Atrioventricular block, often shortened to AV block or heart block, is a disruption in the electrical signaling between the upper chambers of the heart (the atria) and the lower chambers (the ventricles). Normally, every heartbeat starts as an electrical impulse in the atria that travels through a specialized gateway called the AV node before reaching the ventricles and triggering their contraction. When that relay is slowed or interrupted, you get AV block, and the severity ranges from a barely noticeable delay on an electrocardiogram to a life-threatening disconnect where the ventricles beat on their own at dangerously slow rates. The condition can be benign, requiring nothing more than occasional monitoring, or it can demand an emergency pacemaker within hours.
How the AV Node Creates a Deliberate Pause
The AV node is not simply a wire between the atria and ventricles. It is a small cluster of specialized cells that intentionally slows conduction, creating the brief pause between atrial and ventricular contraction that lets the ventricles fill with blood before squeezing. Research on AV node physiology shows that the compact “N zone” within the node is the main source of this delay, accounting for the majority of conduction time from atrium to His bundle.1PubMed. Morphology and electrophysiology of the mammalian atrioventricular node When disease, drugs, or structural damage disrupts any point in this relay system, the result is some degree of AV block.
First-Degree AV Block
First-degree AV block is the mildest form. Every electrical impulse still gets through from the atria to the ventricles, but each one takes longer than normal. On an ECG, this shows up as a PR interval longer than 200 milliseconds. Most people with first-degree block feel nothing and discover it incidentally during a routine exam.
Whether first-degree block is truly harmless has been debated. A large community-based study found that people with first-degree block had roughly double the adjusted risk of developing atrial fibrillation and about triple the risk of eventually needing a pacemaker compared to those with normal conduction.2JAMA. Long-term Outcomes in Individuals With Prolonged PR Interval or First-Degree Atrioventricular Block However, a separate population study from Europe found that a prolonged PR interval was not associated with increased mortality or hospitalizations for coronary artery disease, heart failure, atrial fibrillation, or stroke after adjusting for age and sex.3European Heart Journal. Prognostic significance of prolonged PR interval in the general population The discrepancy likely reflects differences in the study populations and how thoroughly other risk factors were accounted for. The practical takeaway is that first-degree AV block alone, in an otherwise healthy person, does not require treatment, but it is worth mentioning to your doctor so they can keep an eye on it over time, particularly if you develop symptoms like fatigue or lightheadedness.
Second-Degree AV Block
Second-degree block means some impulses get through and some don’t. There are two classic patterns, and the distinction between them matters enormously for treatment.
Mobitz Type I (Wenckebach)
In Mobitz type I, the delay between atria and ventricles gets progressively longer with each beat until one impulse is finally dropped entirely. Then the cycle resets. This pattern usually reflects a problem at the level of the AV node itself, which tends to be a relatively benign location for block. In young, healthy people and trained athletes, type I block often has no clinical significance and may simply be a sign of high vagal tone (the body’s rest-and-digest nervous system putting the brakes on the heart during sleep or relaxation).
In children and adolescents, Wenckebach block has been observed to sometimes progress over years. A long-term follow-up study of pediatric patients with this rhythm found that some eventually developed complete heart block, while others remained stable with only first-degree block.4ScienceDirect (The American Journal of Cardiology). Pediatric cardiology Wenckebach atrioventricular block (Mobitz type I) in children and adolescents So even in this “benign” category, ongoing follow-up matters.
Mobitz Type II
Mobitz type II is a different animal. Here, the PR interval stays constant from beat to beat, but then a ventricular beat is suddenly and unpredictably dropped. This pattern almost always means the problem is below the AV node, in the His-Purkinje system, the network of fibers that distributes the electrical signal to the ventricle walls. That location carries a high risk of progressing to complete heart block without warning.
A correctly identified Mobitz type II block is considered a medical emergency. The risk of sudden prolonged asystole (the heart simply stopping for seconds at a time) or dangerous rhythm disturbances makes pacemaker implantation necessary regardless of whether the patient feels symptomatic at the moment.5PubMed Central. Mobitz type II second-degree atrioventricular block: a commonly overdiagnosed and misinterpreted arrhythmia That said, this same review emphasizes that Mobitz type II is frequently misdiagnosed, often confused with type I or other rhythms that look similar on an ECG strip but carry a very different prognosis. Getting the diagnosis right is critical because it determines whether someone needs immediate pacemaker implantation or can safely be observed.
