Third-degree heart block, also called complete heart block, is a condition where the electrical signals that control your heartbeat completely fail to travel from the upper chambers of the heart (atria) to the lower chambers (ventricles). The result is a dangerously slow heart rate, typically below 45 to 50 beats per minute. Without treatment, it has a five-year survival rate of roughly 37%, making it one of the most serious heart rhythm disorders.
How the Electrical Failure Works
A healthy heart relies on a built-in electrical relay system. A natural pacemaker at the top of the heart fires a signal, which travels through a junction point called the AV node and into the ventricles, telling them when to contract. In third-degree heart block, that relay is completely severed. No signals from the upper chambers reach the lower chambers at all.
When the ventricles stop receiving instructions, backup pacemaker cells lower in the heart take over to keep blood pumping. These backup cells fire much more slowly than the normal pacemaker, which is why the heart rate drops so dramatically. Meanwhile, the upper chambers continue beating at their own faster pace, completely out of sync with the ventricles. This disconnect is called atrioventricular dissociation, and it’s the defining feature of the condition.
Where exactly the blockage occurs matters. If it happens near the AV node, the backup rhythm tends to be a bit faster and more stable. If it happens lower in the conduction system, the backup rhythm is slower and less reliable, which carries a higher risk of the heart simply stopping.
What Causes It
The most common causes in adults fall into a few categories. Heart attacks are a frequent trigger, particularly ones that damage the blood supply to the heart’s conduction system. Age-related wear and tear can also gradually destroy the conduction fibers through a process of scarring and calcification. Certain medications that slow heart conduction, including some blood pressure drugs and heart rhythm medications, can push a vulnerable conduction system into complete block.
Other causes include infections that inflame the heart, complications from heart surgery, and infiltrative diseases where abnormal deposits build up in heart tissue. In some cases, no clear cause is identified, and the block is attributed to progressive degeneration of the conduction system over time.
Congenital Complete Heart Block
Rarely, babies are born with third-degree heart block. The most well-understood cause involves the mother’s immune system. In a condition called neonatal lupus, antibodies from the mother cross the placenta around 12 weeks of gestation and attack cells in the developing conduction system. This triggers inflammation that eventually scars the tissue, permanently disrupting the electrical pathway. The mother may have lupus or Sjögren’s syndrome, but roughly half of affected mothers have no symptoms of autoimmune disease at the time of diagnosis. The condition is typically discovered when a slow fetal or newborn heart rate is detected in an otherwise structurally normal heart.
Symptoms and Warning Signs
Most people with complete heart block are noticeably unwell. The severely reduced heart rate means the body isn’t getting enough blood flow, which produces a cluster of symptoms tied to poor circulation: dizziness, lightheadedness, profound fatigue, and shortness of breath, especially with exertion. Fainting episodes are common and can occur without warning. These sudden blackouts, sometimes called Stokes-Adams attacks, happen when the ventricles briefly stop beating altogether before the backup pacemaker kicks in.
Some people experience chest pain, confusion, or a sense that something is seriously wrong. In severe cases, the reduced cardiac output can lead to heart failure, with fluid building up in the lungs and legs. Because the heart rate is so low, even mild physical activity can feel exhausting.
How It’s Diagnosed
An electrocardiogram (ECG) is the primary diagnostic tool, and the pattern it reveals is distinctive. The ECG shows two independent rhythms happening simultaneously: the upper chambers beating at one rate and the ventricles beating at a completely different, slower rate. There is no consistent relationship between the two. In a healthy heart, every signal from the upper chambers produces a corresponding ventricular beat. In third-degree block, those two rhythms have nothing to do with each other.
This sets it apart from second-degree heart block, where some signals still get through. In second-degree block, you can see a pattern: signals are occasionally dropped or progressively delayed, but there’s still a relationship between the upper and lower chambers. In third-degree block, that relationship is gone entirely. The diagnosis is usually straightforward for trained clinicians looking at a standard 12-lead ECG.
Treatment: Why Pacemakers Are Standard
Third-degree heart block almost always requires a permanent pacemaker. The 2018 guidelines from the American College of Cardiology and the American Heart Association are clear: permanent pacing is recommended for anyone with third-degree heart block that isn’t caused by a reversible or temporary condition, regardless of whether they have symptoms. This is not a “wait and see” situation.
In an emergency, temporary measures can bridge the gap. External pacing pads placed on the chest can electrically stimulate the heart while a permanent solution is arranged. Certain medications can also temporarily raise the heart rate, though their effectiveness varies depending on where the block is located.
A permanent pacemaker is a small device implanted under the skin near the collarbone, with wires threaded into the heart. It monitors the heart’s rhythm continuously and delivers electrical impulses when the heart rate drops too low. The procedure typically takes one to two hours, and most people go home the same day or the next morning. Recovery involves limiting arm movement on the implant side for a few weeks while the leads settle into position.
The only exception to the pacemaker recommendation is when the block is clearly caused by something reversible, like a medication that can be stopped or an acute infection that will resolve. In those cases, the block may correct itself once the underlying cause is treated.
Living With Complete Heart Block
With a pacemaker in place, most people return to normal or near-normal activity levels. The device essentially replaces the broken relay, ensuring the ventricles receive the signals they need to beat at an appropriate rate. Modern pacemakers are small, long-lasting (batteries typically last 10 to 15 years), and adjust their pacing rate based on your activity level.
You’ll need periodic checkups, usually every 6 to 12 months, where a technician wirelessly reads data from the pacemaker to check battery life, lead function, and how often the device is pacing. Many newer pacemakers can transmit this data remotely from your home. Eventually, the device will need to be replaced when the battery runs low, which is a simpler procedure than the original implant since the leads are already in place.
Who’s Most at Risk
Third-degree heart block is uncommon but not rare. A large population study tracking participants over 17 years found that high-degree heart block (which includes both advanced second-degree and complete heart block) occurred at a rate of roughly 2.4 to 2.8 cases per 1,000 person-years, with rates increasing sharply with age. As the population ages, the number of cases in the United States is projected to rise from about 379,000 in 2020 to 535,000 by 2060, with the largest increase among older adults. The condition affects roughly 0.02% of the U.S. population at any given time, but it carries outsized consequences when it occurs.