Heart block is a condition where electrical signals that coordinate your heartbeat are delayed or partially blocked as they travel from the upper chambers of your heart to the lower chambers. In a healthy heart, each electrical impulse originates at the top, passes through a relay point called the AV node, then travels down a network of specialized fibers to trigger the lower chambers to pump blood. When any part of that pathway is damaged or disrupted, the result is heart block. It ranges from a minor timing delay you’d never notice to a complete disconnection that requires a pacemaker.
How Your Heart’s Electrical System Works
Your heart has its own built-in wiring. An electrical signal starts in a cluster of cells at the top of the heart, spreads across the upper chambers (atria) to make them contract, then funnels through a single gateway called the AV node. From there, the signal enters a cable-like structure called the Bundle of His, which splits into branches that carry the impulse deep into the walls of the lower chambers (ventricles). This system ensures the upper and lower chambers beat in a coordinated rhythm, with the atria squeezing first and the ventricles following a fraction of a second later.
Heart block happens when signals slow down or stop at some point along this route. The block can occur at the AV node itself, within the Bundle of His, or in the branching fibers below it. Where the disruption happens matters: blocks at the AV node tend to be more stable, while blocks lower in the system are more likely to cause dangerous rhythm problems because there are fewer backup pacemaker cells available to keep the ventricles beating.
First-Degree Heart Block
First-degree heart 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 prolonged PR interval, the gap between the electrical activity in the atria and the ventricles. Normally that interval is between 120 and 200 milliseconds. In first-degree block, it stretches beyond 200 milliseconds, and it’s considered “marked” when it exceeds 300 milliseconds.
Most people with first-degree heart block have no symptoms at all. It’s often discovered incidentally during a routine EKG. The delay can come from the AV node, the conduction fibers below it, or even the atrium itself. In many cases it’s benign and doesn’t require treatment, though it can sometimes be a sign of underlying heart disease or a medication side effect worth investigating.
Second-Degree Heart Block
In second-degree heart block, some electrical signals make it through to the ventricles and others don’t. There are two distinct patterns, and they carry very different levels of risk.
Mobitz Type I (Wenckebach)
In this pattern, each successive signal takes a little longer to pass through the AV node until one signal fails to get through entirely. Then the cycle resets. On a heart monitor, this creates a distinctive “grouped beating” pattern: a cluster of heartbeats, a pause, then another cluster. The block is typically located within the AV node, and the condition is generally not an emergency. Many people with Type I block are asymptomatic and may not need treatment at all.
Mobitz Type II
Type II is more serious. The timing between the atria and ventricles stays constant, but signals drop out unpredictably. The problem usually lies below the AV node, in the Bundle of His or the fibers branching from it. This is a critical distinction because when conduction fails at that lower level, there may be no reliable backup rhythm to keep the ventricles beating. The block can progress suddenly to a complete block, potentially causing prolonged pauses or dangerously slow heart rates without warning. People with Mobitz Type II block typically need a pacemaker and should not leave medical care without an intervention plan in place.
Third-Degree (Complete) Heart Block
Third-degree heart block means no electrical signals are getting through from the upper chambers to the lower chambers. The atria and ventricles beat completely independently of each other. The ventricles may continue to beat on their own using backup pacemaker cells, but these backup rhythms are significantly slower than normal, often producing severe bradycardia. The heart rate may drop low enough that the brain and organs don’t receive adequate blood flow, leading to fainting, extreme fatigue, or collapse. Complete heart block is a medical emergency that almost always requires permanent pacing.
Symptoms by Severity
First-degree heart block rarely produces any noticeable symptoms. You could have it for years without knowing. Second-degree block can go either way: Type I is often silent, while Type II may cause dizziness, lightheadedness, or episodes of near-fainting as the heart intermittently skips beats.
