An EKG is a frequently used heart function test. In the event of a heart attack, an EKG can reveal important information about the situation’s severity and what treatment may be needed.

An electrocardiogram (EKG) is a test of the heart’s electrical activity that assesses the heart’s rate and rhythm in real time. An EKG is one of several tests you may undergo if you appear to be having a heart attack.

An EKG can reveal a heart rhythm disturbance (arrhythmia), blockage in the coronary arteries, and heart muscle damage that may have been caused by a heart attack.

Based on the results of your EKG, particularly if a heart attack has been diagnosed, your healthcare team may recommend additional tests or immediate treatment.

This article explains more about what an EKG shows, how the results influence treatment recommendations, and what other types of EKGs may be appropriate in certain situations even if a heart attack isn’t suspected.

An EKG is one of several tests that can indicate a heart attack. A heart attack occurs when blood flow through one or more of the arteries, which supply blood to the heart muscle, is blocked. A heart attack is often the result of plaque in a coronary artery that breaks off and blocks circulation within the heart.

The National Heart, Lung, and Blood Institute suggests that an EKG may be the most common test administered to someone in a hospital emergency department for heart attack symptoms.

If the EKG indicates a heart attack is occurring, other tests or treatments will follow. A normal EKG reading may also lead to more tests to see what else could be causing symptoms.

Additionally, an EKG may sometimes be used to detect a previous heart attack. Unusual or abnormal electrical patterns detected in the EKG can suggest that heart muscle tissue may have been injured by a previous heart attack.

An EKG looks like wavy lines on a narrow strip of graph paper, filled with small and large boxes.

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Getty Images/Vladimir Godnik

The small boxes are 1 millimeter (mm) wide and represent 0.04 seconds of your heart rate. The larger boxes are 5 mm across (or 5 small boxes wide) and represent 0.20 seconds.

The electrical activity of a single heartbeat is represented by five points or waves on an EKG. The waves are identified using the letters “PQRST,” instead of “ABCDE,” to allow for more points or waves to be added if necessary. The location and shape of each wave along the baseline indicate a specific characteristic of how your heart is functioning.

The important parts of an EKG to know are:

  • P wave: A P wave represents the electrical activity occurring when the two atria (the heart’s top chamber) are contracting. The P wave marks the beginning of the electrical activity that triggers a heartbeat, and it usually looks like a small bump on an EKG.
  • QRS complex: According to 2004 research, the QRS complex depicts the electrical activity through the ventricles (lower chambers) when they’re contracting. On the EKG, the complex looks like a tall spike (R wave), while two “valleys” or “dips” (Q and S waves) may appear on either side of the spike. The Q and S waves, however, aren’t always present.
  • T wave: The T wave represents the electrical activity of the ventricles when they’re at rest. The T wave is the last wave to appear before the P wave appears again.

If you’re having a heart attack, abnormal EKG results show an “ST elevation” in which the ST segment (the segment between the S and T waves) is elevated above the baseline.

A normal EKG would show a consistent and predictable heart rhythm and a heart rate between 60 and 100 beats per minute. It’s unlikely that you could be having a heart attack with a normal EKG.

If your EKG results are normal, but you’re still having symptoms such as chest pain, for example, additional tests may be performed to determine their cause. These tests may include:

Abnormal EKG results may show that your heart is having extra beats or significant pauses, or is going into an irregular rhythm (arrhythmia) such as atrial fibrillation. Other potential problems related to an abnormal EKG include:

If the healthcare team determines that you’re having a heart attack based on your symptoms, the results of an EKG and other tests, and a physical examination, you may be treated immediately.

Immediate treatments may include thrombolytic therapy (medications to break up a blood clot in the coronary arteries) or more invasive procedures such as a stent. If your heart is severely blocked, you may require coronary artery bypass grafting (CABG), which is a type of open heart surgery.

A normal EKG reading doesn’t require any treatment unless other tests suggest that there’s another ongoing heart problem or if one is likely to develop.

Once an EKG is available, a doctor will interpret the reading. If the EKG is inconclusive, other tests may be ordered, including additional EKGs.

Another test that’s often administered is a blood test to check for levels of troponin, a protein that usually isn’t found in the blood unless there has been damage to the heart muscle. A 2019 study suggests that a person’s symptoms, EKG results, and troponin tests can usually confirm that a heart attack is occurring.

Once a heart attack has been diagnosed, the healthcare team will decide what treatment makes the most sense.

Because an EKG is a quick, noninvasive, and relatively low-cost test, it can be administered in many situations, but health experts say that most people don’t need to run an EKG unless they’re having heart attack symptoms.

A 2019 study suggests that, absent any symptoms, an EKG has little value. Likewise, the U.S. Preventive Services Task Force recommends against routine EKGs for individuals with no symptoms, noting that false positives can lead to further unnecessary expensive and anxiety-inducing testing or treatment.

However, if you have other indications such as a history of heart disease or a family history of early heart disease, or if you have a condition such as diabetes, the healthcare team may want to include routine EKGs as part of your usual preventive care.

If you’re having heart attack symptoms, call 911 or local emergency services. Try to get to a hospital emergency room immediately if possible. You’ll most likely have an EKG when you arrive. If you’re taken by ambulance, you may have an EKG on the way.

In any event, EKG results can often show whether you’re having a heart attack or if you have had a previous heart attack.

A normal EKG result suggests your heart is functioning well, but you may need other tests to see what’s causing your chest pain, shortness of breath, or other symptoms.

Remember that an EKG doesn’t send electricity into your body. An EKG simply detects the heart’s electrical activity to give a healthcare team an idea of your current heart health.