Third-Degree (Complete) AV Block
In third-degree block, no electrical impulses cross from the atria to the ventricles at all. The ventricles are left to beat on their own using a backup pacemaker, which is much slower, typically 30 to 40 beats per minute. Experimental studies of complete heart block show what happens to the heart mechanically: cardiac output drops immediately, stroke volume increases as the ventricles fill for longer, and diastolic blood pressure falls.6The American Journal of Cardiology. Experimental study Cardiodynamics of complete heart block Over months, the heart compensates by enlarging and thickening its walls, but this remodeling eventually fails, leading to heart failure if the block is not corrected with pacing.
Symptoms of complete block can range from fatigue and exercise intolerance to fainting spells and cardiac arrest. The clinical urgency depends on the escape rhythm: if the backup pacemaker fires from a point high in the conduction system, the heart rate may be adequate at rest, but if the escape originates low in the ventricles, the rate is unreliable and the risk of sudden death is high.
What Causes AV Block
The causes of AV block span a wide range, from aging-related wear and tear to acute infections and surgical complications.
Age-Related Degeneration
The most common cause in older adults is progressive fibrosis (scarring) of the conduction system, sometimes called Lenègre’s disease or Lev’s disease. Over decades, the specialized conducting fibers are gradually replaced by fibrous tissue. Mouse models of this process show that mutations in the SCN5A gene, which encodes a key sodium channel, lead to age-dependent conduction slowing and extensive fibrosis in the ventricles, closely mimicking what is seen in humans.7PubMed. Mouse model of SCN5A-linked hereditary Lenègre’s disease: age-related conduction slowing and myocardial fibrosis
Infections
Lyme disease is a well-recognized infectious cause. Carditis occurs in roughly 4 to 10 percent of Lyme disease cases, usually appearing a few weeks after the initial tick bite, and can cause varying degrees of AV block, sometimes severe enough to cause prolonged pauses in heartbeat.8PubMed. Lyme carditis: an important cause of reversible heart block The encouraging part is that Lyme-related heart block is typically reversible with antibiotic treatment. Chagas disease, caused by the parasite Trypanosoma cruzi, is another important infectious cause, particularly in Central and South America, where it can cause chronic, irreversible conduction damage.
Heart Attack
AV block can complicate a heart attack, particularly when the blood supply to the AV node or the conduction system is disrupted. Heart block is more common with inferior (bottom-wall) heart attacks than anterior (front-wall) heart attacks, occurring in about 7 percent of inferior infarctions compared to about 3 percent of anterior ones.9PubMed Central. Acute myocardial infarction complicated by heart block in the elderly: prevalence and outcomes When heart block develops during an infarction, in-hospital mortality risk increases regardless of the infarction’s location. Block caused by an inferior heart attack tends to be at the AV node level and often resolves as the heart heals, whereas block from an anterior infarction typically involves the His-Purkinje system and is more likely to be permanent.
Medications
Several common cardiac drugs can slow AV conduction: beta-blockers, certain calcium channel blockers, digoxin, and some antiarrhythmic agents. For years, the assumption was simple: stop the drug, and the block resolves. Reality is more complicated. While drug discontinuation did lead to resolution of AV block in about 72 percent of cases in one review, roughly a quarter of those patients later had the block come back even though they stayed off the medication.10PubMed. Management of patients with drug-induced atrioventricular block The same study found that the recurrence pattern varied by drug: carvedilol-related blocks rarely recurred after stopping the drug, while metoprolol-related blocks frequently persisted or came back.
This raises a question cardiologists increasingly grapple with: did the drug cause the block, or did the drug simply unmask conduction disease that was already developing? A growing body of evidence suggests that truly drug-caused AV block is rare, and that many cases attributed to medications actually reflect underlying conduction system disease that the drug pushed over the edge.11PubMed Central. Clinical Significance and Management of Atrioventricular Block Associated With Bradycardic/Antiarrhythmic Drug Therapy: Drug-Induced or Drug-Revealed? This distinction matters: if the block was merely revealed rather than caused, stopping the drug may not be enough to prevent progression.
