Heart block is a condition where electrical signals that control your heartbeat are delayed or completely stopped as they travel from the upper chambers of your heart to the lower chambers. It ranges from a minor timing delay that causes no symptoms to a complete breakdown in signaling that can slow your heart rate below 40 beats per minute and cause fainting. Heart block is classified into three degrees, each with different implications for your health.
How Your Heart’s Electrical System Works
Your heartbeat starts with a tiny cluster of cells called the sinoatrial node, located in the upper right chamber of your heart. This natural pacemaker fires an electrical impulse that spreads across both upper chambers (atria), causing them to contract and push blood into the lower chambers (ventricles). The signal then funnels through a second checkpoint called the atrioventricular node before traveling down a pathway called the His bundle, which splits into right and left branches that fan out across the ventricles through a network of fine fibers. This triggers the ventricles to contract and pump blood to your lungs and body.
Heart block occurs when something disrupts this relay, specifically at or below the atrioventricular node. The signal might slow down, drop out intermittently, or stop reaching the ventricles entirely. The degree of disruption determines the type and severity.
First-Degree Heart Block
In first-degree heart block, every electrical signal still reaches the ventricles, but each one takes longer than normal. On an electrocardiogram (EKG), this shows up as a PR interval longer than 200 milliseconds, the stretch of time between the electrical impulse firing in the atria and arriving at the ventricles. Under normal conditions, this interval is shorter.
First-degree heart block rarely causes symptoms. Many people have it and never know. It can show up as an incidental finding on a routine EKG and typically requires no treatment. It is common in athletes and older adults, and by itself it is not dangerous.
Second-Degree Heart Block
In second-degree heart block, some electrical signals make it through to the ventricles and some don’t, meaning your heart occasionally skips a beat. There are two distinct patterns, and the difference between them matters a lot.
Type I (Wenckebach)
In Type I, the electrical delay gets progressively longer with each heartbeat until one signal fails to reach the ventricles entirely. Then the cycle resets and starts over. This pattern is often temporary and can occur during sleep or in well-conditioned athletes. It rarely causes significant symptoms and usually doesn’t need treatment unless you’re fainting or feeling lightheaded regularly.
Type II (Mobitz Type II)
Type II is more serious. Signals drop without any progressive delay beforehand, making the skipped beats unpredictable. This type frequently produces a 2:1 pattern, where only every other electrical signal reaches the ventricles, effectively halving your heart rate. Type II carries a real risk of progressing to complete heart block, and current guidelines from the American College of Cardiology and American Heart Association recommend a permanent pacemaker for this type regardless of whether you have symptoms.
Third-Degree (Complete) Heart Block
In third-degree heart block, no electrical signals pass from the atria to the ventricles at all. The upper and lower chambers of your heart beat completely independently of each other. The ventricles still contract, but only because backup pacemaker cells lower in the heart fire on their own at a much slower rate, typically below 40 beats per minute.
At rates that low, many people experience dizziness, fainting, chest pain, extreme fatigue, and shortness of breath. Some tolerate rates around 40 beats per minute surprisingly well, but below that, symptoms become likely and can include seizures from reduced blood flow to the brain. Complete heart block requires a permanent pacemaker in nearly all cases when the cause isn’t reversible.
Common Causes
Heart block can be either something you’re born with or something that develops over time. The causes differ significantly between the two.
Congenital heart block is rare, occurring in roughly 1 in 12,500 to 20,000 live births. The leading cause is neonatal lupus, which accounts for 80% to 95% of cases diagnosed in the first 28 days of life. This happens when a mother with certain autoimmune antibodies (anti-Ro/SSA or anti-La/SSB) passes them to the fetus during pregnancy. These antibodies can damage the developing conduction system. The mother may not even know she carries these antibodies, as many have no lupus symptoms themselves.
Acquired heart block, the far more common type, develops later in life. Aging is the single biggest factor: years of wear and gradual fibrosis of the conduction tissue can slow or block signals. Other causes include coronary artery disease, heart attacks (especially those affecting the bottom wall of the heart), infections like Lyme disease, inflammatory conditions like sarcoidosis, and certain medications that slow conduction through the atrioventricular node, particularly beta-blockers, calcium channel blockers, and some heart rhythm drugs. Surgical procedures on the heart, especially valve replacements, can also injure the conduction pathway.
Symptoms by Severity
First-degree heart block almost never produces symptoms you’d notice. Second-degree Type I may cause occasional lightheadedness or a fluttering sensation, but many people feel nothing at all.
More advanced forms of heart block, particularly Type II and complete block, produce symptoms tied to a slow heart rate. These include:
- Dizziness or near-fainting, especially when standing up or exerting yourself
- Fainting (syncope), which can happen without warning
- Fatigue that doesn’t improve with rest
- Shortness of breath during activities that previously felt easy
- Chest pain, particularly if your heart isn’t pumping enough blood to meet its own oxygen needs
- Heart palpitations, a feeling that your heart is pounding, racing, or skipping
Symptoms can come and go if the block is intermittent, which makes it tricky to catch. Some people feel fine for weeks, then experience sudden episodes of dizziness or fainting when the block worsens temporarily.
How Heart Block Is Diagnosed
A standard 12-lead EKG is the first tool used and can identify heart block if it’s happening at the moment of the test. The characteristic patterns, a prolonged PR interval, dropped beats, or complete dissociation between atrial and ventricular signals, are visible on the tracing. But heart block can be intermittent, and a 10-second EKG snapshot may miss it entirely.
When heart block is suspected but not caught on a standard EKG, longer monitoring fills the gap. A Holter monitor records your heart rhythm continuously for 24 to 48 hours, though even this window catches the problem only about 15 to 28% of the time if the episodes are infrequent. For less frequent symptoms, external loop recorders can monitor for up to 30 days. These devices continuously analyze your rhythm and automatically save recordings when they detect something abnormal, even capturing the electrical activity from a few minutes before the event started.
If episodes are rare, spaced months apart, an implantable loop recorder can be placed just beneath the skin of your chest in a quick procedure. These tiny devices monitor continuously for up to three years and don’t require you to do anything when symptoms occur, which is especially useful if fainting prevents you from pressing a button on an external device.
Treatment and Pacemakers
Treatment depends entirely on the type and cause. First-degree heart block and second-degree Type I rarely need intervention. If a medication is slowing your conduction, adjusting or stopping it may resolve the problem. Infections like Lyme disease can cause temporary heart block that clears with antibiotic treatment.
For second-degree Type II and third-degree heart block, a permanent pacemaker is the standard treatment when the cause isn’t reversible. A pacemaker is a small device implanted under the skin near your collarbone, with one or two thin wires threaded through a vein into your heart. It monitors your rhythm continuously and delivers a tiny electrical pulse to trigger a heartbeat whenever your 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.
Modern pacemakers last 10 to 15 years before the battery needs replacing, and they’re checked periodically, often remotely from home using a bedside transmitter. After the initial recovery period of a few weeks, during which you’ll avoid raising your arm above your shoulder on the implant side, most people return to normal activities with no significant restrictions. The improvement in symptoms, particularly fatigue, dizziness, and fainting, is often dramatic and immediate.