, "It's a foam circle with a metal thing in the middle." I said, "Peel it off, please." He did and showed it to me.

"An EKG lead, of all things!" I said. He asked what he should do with it. I said throw it away, that they are disposable.

So if you have a 12-lead EKG done, make sure you get all the pads off.

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