A pulmonary embolism (PE) is a blood clot in one of the arteries in the lungs. It not only blocks blood flow in the lungs, but it can affect heart function by making the right side of the heart work harder than usual.

A PE can be a life threatening condition, but it’s often treatable if diagnosed early and accurately.

An electrocardiogram (ECG) is one of several tests that doctors might use to learn more information about a PE. An ECG is a noninvasive screening that involves electrodes placed on the skin that can monitor the heart’s electrical activity and pick up any deviations from the heart’s usual rhythm.

While an ECG cannot help diagnose a PE, it can reveal problems in the heart that could suggest a PE, especially if a patient has other symptoms. In particular, it could show problems with the right ventricle of the heart.

Reading an ECG

An ECG readout represents the pattern of electrical activity in the heart as a line of waves. The key points on those waves are labeled P, Q, R, S, and T.

The distances between these points and their positions above and below the baseline combine to reveal the speed and rhythm of the beating heart. They also indicate the strength and pace of the electrical impulses moving throughout the heart muscle.

The specific signature of the PQRST waves can tell doctors a lot about your heart. An ECG can reveal if your heart rate is elevated or too slow or if there is an irregular rhythm (arrhythmia).

Electrodes placed on the chest also measure different information than those placed on the limbs. But each electrode or lead is important in presenting a complete picture of how electrical impulses move through the heart and affect its function.

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ecg reading of a typical sinus rhythmShare on Pinterest
Illustration by Maya Chastain

An ECG cannot, by itself, diagnose a pulmonary embolism. A 2017 study suggests that about 25 percent of ECG results appear standard in people with a blood clot in their lungs.

Some of the more common conditions an ECG can uncover include:

Sinus tachycardia

Sinus tachycardia is one of the more common arrhythmias associated with PE. Sinus tachycardia occurs when the sinus node emits electrical impulses that make the heart beat too fast. Sinus tachycardia is present in about 30 percent of PE cases.

Treating the underlying cause of sinus tachycardia, whether it’s a PE, anemia, thyroid disease, or another trigger, often allows the heart to maintain a healthy rate and rhythm.

Right bundle branch block

A blockage of electrical signals in the right side of the heart is called a right bundle branch block (RBBB). A PE could cause an RBBB by causing the right ventricle to work unusually hard.

This reduces blood flow to the right bundle branches, which are the special fibers that carry the electrical signals from the right atrium to the right ventricle.

The presence of RBBB suggests an especially large and dangerous blood clot.

Right ventricular strain

Right ventricular (RV) strain means there’s a problem with the muscle in the heart’s right ventricle. A 2019 study suggests that an ECG indicating RV strain in people with shortness of breath is “highly suggestive” of a PE.

Right atrial enlargement

When blood flow from the right side of the heart to the lungs is made more difficult by a blood clot in the lungs, both the right ventricle and right atrium can become enlarged. These changes can make the heart less effective in pumping blood out to the rest of the body, leading to heart failure or death.

Atrial fibrillation

One of the most common arrhythmias, atrial fibrillation (AFib), can be both a result and a cause of PE, according to a 2017 review.

Someone with AFib is at higher risk of developing a blood clot in the heart that could make its way to the lungs and block an artery there. If a clot forms in the lungs and forces the heart to work harder, the additional burden on the heart could cause the onset of AFib.

Pulmonary embolism and the S1Q3T3 pattern

The S1Q3T3 pattern is a common ECG finding when a PE is present. But it does not always indicate PE.

An ECG reading with this pattern shows:

  • a pronounced S wave in lead 1
  • a pronounced Q wave in lead 3
  • an upside-down T wave in lead 3

The pattern suggests excessive strain on the right side of the heart.

Certain ECG anomalies associated with PE, such as an S1Q3T3 pattern with RBBB, may also be reflected in cor pulmonale. This is a disease of the right ventricle that may or may not be triggered by PE.

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An ECG is not an important part of diagnosing a PE, but it can give doctors more information.

To diagnose a PE, a doctor will consider several imaging tests along with your medical history and current symptoms. An ECG will provide clues as to how the heart is functioning, which can influence outlook and treatment.

Anomalies that show up on an ECG may indicate the severity of a PE and help determine whether emergency treatment is necessary.

A 2017 review of studies found that ECG anomalies predicted a negative outcome for PE patients. Those with S1Q3T3 patterns or signs of RBBB were more likely to die in hospital. Sinus tachycardia and AFib were the strongest predictors of 30-day mortality.

An ECG is a relatively simple test to perform. Doctors often request an ECG when there is a suspicion of any cardiovascular trouble. The test includes the following steps:

  1. A doctor, nurse, or technician will place up to 12 electrodes on your chest and limbs.
  2. The electrodes transmit the heart’s electrical activity to the ECG monitor.
  3. The monitor converts the information into wave patterns.
  4. The patterns are usually printed on paper, giving doctors a hard copy to analyze.
  5. Your doctor examines the wave patterns to look for signs of irregular rates or rhythms.

The entire procedure can take less than 10 minutes. It is painless and requires no anesthesia or special preparation.

Why is pulmonary embolism so difficult to diagnose?

One of the main challenges in diagnosing a PE is that its symptoms, such as chest pain, shortness of breath, and lightheadedness, are common in several other cardiovascular conditions.

Also, standard screenings such as an ECG or chest x-ray cannot reveal whether a blood clot is present in the lungs. However, they can help create a comprehensive assessment of a person’s heart and lung health.

What tests do doctors use to diagnose pulmonary embolism?

A 2021 study suggests that computed tomographic pulmonary angiography is the “gold standard” test used to diagnose a PE. The screening combines a CT scan with an angiogram.

A CT scan uses special x-ray equipment to create cross-sectional images of your body. An angiogram uses a contrast dye to reveal detailed images of blood flow through the arteries and veins.

Some tests identify the location and size of a blood clot in the lungs. Others help to better evaluate how the heart and lungs are functioning. These tests can sometimes reveal the severity of the PE or rule out a particular cause of symptoms.

Common tests used to diagnose a PE include:

Can I take an ECG at home?

There are a variety of at-home ECG monitors you can purchase. There are also portable monitors, such as a Holter monitor, that a doctor can prescribe to track your heart’s electrical activity 24/7.

While these devices can be helpful, especially in picking up unusual heart rhythms when you’re away from the doctor’s office, they cannot exclusively detect a PE.

A PE can trigger complications ranging from arrhythmias to sudden cardiac arrest. Getting an accurate diagnosis as soon as possible can be a lifesaver.

An ECG can help determine if an arrhythmia or other change in heart function is present. It’s part of a comprehensive evaluation of your heart and lung health.

If your doctor suspects or has already diagnosed a PE, a simple, painless ECG can give them vital information about your heart’s function and any complications brought on by a blood clot in the lungs.