An electrocardiogram (ECG or EKG) is a test that measures the electrical activity of your heart. Abnormal patterns of activity suggest that part of your heart may have been damaged from a heart attack or some other heart condition.

It’s estimated that in the United States, one person has a heart attack every 40 seconds. Heart attacks typically cause symptoms such as chest pain, trouble breathing, and dizziness. It’s also possible to have a silent heart attack and experience no symptoms at all.

In this article, we’ll examine how accurate EKGs are for diagnosing a previous heart attack, if they can predict future heart attacks, and other tests to assess your heart’s health.

An EKG is a safe, fast, noninvasive way of checking your heart’s electrical activity.

Your heartbeat is controlled by an electrical system that controls when your heart’s upper chambers (atria) contract and then when your heart’s lower chambers (ventricles) beat. Normally, this happens in a synchronized pattern and at a predictable speed.

Changes in how your heart beats can be an indication of cardiac problems, such as a heart attack or arrhythmia.

According to the American Heart Association, an EKG provides two important pieces of information about your heart health:

  • It measures the time it takes for an electrical wave to move from the heart’s atria to the ventricles, which reveals whether the electrical activity is too fast or slow, or whether it’s chaotic.
  • It also measures the amount of electrical activity moving through your heart, which can help your doctor determine if your heart is working too hard.

An EKG involves the placement of small electrodes on your chest and limbs. The electrodes are removable stickers attached by wires to the EKG machine, which records electrical signals from your heart and displays them on a monitor.

Printouts show the electrical pattern of your heart. This is done so that there’s a hard copy record of how your heart was behaving at that moment. This is important because some changes to your heart’s electrical activity are temporary. That’s why it’s helpful to have evidence of changes from your heart’s usual electrical patterns.

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Illustration by Sophia Smith

An EKG is one of several tests that can help diagnose a heart attack. It’s usually one of the first screenings performed when someone goes to the hospital with heart attack symptoms.

Because damaged heart tissue tends to disrupt the regular flow of electrical energy throughout the heart, an EKG can instantly detect if there is a problem.

In addition to indicating the possibility of some heart muscle damage, an abnormal EKG reading may also be the result of reduced blood flow through the coronary arteries. This is often the trigger for most heart attacks.

An EKG may also diagnose an abnormal heart rhythm, called an arrhythmia.

In addition to an EKG, a blood test is also done if a heart attack is suspected. Damaged heart tissue usually releases certain proteins called troponins. Unusually high levels of troponin T and troponin I are often a sign of a heart attack.

An EKG can potentially detect that you had a heart attack years ago without knowing it. Abnormal electrical patterns during the test suggest that part of your heart may have been damaged from lack of oxygen.

Not all heart attacks produce noticeable symptoms. If you’ve had a silent heart attack, you may not know it occurred until you have an imaging test like an EKG, MRI, CT scan, or ultrasound.

An EKG is one tool that doctors use to find evidence of previous heart attacks, but it’s best used when combined with other diagnostic techniques like blood tests and imaging. It’s relatively common for EKG results to give a false positive.

One study measured the accuracy of an EKG for diagnosing a previous heart attack compared to a cardiac MRI. The researchers found that EKGs had:

  • Poor sensitivity. The EKG only correctly identified a previous heart attack 48.4 percent of the time when compared with an MRI.
  • Good specificity. The EKG correctly identified that no previous heart attack had occurred 83.5 percent of the time when compared with MRI.
  • Positive predictive accuracy. People with EKG results that suggested they had a heart attack had a 72 percent chance of actually having had a heart attack.
  • Negative predictive accuracy. People with EKG results that suggested they didn’t have a heart attack had a 64.2 percent probability of not actually having had a heart attack.

The poor sensitivity and the modest negative predictive accuracy of the EKG findings suggest that EKG alone may not be the best way to diagnose a previous heart attack.

An EKG can potentially predict future risk of heart attack by uncovering abnormalities in the electrical activity of your heart.

A 2019 study found that EKG-based risk scores for cardiovascular disease are as good as — or sometimes better than — risk scores based on a patient’s history.

In another 2019 study, researchers found that people with atherosclerotic disease or multiple risk factors for heart attack often had abnormal EKG readings.

Using EKG results alone isn’t an effective predictor of a future heart attack in those at low risk. It’s possible to have a heart attack despite a normal EKG reading.

A limitation of EKG is that it cannot show an asymptomatic blockage in your arteries that may put you at risk of a future heart attack. EKGs are best used as a predictor of a future heart attack in combination with other tests.

Researchers have found evidence that the combined results from five different tests may improve the ability to predict the risk of developing heart disease compared to evaluating blood pressure, cholesterol, diabetes, and smoking history.

The five tests are:

A number of other tests may also be used to help detect a previous heart attack. These include:

  • A Holter monitor. A Holter monitor is a type of EKG that measures the electrical activity of your heart over a period of 24 hours or longer as you go about your daily activities. Electrodes attached to your chest send information about the electrical activity of your heart to a small, battery-powered device.
  • Blood tests. A blood test can look for certain markers that indicate you’ve had a heart attack. One substance commonly used as a marker of a heart attack is troponin. Levels of this protein remain elevated in the blood for up to 2 weeks after a heart attack.
  • Coronary CT angiogram. A coronary CT angiogram uses X-rays to produce an image of the arteries that bring blood to your heart. A special dye is injected into your bloodstream that allows a doctor to view the dye as it flows through your arteries.
  • Cardiac catheterization. During a cardiac catheterization, a long tube called a catheter is inserted through a puncture in your skin and inserted into an artery leading to your heart. A contrast dye is injected into your bloodstream so that the doctor can examine your heart.
  • Echocardiogram. An echocardiogram uses ultrasound waves to show a live image of your heart. The image can tell a doctor if one part of your heart isn’t pumping blood as well as it should.
  • MRI. A cardiac MRI uses strong magnetic fields and radio waves to produce a 3-D image of your heart. An MRI allows the doctor to identify a lack of blood flow to a certain area, or to see if part of your heart is damaged.

A silent heart attack is a heart attack that has few or no symptoms. If you’ve had a silent heart attack, you may have an elevated risk of developing another heart attack or heart failure. You may also have an elevated risk of mortality because the lack of symptoms often delays medical treatment.

When symptoms are present, they’re often so mild that they don’t seem particularly concerning. Fatigue, mild chest pain that feels like indigestion, and flu-like symptoms are all possible signs.

Silent heart attacks are caused by a lack of blood flow to your heart like traditional heart attacks. Improving your overall cardiovascular health and regularly getting checkups can help minimize your risk.

When to seek emergency care

If you think you may have had a heart attack, seek emergency care right away. Getting immediate treatment can help minimize the damage to your heart.

If you experience any of the following symptoms, seek emergency attention:

  • chest pain or discomfort in the center or left side of your chest that lasts more than a few minutes
  • pain in your jaw, back, or neck
  • pain in one or both arms or shoulders
  • shortness of breath
  • lightheadedness
  • feeling weak or faint

An EKG can help identify a previous heart attack by screening for abnormalities in the electrical activity of your heart. EKG results are often best used in combination with blood tests and imaging techniques to reduce the chances of a false positive.

It’s still not clear how effective EKGs are at identifying your future heart attack risk. Research suggests that they may be best used in combination with blood tests and a calcium coronary scan for this purpose.

If you think you may have had a heart attack, it’s important to seek medical attention immediately. The sooner you get proper treatment, the better your outlook.

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