AV Block in Athletes
Endurance athletes frequently have ECG findings that would be considered abnormal in a sedentary person, including first-degree AV block and Mobitz type I. Years of intense training ramp up vagal tone, which slows the heart rate and AV conduction at rest. A study of athletes with baseline AV conduction abnormalities found that in all eight athletes tested, the abnormalities disappeared after pharmacological blockade of the autonomic nervous system, confirming a functional vagal mechanism rather than structural disease.12PubMed Central. Intrinsic Versus Extrinsic Sinus and Atrioventricular Nodes Dysfunction in Athletes
The challenge arises when an athlete presents with more advanced block or symptoms like fainting. A case report of an endurance athlete with 2:1 AV block and a history of syncope illustrates the diagnostic gray zone: the resting ECG showed first-degree block, but continuous monitoring revealed more advanced block, raising doubt about whether the finding was physiological or pathological. In that case, an invasive electrophysiology study was needed to confirm that the block was occurring at the AV node level (physiological) rather than below it (pathological).13PubMed Central. A case report of profound atrioventricular block in an endurance athlete: how far do you go? The principle is that athletes with AV block should be evaluated to rule out structural conduction disease before the finding is dismissed as a normal adaptation.
Congenital and Autoimmune AV Block
Some babies are born with complete AV block. The most common cause is the transfer of maternal autoantibodies, particularly anti-SSA/Ro and anti-SSB/La antibodies, from a mother with an autoimmune condition like lupus or Sjögren syndrome to the developing fetus. Research into the mechanism suggests that these antibodies interfere with the normal clearance of dying heart cells during fetal development, causing inflammation and scarring that permanently damages the conduction system.14JCI Insight. Impaired clearance of apoptotic cardiocytes is linked to anti-SSA/Ro and -SSB/La antibodies in the pathogenesis of congenital heart block Affected infants often need a pacemaker in infancy or early childhood.
Structural congenital heart disease can also lead to AV block. Congenitally corrected transposition of the great arteries, a condition where the ventricles are anatomically swapped, carries a particular predisposition. In one series of adults with this condition, nine patients developed progressive AV block unrelated to any surgical intervention, suggesting the abnormal anatomy itself gradually damages the conduction pathway.15Journal of the American College of Cardiology. Congenitally corrected transposition of the great arteries in the adult: Functional status and complications
Genetic Forms of Conduction Disease
Familial forms of progressive conduction disease also exist. Mutations in the lamin A/C gene, which encodes proteins important for the structural integrity of cell nuclei, have been identified as causes of hereditary conduction system disease that progresses over time, often accompanied by dilated cardiomyopathy.16New England Journal of Medicine. Missense mutations in the rod domain of the lamin A/C gene as causes of dilated cardiomyopathy and conduction-system disease Affected families typically show a pattern of sinus node dysfunction, AV block, and atrial arrhythmias that worsen with each decade. Genetic testing can identify these mutations and guide screening of at-risk family members before symptoms appear.
Heart Block After Valve Replacement Procedures
One of the most common settings where new AV block arises today is after transcatheter aortic valve replacement (TAVR), a minimally invasive procedure to replace a diseased aortic valve. A 2014 meta-analysis of over 11,000 TAVR patients found that about 17 percent required a permanent pacemaker, with the strongest risk factors being pre-existing right bundle branch block and AV block that occurred during the procedure itself.17PubMed Central. Mechanisms of Heart Block after Transcatheter Aortic Valve Replacement The physical reason is straightforward: the conduction system runs close to the aortic valve annulus, and placing a prosthetic valve can compress or injure these fibers.
The most important modifiable factor is how deep the valve is implanted below the aortic annulus. Deeper implantation compresses more of the conduction system. Other factors include the size of the valve relative to the patient’s anatomy, the amount and location of calcium deposits on the native valve, and whether a self-expanding or balloon-expandable valve is used, with self-expanding designs historically associated with higher pacemaker rates.18PubMed Central. The incidence and impact of cardiac conduction disturbances after transcatheter aortic valve replacement Improving implantation technique with CT-based planning has helped reduce pacemaker rates in recent years, though the risk remains significant.
How AV Block Is Diagnosed Beyond the ECG
A standard 12-lead ECG is the first step and often sufficient for diagnosing the type and degree of block. Ambulatory monitoring (Holter monitors or longer-term event recorders) catches intermittent block that might not appear during a brief office visit. For cases where the level of block is uncertain, an electrophysiology study (EPS) can pinpoint exactly where conduction fails.
