What is a 12-Lead ECG?

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 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.

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.

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.