Low Voltage ECG: What Does It Mean and What Causes It?

A low voltage ECG means the QRS complexes, the tall spikes made when the lower heart chambers contract, are smaller than expected across many leads. It describes how the electrical signal looks at the skin, not a disease. Sometimes the cause is simply what lies between the heart and the electrodes; sometimes it prompts tests for fluid around the heart, lung disease, thyroid problems or heart muscle changes.

Research studies commonly define low QRS voltage as complexes of 0.5 mV or less in all limb leads, or 1.0 mV or less in all chest leads. Definitions vary slightly between studies and machines, and the label may come from an automated reading. MedlinePlus notes that ECG results cannot diagnose a heart condition on their own, so the finding is always read alongside symptoms, examination and other tests.

Fluid around the heart can occasionally build up enough to press on it. This is called cardiac tamponade and is an emergency. Cleveland Clinic advises going to an emergency department immediately with any of these symptoms:

• Chest pain, tightness or heaviness, especially if it spreads to the arms, neck, jaw or back.
• Trouble breathing or fast breathing.
• Fainting, dizziness or light-headedness.
• Pale, grey or blue skin, lips or nails.
• Palpitations, or a resting pulse above 100 beats per minute.

Yes. The ECG measures tiny electrical signals through the skin. Tissue between the heart and the electrodes can make the waves look smaller. Research on low QRS voltage counts obesity, defined in one study as a body mass index of 30 or more, among the classic explanations.

This is one reason clinicians compare the result with your build, earlier ECGs and how you feel. A low voltage pattern in someone with no symptoms may lead to nothing more than a note in the record, but that decision belongs to the clinician who knows the full picture.

Low voltage on its own is a description, not a verdict on how well the heart works. How much it matters depends on the reason behind it and on how you feel. A low voltage pattern explained by body build or a long-standing lung condition is very different from one that appears alongside new breathlessness, swelling or chest discomfort.

Clinicians usually ask three things: is this new compared with earlier ECGs, are there symptoms, and does the examination suggest fluid around the heart or lungs? A new change, or one that comes with symptoms, is more likely to lead to further tests. MedlinePlus notes that people often need other heart tests before a diagnosis can be made, so a single tracing is a starting point rather than an answer.

If you were not told what the finding means for you, contact the team that arranged the ECG and ask, rather than waiting for the next routine appointment.

Air and fluid also dampen the signal. Research lists over-inflated lungs, emphysema and other obstructive lung disease as classic causes of low QRS voltage, as well as moderate or large pericardial effusions, which are collections of fluid in the sac around the heart.

Cleveland Clinic explains that many pericardial effusions cause no symptoms and are sometimes found by chance, while larger or fast-building ones can cause breathlessness, chest pressure and palpitations. Because fluid around the heart can be checked directly, an echo is a common next step when it is suspected.

An underactive thyroid can be part of the picture. A Danish primary-care study found that both subclinical and overt hypothyroidism were associated with low voltage on the ECG. Cleveland Clinic also lists hypothyroidism as a cause of pericardial effusion, and a thyroid blood test (TSH) may be among the tests used to look for the cause of fluid around the heart.

If your thyroid has not been checked recently, it is reasonable to ask whether it should be. Do not start, stop or change any thyroid or heart medicine yourself; discuss any change with your prescriber.

An echocardiogram is an ultrasound of the heart. It shows the heart muscle, the valves and any fluid around the heart. MedlinePlus describes echo as the test of choice when tamponade is suspected, and it is one of the main tests for pericardial effusion.

Echo is also used when clinicians want to look at the heart muscle itself. For example, MedlinePlus notes that cardiac amyloidosis, a condition where abnormal protein builds up in the heart, may show low voltage on the ECG, and echo, MRI and sometimes a biopsy can be used to investigate it. Other possible tests include a chest X-ray, blood tests such as TSH or BNP, and a repeat ECG. Which tests make sense depends on your symptoms and history.

Take the ECG report, any earlier ECGs, and a list of your symptoms and medicines. Useful questions include:

• Was low voltage seen in the limb leads, the chest leads or both?
• Was the low voltage reading confirmed by a clinician, or is it a computer label?
• How does this ECG compare with my earlier ones?
• Could my build, a lung condition or my thyroid explain it?
• Do I need an echo to look for fluid around the heart or changes in the heart muscle?
• Which symptoms should make me seek urgent care while I wait for tests?

On SameCase, patients and verified doctors share de-identified cases. Reading how others with a low voltage ECG were assessed, for example which tests were ordered and what questions they asked, can help you prepare for your own appointment.

Another person's ECG cannot tell you what yours means. We see similar cases as a way to organise your questions, not as a diagnosis or a substitute for your own care team.