It’s estimated that in America, one person will 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.

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, such as from a heart attack.

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 can potentially reveal 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 compared to an MRI.
  • Good specificity. The EKG correctly identified that no previous heart attack had occurred 83.5 percent of the time compared to 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 a 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 low-risk individuals. 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 which 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 at the UT Southwestern Medical Center found evidence that the combined results from five tests better predict the risk of developing heart disease compared to evaluating blood pressure, cholesterol, diabetes, and smoking history.

The five tests were:

A number of other tests may also be used to help detect a previous heart attack. Here are common tests a doctor may recommend.

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. During the test, electrodes attached to your chest send information about the electrical activity of your heart to a small, battery-powered device.

Your doctor may recommend using a Holter monitor if they need more information than they can get from a traditional EKG.

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 the doctor to view the dye as it flows through the 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 the doctor if one part of your heart isn’t pumping blood as well as the others.

MRI

A cardiac MRI uses strong magnetic fields and radio waves to produce a three-dimensional 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.

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.