An AV block (atrioventricular block) is a partial or complete interruption of electrical signals traveling from the upper chambers of your heart to the lower chambers. Your heart relies on a precise electrical relay system to coordinate each beat: a signal fires in the upper chambers (atria), passes through a central junction called the AV node, and then travels down into the lower chambers (ventricles) to trigger a contraction. When that relay is delayed or blocked, the ventricles may beat too slowly, skip beats, or lose coordination with the upper chambers entirely.
AV blocks range from mild and symptom-free to life-threatening, depending on how much of the electrical signal gets through. They’re classified into three degrees, each with a distinct pattern and level of risk.
How the Heart’s Electrical System Works
Every heartbeat starts with an electrical impulse in the atria. That impulse travels to the AV node, a small cluster of cells that acts as a gatekeeper between the upper and lower chambers. The AV node briefly slows the signal, giving the atria time to finish contracting and push blood into the ventricles. The signal then continues down a pathway called the His bundle, which splits into left and right branches that spread the impulse across both ventricles, triggering them to pump blood to your lungs and body.
An AV block can occur at the AV node itself, at the His bundle, or in the branches below it. Where the block happens matters: blocks at the AV node tend to be more stable and less dangerous, while blocks lower in the system are more likely to cause serious symptoms and require treatment.
First-Degree AV Block
In first-degree AV block, every electrical signal still reaches the ventricles, but each one takes longer than normal to get there. On an ECG, the delay shows up as a prolonged PR interval, the gap between the atrial signal and the ventricular response. A normal PR interval is 120 to 200 milliseconds. In first-degree block, it exceeds 200 milliseconds. When it stretches past 300 milliseconds, it’s considered a “marked” first-degree block.
Most people with first-degree AV block have no symptoms at all and never need treatment. It affects about 6% of adults over 60 and carries very low mortality on its own. It’s often discovered incidentally during a routine ECG.
Second-Degree AV Block
Second-degree AV block means some signals get through to the ventricles and some don’t, causing dropped beats. There are two distinct types, and the difference between them matters significantly.
Mobitz Type I (Wenckebach)
In Mobitz type I, the delay through the AV node gets progressively longer with each beat until one signal fails to make it through entirely, dropping a beat. The cycle then resets, starting with a shorter delay before gradually lengthening again. This repeating pattern is called the Wenckebach phenomenon. The block almost always occurs at the AV node when the ECG shows a narrow, normal-looking heartbeat pattern. Mobitz type I is generally considered less dangerous and may not require treatment unless it causes significant symptoms.
Mobitz Type II
Mobitz type II is more concerning. The delay before each beat stays constant, but signals are intermittently blocked without warning. There’s no gradual buildup. Beats simply drop. This type is always considered abnormal and occurs lower in the conduction system, at the His bundle in about 25% of cases and in the bundle branches in the rest. People with Mobitz type II are more likely to experience dizziness, lightheadedness, or fainting. It also carries a higher risk of progressing to complete heart block, which is why it typically requires closer monitoring or intervention.
Third-Degree (Complete) AV Block
Third-degree AV block is the most severe form. No electrical signals pass from the atria to the ventricles. The upper and lower chambers beat completely independently of each other. On an ECG, this shows up as P waves (atrial signals) and QRS complexes (ventricular signals) firing at their own separate rates with no relationship between them. There are more P waves than QRS complexes because the ventricles beat more slowly without proper signaling.
When the ventricles lose their normal driving signal, backup pacemaker cells take over. If these backup cells are located higher in the conduction system, the heart rate may be somewhat adequate, around 40 to 60 beats per minute. If they’re located lower, the rate can drop to 20 to 40 beats per minute, which is often too slow to maintain adequate blood flow. Without treatment, complete heart block has a five-year survival rate of only about 37%. It is rare, affecting roughly 0.02% of the U.S. population, but it requires urgent treatment, almost always with a pacemaker.
Symptoms Across All Types
First-degree block rarely produces noticeable symptoms. As the degree of block increases, symptoms become more likely and more serious. Common symptoms include fatigue, dizziness, lightheadedness, heart palpitations, shortness of breath, and chest pain. Fainting (syncope) can occur when the heart rate drops low enough that the brain doesn’t get sufficient blood flow. In severe or untreated cases, particularly complete heart block, sudden cardiac arrest is possible.
Some people with second-degree or even third-degree block are surprisingly asymptomatic, at least initially. Symptoms often depend on how slow the ventricular rate becomes and whether the person has other heart conditions.
Common Causes
AV block can result from structural damage to the heart’s conduction system or from medications that slow electrical conduction. The most common causes include:
- Aging: The conduction fibers can gradually degenerate with age, which is why AV block prevalence rises in older adults.
- Heart disease: Coronary artery disease, heart attacks, and inflammation of the heart muscle can damage conduction pathways.
- Medications: Several drug classes can slow or block AV conduction. Beta-blockers cause AV block in up to 25% of users. Calcium channel blockers like diltiazem and verapamil carry rates of roughly 4 to 16%. Digoxin, amiodarone, and certain antidepressants are also associated with AV block. Even beta-blocker eye drops used for glaucoma can occasionally cause it.
- Surgery or procedures: Cardiac surgery or catheter-based procedures near the AV node can damage conduction tissue.
- Congenital block: Some people are born with AV block, though this is uncommon.
When medications are the cause, the block often resolves after the drug is stopped or the dose is reduced.
How AV Block Is Diagnosed
A standard 12-lead ECG is the primary tool for diagnosing AV block. It captures the heart’s electrical activity and can reveal the characteristic patterns of each type: a prolonged PR interval in first-degree, progressive lengthening with dropped beats in Mobitz I, fixed intervals with unexpected drops in Mobitz II, or complete dissociation in third-degree block.
The challenge is that AV block can come and go. If your symptoms are intermittent, a single ECG snapshot may look perfectly normal. In that case, your doctor may use a Holter monitor, a portable device you wear for 24 to 48 hours that continuously records your heart’s rhythm as you go about your day. This extended monitoring catches irregularities that a brief office ECG would miss. For symptoms that occur even less frequently, longer-term monitors worn for weeks or implantable loop recorders can be used.
Treatment Options
Treatment depends entirely on the type and severity of the block. First-degree AV block almost never requires treatment. Mobitz type I second-degree block is usually monitored without intervention unless it causes symptoms like fainting or significant fatigue.
Mobitz type II and third-degree AV block are treated more aggressively because of the risk of dangerously slow heart rates or progression to cardiac arrest. The definitive treatment for these is a pacemaker, a small device implanted under the skin of the chest that sends electrical impulses to the heart when it detects the rate dropping too low. Pacemaker implantation is a relatively short procedure, typically taking one to two hours, and most people go home the same day or the next.
If a medication is causing the block, stopping or adjusting that drug is the first step. In acute situations where the heart rate is critically low, temporary measures can be used to support the heart rate until a permanent pacemaker is placed.