An AV block (atrioventricular block) is a delay or complete interruption in the electrical signals that travel from the upper chambers of your heart to the lower chambers. In a healthy heart, each heartbeat begins with an electrical impulse in the upper chambers that passes through a relay point called the AV node before reaching the lower chambers. When that relay point malfunctions, your heart’s two sets of chambers lose their coordination, and your heart may beat too slowly to deliver enough blood to your body.
AV blocks range from mild (barely noticeable) to life-threatening, depending on how much the signal is disrupted. There are three degrees, each with distinct characteristics and different implications for your health.
How Your Heart’s Electrical System Works
Every heartbeat starts with a tiny burst of electricity from a cluster of cells called the SA node, located at the top of the heart. That signal spreads across the upper chambers (atria), causing them to squeeze and push blood downward. The signal then reaches the AV node, which sits near the center of the heart and acts as a gatekeeper. It deliberately pauses the signal for a fraction of a second so the upper chambers have time to fully empty before the lower chambers contract.
After that brief pause, the signal travels down a bundle of nerve fibers running through the wall that separates the two lower chambers, then fans out into a network of smaller fibers that trigger the lower chambers (ventricles) to contract and pump blood to your lungs and body. An AV block occurs when something goes wrong at or below the AV node, slowing or stopping the signal before it reaches the ventricles.
First-Degree AV Block
First-degree AV block is the mildest form. The electrical signal still reaches the ventricles every time, but it takes longer than normal to get through the AV node. On an ECG, this shows up as a PR interval (the time between the upper and lower chambers firing) longer than 0.20 seconds. Normal is 0.12 to 0.20 seconds. When the delay stretches beyond 0.30 seconds, it’s considered a “marked” first-degree block.
Most people with first-degree AV block have no symptoms at all. It’s typically discovered incidentally during a routine ECG. It doesn’t usually require treatment, though it can sometimes signal an underlying issue worth monitoring.
Second-Degree AV Block
In second-degree AV block, some electrical signals make it through to the ventricles and some don’t, meaning your heart occasionally skips a beat. There are two subtypes, and the distinction matters because they carry different risks.
Type I (Wenckebach)
With Type I, the delay through the AV node gets progressively longer with each heartbeat until one signal fails to get through entirely. Then the cycle resets and starts over. This pattern tends to originate within the AV node itself and typically produces a normal-looking heartbeat on an ECG when a signal does get through. Type I is generally considered less dangerous and may not need treatment unless it causes significant symptoms.
Type II (Mobitz II)
Type II is more concerning. The signal passes through normally for several beats, then suddenly drops without any progressive lengthening beforehand. This pattern suggests the problem is below the AV node, deeper in the conduction system. Type II carries a higher risk of progressing to complete heart block and often warrants a pacemaker.
Sometimes the block follows a fixed ratio, like every other signal being dropped (2:1 block) or two out of every three being dropped (3:1 block). When this happens, it can be difficult to tell whether the underlying pattern is Type I or Type II without watching the rhythm over a longer period.
Third-Degree (Complete) AV Block
Third-degree AV block is the most severe form. No electrical signals pass from the upper chambers to the lower chambers at all. The atria and ventricles become completely disconnected electrically, each beating at its own pace. The upper chambers typically maintain a rate of 60 to 100 beats per minute, while the lower chambers generate their own backup rhythm at a much slower 30 to 40 beats per minute.
That backup rhythm keeps you alive, but it’s often too slow to meet your body’s needs, especially during physical activity. The backup can originate from different levels of the conduction system. When it comes from higher up, near the AV node, the heartbeat pattern looks relatively normal on an ECG. When it comes from lower in the system, the pattern is wider and more abnormal, which generally signals a less reliable rhythm. Complete heart block almost always requires a pacemaker.
What Causes AV Block
The most common cause in older adults is age-related wear on the heart’s electrical wiring, where the conduction fibers gradually become scarred or stiff. Heart attacks are another major cause: when blood flow to the area around the AV node is cut off, the damage can disrupt signal transmission. The presence of complete heart block during a heart attack is an independent predictor of higher mortality, particularly when the heart attack affects the front wall of the heart.
Several medications can slow conduction through the AV node. Beta-blockers, certain calcium channel blockers, some anti-arrhythmia drugs, and digoxin have traditionally been considered reversible causes. However, research shows the relationship is more nuanced than once thought. At normal doses, these medications rarely cause AV block on their own. They’re more likely to unmask an existing conduction problem that hasn’t yet caused trouble. When someone develops AV block while on these drugs, the block frequently recurs even after the medication is stopped, suggesting underlying disease was the real culprit.
High potassium levels, thyroid disorders (particularly an underactive thyroid), infections affecting the heart, and certain autoimmune conditions can also trigger AV block. Heart surgery and catheter-based valve procedures occasionally cause it as well.
Symptoms to Recognize
First-degree AV block rarely causes symptoms. Second-degree block may or may not produce noticeable effects depending on how many beats are dropped and how often. Third-degree block almost always causes symptoms because the heart rate drops so significantly.
The hallmark symptoms are dizziness, lightheadedness, and fainting (syncope). Fainting happens because the slow heart rate can’t push enough blood to the brain, especially when you stand up or exert yourself. Many people also experience persistent fatigue that worsens over weeks or months, poor exercise tolerance, and shortness of breath. In some cases, symptoms develop gradually. A person might feel increasingly tired and dizzy over a period of months before a sudden collapse prompts medical evaluation.
Chest discomfort or a sensation that your heart is beating irregularly can also occur. In severe cases, particularly with complete heart block, prolonged periods of very slow heartbeat can lead to loss of consciousness without warning.
How AV Block Is Diagnosed
A standard 12-lead ECG is the primary tool. It captures a snapshot of your heart’s electrical activity and can identify the degree and type of block if it’s happening at the time of the test. The challenge is that some AV blocks come and go, so a normal ECG doesn’t always rule one out.
For intermittent or suspected AV block, your doctor may use ambulatory monitoring. A traditional Holter monitor records your heart rhythm continuously for 24 to 48 hours. Patch-based monitors can extend that to 14 days or longer, and studies show these longer recording periods catch more arrhythmias than a standard 24-hour Holter. For very infrequent episodes, an implantable loop recorder (a tiny device placed just under the skin of your chest) can monitor your heart rhythm for up to three years.
If the block is linked to a heart attack or suspected structural heart disease, additional testing like echocardiography or cardiac catheterization may be used to identify the underlying cause.
Treatment and Pacemakers
Treatment depends entirely on the type and severity. First-degree AV block with no symptoms typically requires only periodic monitoring. Second-degree Type I (Wenckebach) is usually managed the same way unless symptoms are present.
Second-degree Type II and third-degree block are treated with a permanent pacemaker in most cases. A pacemaker is a small device implanted under the skin near the collarbone, with thin wires threaded into the heart. It monitors your heart rhythm and delivers electrical impulses when the heart rate drops below a set threshold. The procedure takes about 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 place.
When a reversible cause is identified, such as a medication, high potassium, or a thyroid disorder, correcting that issue may resolve the block. But because underlying conduction disease is often present even when a reversible trigger is found, close follow-up is important. Recurrence rates after drug discontinuation are higher than previously recognized, and many of these patients eventually need a pacemaker anyway.