An electrocardiogram — abbreviated as EKG or ECG — is a test that measures the electrical activity of the heartbeat. With each beat, an electrical impulse (or “wave”) travels through the heart. This wave causes the muscle to squeeze and pump blood from the heart. A normal heartbeat on ECG will show the timing of the top and lower chambers.

The right and left atria or upper chambers make the first wave called a “P wave” — following a flat line when the electrical impulse goes to the bottom chambers. The right and left bottom chambers or ventricles make the next wave called a “QRS complex.” The final wave or “T wave” represents electrical recovery or return to a resting state for the ventricles.

An ECG is a noninvasive, painless test with quick results. During an ECG, sensors (electrodes) that can detect the electrical activity of your heart are attached to your chest and sometimes your limbs. These sensors are usually left on for just a few minutes.

Other types of heart rhythm monitoring

If you have a heart rhythm irregularity that tends to come and go, it may not be captured during the few minutes a standard ECG is recording. In this case your doctor may recommend another type of heart rhythm monitor.

  • Holter monitor. A Holter monitor is a small, wearable device that records a continuous ECG, usually for 24 to 48 hours. Wires from electrodes on your chest go to a battery-operated recording device carried in your pocket or worn on a belt or shoulder strap. While you’re wearing the monitor, you’ll be able to go about your normal activities, as long as you keep the electrodes and device dry. In addition, your doctor will likely ask you to keep a diary of what you’re doing when symptoms occur and the time. Your doctor will compare the diary with the electrical recordings to try to figure out the cause of your symptoms.
  • Event monitor. If your symptoms don’t occur often, your doctor may suggest that you wear an event monitor. This portable device is similar to a Holter monitor, but it records only at certain times for a few minutes at a time. And you can wear it longer than a Holter monitor, typically 30 days.With many event monitors, you activate them by pressing a record button when you have symptoms or a fast heart rate. Other monitors automatically sense abnormal heart rhythms and then start recording. You then send the ECG readings to your doctor through your phone. He or she uses the recorded electrical signals to look at your heart rhythm at the time of your symptoms.
  • Stress test. If your symptoms occur most often during exercise, your doctor may ask you to walk on a treadmill or ride a stationary bike during an ECG. This is called a stress test. If you have a medical condition that makes it difficult for you to exercise, your doctor may inject you with a drug that mimics the effect of exercise on your heart.
  • Implantable loop recorder. This is a continuous heart rhythm monitoring device. Like a Holter monitor, it continuously records your heart rhythms, but for a much longer time. An implantable loop recorder is inserted under the skin in the chest area during minor surgery and can be left in place for up to three years.

How it works

Each beat of your heart is triggered by an electrical impulse normally generated from special cells in the upper right chamber of your heart (pacemaker cells). An electrocardiogram records the timing and strength of these signals as they travel through your heart.

An electrocardiogram is also called a 12-lead EKG or 12-lead ECG because it gathers information from 12 different areas of the heart. These views are created by electrodes, typically 10, placed on the skin of your chest and sometimes your limbs. The electrical activity is recorded as waves on a graph, with different patterns corresponding to each electrical phase of your heartbeat.

A standard ECG can record an abnormal heart rhythm only if it happens during the test. Some abnormal heart rhythms come and go, so your doctor may have you undergo a different type of heart rhythm monitoring, such as with a Holter monitor, to help diagnose the cause of your symptoms.