Heart conduction problems happen when the electrical signals that coordinate each heartbeat are delayed, disrupted, or completely blocked somewhere along their path. The causes range from natural aging and reduced blood flow to medications, infections, and inherited genetic conditions. Understanding what went wrong, and where, helps explain why some conduction problems are temporary and others require a permanent pacemaker.
How Your Heart’s Electrical System Works
Every heartbeat starts with a tiny burst of electricity in the sinoatrial (SA) node, a cluster of specialized cells in the upper right chamber of your heart. This node acts as your heart’s natural pacemaker, firing signals that tell the upper chambers to contract and push blood downward. The signal then reaches the atrioventricular (AV) node, which briefly pauses it, giving the upper chambers time to fully empty before the lower chambers take over.
From the AV node, the signal travels through a highway of nerve fibers called the bundle of His, which splits into left and right branches running down the wall between the lower chambers. These branches fan out into a web of tiny Purkinje fibers that deliver the signal across the entire lower heart, triggering the powerful contraction that sends blood to your lungs and body. A conduction problem can develop at any point along this chain, and where it occurs determines how it affects your heart rhythm and how serious it is.
Aging and Fibrosis
The most common cause of conduction problems is simply getting older. Over decades, the heart gradually accumulates scar-like tissue made of collagen, a stiff structural protein. In healthy aging, the amount of collagen in the heart can increase by up to 200% compared to younger hearts. This collagen also becomes more cross-linked and rigid over time, partly driven by compounds that accumulate with age, making it harder for electrical signals to pass through the tissue normally.
This process, called fibrosis, slows the speed at which signals travel and disrupts the normally smooth, uniform pattern of electrical conduction. Fibrosis is considered a major driver of age-related rhythm problems, including atrial fibrillation. Research shows that both the buildup of collagen and the loss of gap-junction proteins (the molecular connectors that let electrical current flow between heart cells) are needed to meaningfully reduce conduction speed. Calcium deposits can also form on the heart’s valves and nearby structures with age, adding another layer of stiffness that may interfere with conduction pathways close to the valve rings.
Heart Attacks and Blocked Arteries
The heart’s conduction system has its own blood supply, and a heart attack can cut it off. The SA node gets its blood from the right coronary artery in about two-thirds of people and from the left circumflex artery in roughly one-fifth. When the artery feeding the SA node is blocked, the result can be a dangerously slow heart rate, pauses in the heartbeat, or even a brief period where the heart stops firing altogether.
The AV node is similarly vulnerable. Its blood supply branches off near the bottom of the right coronary artery in most people, which is why AV block is a relatively common complication of heart attacks affecting the heart’s lower wall. Beyond direct blood-flow loss, a blockage in the right coronary artery also triggers the release of acetylcholine, a chemical that further slows conduction through the AV node.
Heart attacks in the front wall of the heart, caused by blockages in the left anterior descending artery, can damage the bundle branches instead. A blockage high up in that artery can knock out the right bundle branch and the left front fascicle simultaneously, because they share the same blood supply from the first major branch. Damage to the left bundle branch as a whole typically requires severe disease in two or three coronary arteries, because its front and back portions are fed by different vessels.
Medications That Slow Conduction
Several widely prescribed drug classes can slow or block electrical conduction through the heart, particularly at the AV node. The most common culprits are beta-blockers, certain calcium channel blockers (the types used for heart rate control rather than blood pressure), and digoxin. All three slow signal transmission through the AV node, and at normal doses, they generally leave the rest of the conduction system alone.
Antiarrhythmic drugs designed to treat abnormal rhythms can also cause conduction problems, particularly in people who already have subtle, undetected disease in their conduction system. Some of these medications slow conduction below the AV node, through the bundle branches, and can unmask underlying problems that weren’t visible before the drug was started. In many cases, the conduction block reverses when the medication is stopped or reduced, which is why drug-related conduction problems are often considered reversible.
