Complete heart block happens when electrical signals from the upper chambers of the heart completely fail to reach the lower chambers. The most common causes are age-related wear on the heart’s wiring system and damage from heart attacks, though infections, medications, autoimmune conditions, and heart surgery can also trigger it. Understanding the specific cause matters because some forms are reversible while others require a permanent pacemaker.
How the Heart’s Electrical System Breaks Down
Your heart has a built-in pacemaker called the SA node, which fires electrical signals to make the upper chambers (atria) contract. Those signals travel through a relay station called the AV node before reaching the lower chambers (ventricles) through a network of specialized fibers. This brief delay at the AV node ensures the upper chambers finish squeezing blood downward before the lower chambers contract.
In complete heart block, that relay is entirely severed. The SA node keeps firing normally, but none of its signals reach the ventricles. To keep you alive, the ventricles activate a backup rhythm on their own, typically beating at only 30 to 40 beats per minute compared to the normal 60 to 100. This backup rhythm is slower and less reliable, which is why complete heart block can cause fainting, fatigue, and in some cases cardiac arrest.
Age-Related Fibrosis of the Conduction System
The single most common cause of complete heart block in older adults is gradual scarring and stiffening of the heart’s electrical wiring. First described in 1964 by cardiologist Maurice Lev, this process involves fatty and fibrous tissue slowly replacing the specialized conduction fibers. It typically begins around the fourth decade of life and worsens over years or decades. The scarring targets the bundle of His and the Purkinje fibers, the very pathways that carry signals from the AV node into the ventricles.
Because this degeneration is gradual, patients often progress through milder forms of heart block before reaching the complete form. An older adult who has been told they have a “prolonged PR interval” on an EKG may be showing early signs of this same process.
Heart Attacks
A heart attack can damage the conduction system directly by cutting off its blood supply. About 19% of patients with an inferior wall heart attack (affecting the bottom of the heart) develop a high-grade block, and in two-thirds of those cases, the block appears within the first 24 hours. The AV node gets its blood from an artery that also supplies the inferior wall, so blockages in that artery can knock out conduction and heart muscle at the same time.
Heart block caused by an inferior heart attack often resolves on its own as blood flow is restored, because the AV node itself is the site of the problem and it has a relatively good capacity to recover. When a heart attack hits the front wall of the heart instead, the block tends to occur lower in the conduction system and carries a worse prognosis, frequently requiring a permanent pacemaker.
Medications That Slow Conduction
Several widely prescribed drug classes can push a borderline conduction system into complete heart block. The main offenders are:
- Beta-blockers (used for high blood pressure, heart failure, and anxiety)
- Calcium channel blockers like diltiazem and verapamil (used for blood pressure and heart rhythm problems)
- Digoxin (used for heart failure and atrial fibrillation)
- Amiodarone and other antiarrhythmics (used to control irregular heart rhythms)
The risk is highest when these drugs are combined. One well-documented pattern is digoxin paired with a beta-blocker, where each drug independently slows AV conduction and together they can stop it entirely. Certain antidepressants, the Alzheimer’s medication donepezil, and some antibiotics have also been linked to complete heart block in vulnerable patients. The good news is that drug-induced heart block is usually reversible once the offending medication is stopped or its dose is adjusted.
Infections: Lyme Disease and Myocarditis
Lyme disease is the most important infectious cause of complete heart block in younger, otherwise healthy adults. The bacterium that causes Lyme disease can invade the heart’s conduction tissue, creating inflammation that disrupts signaling. In a review of over 100 cases of Lyme carditis, 49% presented with complete heart block. The encouraging part is that this form of heart block typically reverses completely with antibiotic treatment, often progressing back through milder degrees of block before normal conduction returns.
Other infections that inflame the heart muscle (myocarditis) can also cause complete heart block. Viral infections are the most common culprit, though bacterial, fungal, and parasitic infections have all been reported. Chagas disease, caused by a parasite found primarily in Latin America, is a notable cause of irreversible conduction damage.
Autoimmune Conditions
Neonatal Lupus
Babies can be born with complete heart block if their mother carries certain antibodies associated with lupus or Sjögren’s syndrome, specifically anti-Ro and anti-La antibodies. These antibodies cross the placenta during pregnancy and attack proteins on the surface of fetal heart cells that are undergoing normal programmed cell death. What should be a quiet recycling process instead triggers inflammation, attracting immune cells that release signals causing scar tissue to form in the conduction system. The damage is permanent. The scarring, calcification, and tissue death in the fetal conduction system cannot be reversed after birth, and many of these children need a pacemaker.
Cardiac Sarcoidosis
Sarcoidosis is an inflammatory disease that forms clusters of immune cells called granulomas in various organs. When it affects the heart, complete heart block is one of its hallmark presentations and is considered a major diagnostic criterion across multiple international guidelines. Cardiac sarcoidosis should be suspected when a younger adult develops unexplained heart block, particularly if they have a history of sarcoidosis in the lungs, skin, or eyes. The peak age at diagnosis is around 60, with about two-thirds of patients being women.
Heart Surgery and Valve Procedures
Any surgery near the heart’s conduction pathways carries a risk of complete heart block. The AV node and bundle of His sit close to the aortic and mitral valves, making valve replacement surgery a common setting for this complication. Transcatheter aortic valve replacement (TAVR), a less invasive procedure that threads a new valve into the heart through a blood vessel, causes complete heart block in roughly 10% of cases. A study of over 35,500 TAVR procedures found the rate actually increased from 8.4% to 11.8% over a two-year period as the procedure became more widely used.
Surgical repair of congenital heart defects in children, particularly those involving the septum between the chambers, also carries a risk of conduction damage. In many post-surgical cases, the block is permanent and requires a pacemaker before the patient leaves the hospital.
How Complete Heart Block Is Identified
An EKG is the primary tool. The signature finding is “AV dissociation,” meaning the electrical signals from the atria and the ventricles are marching to completely independent rhythms. On the EKG tracing, the atrial signals (P waves) appear at a normal rate of 60 to 100 per minute, while the ventricular signals (QRS complexes) are much slower, typically 30 to 40 per minute. Crucially, there is no consistent timing relationship between the two. The atria fire on their own schedule and the ventricles fire on theirs.
Once complete heart block is confirmed on EKG, identifying the cause determines what happens next. Blood tests can check for Lyme disease, inflammatory markers, or signs of a heart attack. Cardiac imaging can look for sarcoidosis or structural damage. A medication review may reveal a drug-related cause. If the block occurs below the AV node in the specialized conduction fibers, a permanent pacemaker is generally recommended even if the patient has no symptoms, because these lower blocks carry a risk of sudden, unpredictable loss of the backup rhythm.
Reversible Versus Permanent Causes
The distinction that matters most is whether the cause can be fixed. Drug-induced heart block usually resolves when the medication is stopped. Lyme carditis responds well to antibiotics. Heart block from an inferior heart attack often recovers within days as the injured tissue heals. In these situations, temporary pacing can bridge the gap while the underlying cause is treated.
Age-related fibrosis, damage from an anterior heart attack, post-surgical injury, congenital autoimmune block, and advanced cardiac sarcoidosis are generally irreversible. For these causes, a permanent pacemaker is the standard treatment. Current guidelines recommend permanent pacing for any complete heart block that occurs below the AV node, regardless of symptoms, because of the risk of the backup rhythm suddenly failing.