A doctor may refer to an electrocardiogram as an “ECG” or “EKG.” It uses electrodes to measure the electrical activity of the heart. This can help medical professionals detect heart abnormalities, such as heart damage or abnormal heart rhythms.

One common health condition that an ECG can help diagnose is coronary artery disease (CAD). In someone with CAD, the blood vessels supplying the heart become blocked or narrowed, and this can cause chest pain or a heart attack.

This article discusses how a doctor may use an ECG to diagnose, monitor, and stage CAD.

An artistic representation of an ECG.Share on Pinterest

The heart has different sections, called “chambers.” Electrical impulses cause the chambers to contract in a rhythmic cycle to keep blood flowing through the heart.

An ECG is a test that measures and records the heart’s electrical impulses to show the strength of these impulses as they travel through the heart, as well as heartbeat’s speed and rhythm.

A doctor uses an ECG to investigate:

  • signs of damage to the heart
  • abnormal heartbeat rhythms, known as arrhythmias
  • the effect of electrolyte abnormalities or drugs on the heart’s electrical system.

An ECG produces a graph of spikes and valleys that represent waves of electrical activity in the heart. This electrical activity consists of three waves: P, QRS, and T. The T wave has a feature called the ST segment.

Below, we describe these elements in more detail:

  • P wave: This shows contraction of the heart’s atria, which refers to the two upper chambers.
  • QRS complex: This shows electrical activity running through the heart to the ventricles, which are the two lower chambers.
  • T wave: This shows the electrical reset of the heart in preparation for its next cardiac cycle.
  • ST segment: This shows the end of contraction of the heart’s ventricles and the beginning of repolarization.

Other important markers include the PR and QT intervals. The PR interval refers to the time between the first deflection of the P wave and the first deflection of the QRS complex. The QT interval describes the time between the beginning of QRS complex and the end of T wave.

A doctor looks for changes in the waves that could indicate a problem with the heart, such as CAD. The changes could indicate a range of issues — an ECG is not a test specifically for CAD.

To diagnose CAD, a doctor looks for changes to the ECG waves or segments. Changes that could indicate CAD or a heart attack include:

  • ST segment depression
  • ST segment elevation
  • flattening of the T waves
  • inverted T waves
  • Q waves

How ECG results differ for a healthy or severely damaged heart

A normal ECG reading shows a consistent pattern across the P, QRS, and T waves. In a normal reading, both the ST segment and the T wave shows no signs of flattening, sharp spikes, or depressions.

By contrast, an ECG reading of a severely diseased heart is noticeably different. The T-waves may flatten or have more of a downward slope, while the ST segments may have abnormal elevations or depressions, for example.

Before arriving at a diagnosis of CAD or another health issue, a doctor likely needs to perform additional testing.

An ECG can show both stable CAD and acute coronary syndrome.

Stable CAD

Stable CAD occurs when a person has a “stable blockage” in their arteries. This indicates that treatment and lifestyle changes can restore and prevent future changes in blood flow.

Stable CAD often does not involve changes in the heart’s rhythm. However, a doctor may note ECG abnormalities in the ST segment during a stress test, which shows the heart’s response to exercise.

Acute coronary syndrome

This refers to a number of conditions that cause a full or partial blockage of blood flow to the heart. Without treatment, these blockages can cause permanent heart damage.

An ECG can reveal signs of this syndrome and any associated heart damage. During an acute blockage, a doctor can typically see changes to the ST segment. And Q waves can indicate a sign of a previous heart attack.

Doctors may use additional tests to diagnose or monitor CAD. Examples include:

  • Exercise stress test: This involves using a treadmill or exercise bike that gets faster while an ECG records how the heart responds to the exercise.
  • Chest X-ray: This can show the heart, lungs, and other organs.
  • Echocardiogram: This imaging technique uses sound waves to generate an image of the heart and its structures.
  • Coronary angiogram: Also known as cardiac catheterization, this imaging procedure uses dye and X-rays to visualize the movement of blood through the arteries and check for blockages. A doctor may also request a CT coronary angiogram, which uses a CT scan instead of X-rays.
  • Coronary artery calcium scan: A CT scan of the coronary arteries can also help show a calcium buildup along the inner walls of the arteries.

An ECG measures electrical impulses in the heart and produces a reading in the form of a graph. The graph contains individual segments that show the electrical wave activity in the heart.

An ECG reading can help a doctor determine whether there is a problem with a person’s heart, such as heart damage or an arrhythmia. One type of heart damage that an ECG can help detect is CAD.

In a person with CAD, the blood vessels supplying the heart become blocked or narrowed.

When looking at an ECG reading, a doctor will look for specific changes that could indicate a current or past issue. Though the test can be helpful, a doctor usually wants to run additional tests to make a diagnosis and monitor the condition going forward.