Also known as complete heart block, third-degree heart block is the most serious type of heart block. Considered a medical emergency, third-degree heart block can be fatal without prompt treatment.

The steady beating of your heart relies on the uninterrupted flow of electrical energy starting in the upper chambers (atria) and moving down to the lower chambers (ventricles) of your heart.

Heart block happens when there’s an interruption or blockage of electrical impulses as they move from the atria to the ventricles. Third-degree heart block, also known as complete heart block, is the most serious type of atrioventricular (AV) block. It means there’s a complete separation of the electrical activity in the upper and lower chambers of the heart.

Third-degree heart block is a medical emergency and can be fatal without prompt treatment.

This article takes a closer look at third-degree heart block, what causes it, the symptoms, and treatment.

The degree to which electrical impulses are impeded along their journey in the heart muscle determines the type of heart block. The three types of heart block are defined as follows:

  • First degree: Electrical impulses slow down, but there may be no noticeable symptoms or required treatment. It’s the least severe type of heart block.
  • Second degree: With second-degree heart block, only some electrical signals from the heart’s atria reach the ventricles. As a result, the heart may skip beats and also beat irregularly or at a slower pace. Second-degree heart block can be classified as:
    • Mobitz type I, which may not have symptoms but can be a warning sign of the more serious Mobitz type II
    • Mobitz type II, which impacts the heart’s ability to beat effectively and often requires a pacemaker to help the heart beat regularly
  • Third degree: Impulses no longer reach the ventricles from the atria.

Some type of heart disease can cause third-degree heart block, but it can also have other causes, including:

A 2019 study of more than 6,000 adults suggests that two modifiable risk factors — high blood pressure and high fasting blood glucose (sugar) levels — are associated with an increased risk of heart block. Managing these factors may help reduce heart block risk.

A family history of heart block may also raise the risk of third degree heart block, though a 2021 study suggests that inherited heart block is rare.

Third-degree heart block may also be present at birth. More than half of all cases of congenital heart block may be due to the mother having an autoimmune condition, such as lupus or Sjogren’s syndrome.

Third-degree heart block often presents with a very slow or very faint pulse.

Other symptoms can include:

  • dizziness or lightheadedness
  • fatigue
  • fainting
  • chest pain

Third-degree heart block is a medical emergency. If you or someone else has any of the symptoms of third-degree heart block, call 911 immediately or get to an emergency department right away.

The main tests that are used to diagnose third-degree heart block are:

An ECG relies on electrodes placed on the skin to record the heart’s electrical activity. An electrophysiology study uses a catheter that’s inserted into a blood vessel in the groin and guided up to the heart to test the heart’s electrical system.

Because third-degree heart block can be intermittent, an ECG may not diagnose it. Heart rhythm monitoring can be done with a Holter monitor or a patch monitor. These devices help diagnose an abnormal rhythm by monitoring your heart rate over a period of days or weeks.

Other criteria used to diagnose third-degree heart block include:

  • a review of symptoms
  • medical history
  • physical examination
  • review of current medications

Because third-degree heart block involves a complete break in the heart’s electrical system and can result in low cardiac output and even cardiac arrest, treatment requires urgent pacemaker placement.

A pacemaker is a small battery-powered device that’s implanted in the chest. It contains wires (or leads) that run to the heart and carry electrical signals that keep the heart beating in a normal, predictable manner.

If third-degree heart block is due to a reversible cause, like a medication, addressing the cause may reverse the heart block.

The most important factor with third-degree heart block is to get immediate treatment.

The long-term outlook for someone with third-degree heart block also depends largely on the individual’s overall health and the nature of the underlying heart condition that triggered the heart block.

If a heart attack triggered third-degree heart block, prompt treatment and restoration of healthy heart function and blood flow can sometimes reverse serious heart block.

Third-degree heart block is a potentially life threatening medical emergency. It’s the most serious of the three types of heart block.

A heart attack can often trigger third-degree heart block, but there can be other causes like other types of heart disease or medication side effects.

Getting medical attention as soon as you notice symptoms like a slow pulse, dizziness, fatigue, and fainting is key to increasing the survival outlook for third-degree heart block. Treatment with a pacemaker can often restore a healthy heart rate.