AV block is a condition where electrical signals traveling from the upper chambers of your heart to the lower chambers are delayed or completely interrupted. Your heart relies on a precise electrical system to keep its chambers beating in sync, and when that system breaks down at a specific relay point called the AV node, the result is an atrioventricular (AV) block. There are three degrees of AV block, ranging from a minor timing delay that causes no symptoms to a complete communication breakdown that can be life-threatening.
How Your Heart’s Electrical System Works
Your heart has a built-in pacemaker called the SA node, which fires an electrical signal to start each heartbeat. That signal first triggers the upper chambers (atria) to contract and push blood down into the lower chambers (ventricles). Before reaching the ventricles, the signal passes through the AV node, a small cluster of cells near the center of your heart.
The AV node’s job is to create a brief, consistent delay, just a fraction of a second, so the upper chambers have time to fully empty their blood into the lower chambers before the lower chambers contract. Think of it as a traffic light that holds the signal just long enough to keep everything moving in the right order. AV block happens when this relay station slows the signal too much, lets some signals through but not others, or stops conducting them entirely.
First-Degree AV Block
First-degree AV block is the mildest form. Every electrical signal still reaches the ventricles, but it takes longer than normal to get there. On an EKG, this shows up as a PR interval (the time between the signal leaving the atria and arriving at the ventricles) longer than 0.20 seconds. The normal range is 0.12 to 0.20 seconds.
Most people with first-degree AV block have no symptoms at all. It’s typically discovered incidentally during a routine EKG. It doesn’t usually require treatment and often doesn’t progress to more serious forms. In many cases, it’s considered a normal variant, particularly in younger, physically active people.
Second-Degree AV Block
Second-degree AV block means some electrical signals make it through to the ventricles while others are dropped entirely, causing occasional “missed” heartbeats. There are two distinct types, and the difference between them matters significantly.
Type I (Wenckebach)
In Type I, the delay through the AV node gets progressively longer with each beat until one signal fails to get through completely. After that dropped beat, the cycle resets and starts over. The pattern is predictable: each successive beat takes a little longer to conduct, then one is skipped, then the timing resets to its shortest delay. This type is generally considered less dangerous and often occurs in well-conditioned athletes or during sleep. Many people with Type I have no noticeable symptoms.
Type II (Mobitz Type II)
Type II is a more serious pattern. Signals are dropped without any preceding delay, meaning there’s no gradual warning on the EKG before a beat goes missing. This type is rarely seen without underlying structural heart disease and is often linked to scarring, fibrosis, or reduced blood flow in the heart muscle. People with Type II frequently experience fatigue, shortness of breath, chest pain, lightheadedness, or fainting episodes.
The key concern with Type II is that it often progresses to complete heart block. Because of this risk, a pacemaker is typically recommended as soon as this pattern is identified, regardless of whether symptoms are present.
Third-Degree (Complete) AV Block
Third-degree AV block is the most severe form. No electrical signals from the SA node reach the ventricles at all, creating a complete disconnect between the upper and lower chambers. The atria continue beating at their normal pace, but the ventricles are left to generate their own rhythm using backup pacemaker cells.
These backup rhythms are slower and less reliable than the heart’s primary pacemaker. If the backup originates near the AV node itself, the heart rate may be somewhat adequate. If it comes from the ventricle muscle, the rate is typically much slower, often too slow to support normal activity. Symptoms can include severe fatigue, fainting, dangerously low blood pressure, and in some cases sudden cardiac arrest. Complete AV block almost always requires a permanent pacemaker.
Common Causes
AV block can develop for several reasons. In older adults, the most common cause is gradual scarring and fibrosis of the heart’s conduction tissue, essentially age-related wear on the electrical wiring. A heart attack can damage the AV node or surrounding tissue, sometimes causing temporary or permanent block. Open heart surgery carries a risk of conduction damage as well.
Certain infections can trigger AV block. Lyme disease is a well-known cause, particularly of temporary high-grade block that may resolve once the infection is treated. Medications that slow conduction through the AV node, including beta-blockers, calcium channel blockers, and digoxin, can also cause or worsen AV block. In these cases, adjusting or stopping the medication may resolve the problem.
Symptoms Across the Spectrum
What you feel depends heavily on which degree of block you have and how much it affects your heart’s ability to pump blood. First-degree block almost never causes symptoms. Type I second-degree block may cause occasional awareness of a skipped beat but is often silent. As block severity increases, symptoms become more pronounced and more concerning.
With Type II second-degree block and complete block, the most common symptoms are:
- Fatigue that worsens with physical activity
- Dizziness or lightheadedness, especially when standing
- Fainting or near-fainting episodes
- Shortness of breath
- Chest pain
In the most severe cases, reduced blood flow from complete block can contribute to heart failure over time. Long-term data from patients who developed complete AV block during a heart attack shows higher rates of sudden death, repeat heart events, and worsening heart failure compared to those without block.
How AV Block Is Diagnosed
A standard 12-lead EKG is the primary tool for identifying AV block. It captures the electrical activity of each heartbeat and reveals the characteristic patterns of delayed or dropped conduction. First-degree and persistent higher-degree blocks are often caught on a single EKG recording.
The challenge comes with intermittent block, where the abnormal rhythm comes and goes. If you’re experiencing fainting episodes or unexplained dizziness but a standard EKG looks normal, extended monitoring is the next step. A 24-hour Holter monitor records your heart rhythm continuously throughout a full day. For symptoms that occur less frequently, a wearable event monitor or an implantable loop recorder (a small device placed just under the skin) can track your heart rhythm for weeks or even months, catching episodes that a single EKG would miss.
Treatment and Pacemakers
Treatment depends entirely on the type and cause of the block. First-degree AV block and Type I second-degree block rarely need intervention. If a medication is causing or worsening the block, adjusting the dose or switching drugs is often enough. If an infection like Lyme disease is responsible, treating the underlying condition may resolve the block.
For more serious forms, a permanent pacemaker is the standard treatment. Current guidelines from the American College of Cardiology and American Heart Association recommend a pacemaker for anyone with Mobitz Type II, high-grade, or third-degree AV block that isn’t caused by a reversible or temporary condition, even if the person has no symptoms. The reasoning is straightforward: these types carry a real risk of progressing to complete block or sudden cardiac arrest, and a pacemaker eliminates that risk.
For milder forms of AV block, a pacemaker is generally considered only when symptoms clearly correlate with the rhythm abnormality. Living with a pacemaker is a significant but manageable adjustment. Modern devices are small, implanted in a minor procedure, and typically last 10 to 15 years before the battery needs replacing. Most people return to normal activity within a few weeks.