During an EPS, catheters are threaded into the heart to measure conduction intervals directly. A key measurement is the His-to-ventricle (HV) interval, which reflects conduction time below the AV node. An HV interval greater than 70 milliseconds is considered a nonspecific predictor of progression to high-grade block, while an interval exceeding 100 milliseconds is highly predictive but uncommon.19PubMed Central. Electrophysiological Testing for the Investigation of Bradycardias A systematic review of EPS in patients with unexplained fainting found that implantable cardiac monitors detected complete heart block more often during follow-up than surface ECGs did, at about 29 percent versus 7 percent, suggesting that intermittent block can be easy to miss without prolonged monitoring.20PubMed. Electrophysiology studies for predicting atrioventricular block in patients with syncope
EPS has also proven useful after TAVR. In patients who develop a new left bundle branch block after valve implantation, an HV interval greater than 55 milliseconds the day after the procedure predicted subsequent high-grade AV block with a negative predictive value of 90 percent, meaning patients with shorter intervals could be safely monitored rather than receiving a pacemaker immediately.21PubMed Central. Electrophysiology Testing to Stratify Patients With Left Bundle Branch Block After Transcatheter Aortic Valve Implantation
Pacemaker Therapy and Newer Pacing Strategies
For decades, the standard treatment for symptomatic or high-risk AV block has been a permanent pacemaker with a lead placed in the right ventricle. This approach is reliable and well-established, but pacing from the right ventricle creates an abnormal pattern of electrical activation that, over time, can weaken the heart muscle. More than one in ten patients with chronic right ventricular pacing develop pacing-induced cardiomyopathy, a weakening of the heart’s pumping function caused by the pacing itself.22Heart Rhythm. Characterising Pacing-Induced Cardiomyopathy: A Systematic Review and Meta-Analysis Risk factors include a lower baseline heart function, a higher percentage of time spent being paced, and wider paced QRS complexes on the ECG.23PubMed Central. Pacing-induced cardiomyopathy in chronic right ventricular apical pacing: a midterm follow-up study
Conduction system pacing (CSP) is a newer approach that places the pacing lead directly on the heart’s natural wiring, either the His bundle or the left bundle branch area. The goal is to activate the ventricles through their own conduction fibers, preserving the normal squeeze pattern and avoiding the mechanical disadvantage of right ventricular pacing. Two recent randomized trials provide encouraging results. The CSPACE trial found that CSP was associated with significantly lower rates of pacing-induced cardiomyopathy compared to conventional right ventricular septal pacing, with about 5 events per 100 person-years versus roughly 15.24PubMed. Clinical Outcomes of Conduction System Pacing vs Right Ventricular Septal Pacing in Atrioventricular Block: The CSPACE Randomized Controlled Trial The Prague CSP trial similarly found that CSP preserved heart function better, with a smaller decline in ejection fraction and fewer patients experiencing large drops in pumping ability.25PubMed. Prospective randomized trial of conduction system pacing vs right ventricular pacing for patients with atrioventricular block; Prague CSP trial Neither trial showed a significant difference in mortality, but the follow-up periods were relatively short, and preserving heart function is expected to pay dividends over years of pacing.
Smartwatches and Early Detection
Consumer wearable devices are increasingly capable of detecting heart rhythm abnormalities, including AV block. A case report described a patient who had undergone TAVR and was recovering at home during the COVID-19 pandemic when a smartwatch ECG captured advanced AV block, prompting an emergency room visit that confirmed the finding and led to pacemaker implantation.26European Heart Journal – Digital Health. Smartwatch detected high-degree atrioventricular block requiring pacemaker implantation after transcatheter aortic valve replacement during COVID-19 A study comparing smartwatch ECG interpretation with standard 12-lead ECGs found that smartwatches improved the ability to distinguish AV block from simple sinus bradycardia, raising correct classification from about 81 percent to 95 percent.27PubMed. Beyond the wrist: Using a smartwatch electrocardiogram to detect electrocardiographic abnormalities
These devices are not substitutes for clinical-grade monitoring. A single-lead wrist recording cannot distinguish Mobitz type I from type II, or determine the level of block within the conduction system, the sort of distinctions that drive treatment decisions. But as a screening tool, especially for people recovering from cardiac procedures or those with known borderline conduction disease, smartwatches add a layer of surveillance that simply did not exist a few years ago. The rhythm strip they produce can be valuable information to bring to your cardiologist, even if it cannot answer every question on its own.