Third-degree block is where symptoms become hard to ignore. Severe slowing of the heart can cause:
- Syncope (fainting), sometimes called a Stokes-Adams attack, where the heart pauses long enough that blood flow to the brain drops suddenly
- Dizziness or lightheadedness that comes and goes
- Fatigue and exercise intolerance, because the heart can’t speed up to meet your body’s demands
- Shortness of breath, particularly with activity
- Chest pain or pressure in some cases
What Causes Heart Block
The most common cause in adults is gradual, age-related scarring of the heart’s conduction system. Over time, the specialized fibers that carry electrical signals can degenerate, much like wiring in an old house. This degenerative process accounts for many cases of heart block in older adults.
Beyond aging, a range of conditions and exposures can trigger heart block. Heart attacks are a major cause, particularly when the blood supply to the AV node or conduction fibers is disrupted. Coronary artery disease, cardiomyopathy, and heart valve diseases all increase risk. Inflammatory and systemic diseases like sarcoidosis, rheumatic heart disease, and Lyme disease can damage the conduction system. Obstructive sleep apnea and thyroid disease have also been linked to heart block.
Medications are a frequently overlooked trigger. Beta-blockers, calcium channel blockers, digoxin, certain antiarrhythmics, and even some antidepressants, sedatives, and muscle relaxants can slow electrical conduction enough to cause heart block. In these cases, the block may resolve once the medication is adjusted or stopped.
Some people are also born with heart block. An overactive vagus nerve, which naturally slows heart rate, can contribute. Congenital structural abnormalities of the heart are another cause. And in a small number of newborns, heart block develops before birth when certain antibodies from the mother cross the placenta. This autoimmune congenital heart block affects fewer than 2% of babies born to women who carry specific antibodies associated with conditions like lupus or Sjögren’s syndrome.
How Heart Block Is Diagnosed
A standard 12-lead EKG is the first and most important tool. It captures the electrical activity of the heart in real time and can reveal the telltale patterns of each type of block: a prolonged PR interval for first-degree, dropped beats for second-degree, or completely independent atrial and ventricular rhythms for third-degree.
The challenge is that heart block can be intermittent. If it comes and goes, a single EKG snapshot might look perfectly normal. For these cases, doctors use extended monitoring. A Holter monitor records your heart’s rhythm continuously for 24 to 48 hours. Event monitors and implantable loop recorders can track your rhythm over weeks or even months, which is especially useful when heart block causes occasional fainting episodes that are hard to catch in a clinic visit.
In more complex cases, an electrophysiology study may be used. This involves threading thin wires into the heart to measure exactly where in the conduction system the block is occurring. It’s particularly valuable for distinguishing between a block at the AV node (which tends to be more stable) and one in the lower conduction fibers (which is more likely to progress and need treatment).
Treatment and Pacemakers
Treatment depends entirely on the type and severity. First-degree heart block with no symptoms typically needs nothing more than monitoring. The same is often true for Mobitz Type I, as long as you feel fine and the block isn’t getting worse. If a medication is causing the block, adjusting the dose or switching to a different drug may be all that’s needed.
For Mobitz Type II and third-degree heart block, the standard treatment is a permanent pacemaker. This small device, implanted under the skin near your collarbone, sends electrical signals to your heart to keep it beating at a normal rate. Current guidelines recommend permanent pacing for anyone with symptomatic bradycardia from heart block that isn’t caused by a reversible factor. Pacing has been shown to improve quality of life in these patients and reduce the risk of death in those with complete heart block.
The type of pacemaker depends on your specific situation. A dual-chamber device, which paces both the atria and ventricles, is considered appropriate for third-degree block and advanced second-degree block. For people who can’t have traditional lead-based devices (for example, due to limited vein access), leadless pacemakers implanted directly inside the heart are an option. Newer pacing approaches aim to maintain the heart’s natural activation pattern rather than simply stimulating the lower chambers, which can be important for preserving long-term heart function, particularly if you’ll need pacing most of the time.
Living with a pacemaker is straightforward for most people. The device is checked periodically, batteries last years before needing replacement, and most daily activities, including exercise, can resume after a brief recovery period. The pacemaker doesn’t fix the underlying conduction problem, but it effectively takes over the job that the damaged wiring can no longer do reliably.