Infections: Lyme Disease and Beyond
Lyme disease is the most well-known infectious cause of heart conduction problems. When the Lyme-causing bacterium reaches the heart, it colonizes cardiac tissue and triggers an outsized immune response. The immune system can also produce antibodies that mistakenly attack heart tissue through a process called molecular mimicry, where bacterial proteins resemble cardiac proteins closely enough to confuse the immune system.
The result is Lyme carditis, and AV block accounts for about 90% of its cardiac effects. High-grade block, where most or all signals between the upper and lower chambers are blocked, makes up more than two-thirds of those cases. The good news is that Lyme carditis usually resolves with antibiotic treatment and rarely requires a permanent pacemaker. Other infections, including Chagas disease and certain viral infections, can cause similar conduction damage through direct invasion of heart tissue or inflammation.
Infiltrative Diseases
Some diseases cause abnormal substances to build up inside the heart muscle, physically disrupting the conduction pathways. Cardiac amyloidosis is the most common of these infiltrative conditions. In amyloidosis, misfolded proteins deposit throughout the heart, including directly in the SA node, AV node, and bundle branches. This leads to progressive conduction block, with different types of amyloidosis tending to cause different patterns. One form is more likely to cause first-degree AV block (a mild delay), while another more often causes left bundle branch block or higher degrees of AV block.
Cardiac sarcoidosis works through a different mechanism: the immune system forms clusters of inflammatory cells called granulomas within the heart tissue. These granulomas can land anywhere in the conduction system, creating unpredictable blocks. Sarcoidosis is particularly concerning because conduction problems may be the first sign of the disease, appearing before any lung or skin symptoms.
Genetic and Congenital Causes
Some people are born with or develop conduction problems because of inherited genetic mutations. The best-characterized form, called hereditary progressive cardiac conduction defect, has been linked to mutations in genes including SCN5A, SCN1B, and TRPM4. These genes encode proteins that form or regulate the sodium channels responsible for generating and transmitting electrical signals in heart cells. When these channels don’t work properly, conduction slows progressively over a person’s lifetime.
When conduction disease runs in families alongside structural heart defects, mutations in a different set of genes are often involved, including NKX2-5 and TBX5, which play roles in how the heart forms during fetal development. In rare cases, congenital heart block develops before birth when antibodies from a mother with an autoimmune condition cross the placenta and damage the baby’s developing conduction system.
How Conduction Problems Are Classified
Doctors categorize conduction problems based on where the block occurs and how severely signals are disrupted. The PR interval on an electrocardiogram (ECG) measures how long it takes a signal to travel from the upper chambers through the AV node to the lower chambers. The normal range is 0.12 to 0.20 seconds.
In first-degree AV block, every signal still gets through, but it takes longer than 0.20 seconds. This is usually harmless on its own. When the PR interval stretches beyond 0.30 seconds, it’s considered “marked” first-degree block. Second-degree block means some signals make it through and others don’t, creating skipped beats. Third-degree (complete) block means no signals pass from the upper to lower chambers at all, and the lower chambers must rely on their own much slower backup rhythm to keep pumping.
Blocks can also occur in the bundle branches rather than the AV node. A right or left bundle branch block means signals are delayed reaching one side of the lower heart, causing that side to contract slightly out of sync. When blocks develop in multiple branches simultaneously (called bifascicular or trifascicular block), the risk of progressing to complete heart block increases.
When a Pacemaker Becomes Necessary
Not all conduction problems need treatment. First-degree AV block and isolated bundle branch blocks are often monitored without intervention. A permanent pacemaker is most clearly needed for complete third-degree AV block (with or without symptoms), symptomatic second-degree block, and cases where conduction block is triggered by exercise in the absence of a heart attack. Alternating bundle branch block, where the pattern switches between left and right, is also a strong indication because it suggests widespread disease throughout the conduction system.
For less clear-cut situations, the decision depends on whether symptoms like fainting, dizziness, or fatigue can be linked to the conduction problem. A person with a heart rate consistently below 40 beats per minute, for example, may be considered for a pacemaker even if the connection between their slow rate and their symptoms isn’t perfectly established. In cases caused by medications or treatable infections like Lyme disease, doctors typically address the underlying cause first and reserve pacemaker placement for problems that persist afterward.