What Leads To What Heart Block?
Understand the causes of different types of heart block, how heart attacks, infections, medications, and procedures lead to specific blocks, and when treatment or pacemakers are needed.
Written by Dr. J T Hema Pratima
Reviewed by Dr. Rohinipriyanka Pondugula MBBS
Last updated on 25th Oct, 2025
Introduction
If you’ve heard the term “heart block” and wondered what it really means, you’re not alone. Heart block sounds scary—but it simply describes a slowdown or interruption in the heart’s electrical wiring. Some forms are harmless and need only monitoring. Others can cause dizziness, fainting, or even be life threatening and require urgent care. The confusing part is that many different problems—heart attacks, medications, infections like Lyme disease, even routine procedures—can lead to different types of heart block.
In this guide, we’ll make it simple. We’ll explain how the heart’s electrical system works, break down the types of heart block you might see on an ECG, and map which causes typically lead to which kind of heart block. You’ll learn how a heart attack can cause one pattern and why an infection might cause another, what symptoms to watch for, how doctors diagnose the exact cause, and the treatments that help—ranging from stopping a drug to placing a pacemaker. If symptoms persist beyond two weeks or you faint, consult a doctor online with Apollo 24|7 for further evaluation.
Heart Block, Simply Explained
Here’s what you need to know:
Heart block means the electrical signals that tell your heart to beat are delayed or blocked. Normally, a signal starts in the top chamber (atria), passes through the atrioventricular (AV) node, and travels down specialised wiring to the bottom chambers (ventricles). In heart block, that signal is slowed (a delay) or stopped (a drop). The effect depends on how severe the slowdown is and where it occurs.
• Mild delays (first degree AV block) often cause no symptoms and are frequently discovered by chance on an ECG.
• Partial blocks (second degree) may cause intermittent missed beats, leading to dizziness, fatigue, or near fainting.
• Complete block (third degree) prevents signals from reaching the ventricles; a backup “escape” rhythm may be very slow, causing fainting or shortness of breath and requiring urgent treatment.
It’s also important to distinguish AV block from bundle branch block. AV block happens at or near the AV node (between atria and ventricles). Bundle branch blocks happen lower down in the wiring to the ventricles. Both are types of “heart block” in everyday language, but they carry different causes and implications. Understanding this difference is the key to answering “what leads to what heart block.”
How the Heart’s Electrical System Works?
Here’s what you need to know:
Think of your heart as a house wired with a main breaker and two major cables. The sinoatrial (SA) node is the natural pacemaker in the right atrium, setting the rate. Electricity travels across the atria to the AV node—the “gatekeeper.” The AV node briefly slows the signal (to let the ventricles fill with blood), then the signal passes into the His bundle, which splits into right and left bundle branches and finer Purkinje fibres to activate the ventricles.
Where can heart block occur?
• At the AV node (AV nodal block): typically influenced by the autonomic nervous system (vagal tone), medications like beta blockers and diltiazem, or inferior heart attacks. These blocks are often transient and sometimes benign (e.g., Mobitz I).
• Below the AV node (infranodal block: His bundle, bundle branches): more serious because the backup systems are fewer and slower. These are often due to age related scarring, anterior heart attacks, or infiltrative diseases; they carry a higher risk of progressing to complete heart block.
This wiring map becomes your mental model for linking causes to types. AV nodal insults tend to produce PR prolongation or Mobitz I. Infranodal injury tends to cause Mobitz II, bundle branch block, bifascicular patterns, or complete heart block with a wide QRS.
Types of Heart Block at a Glance
Here’s what you need to know:
AV blocks
• First degree AV block: The PR interval is prolonged (>200 ms), but every atrial beat reaches the ventricles. Often benign; common in athletes or with AV nodal slowing drugs.
• Second degree AV block:
o Mobitz I (Wenckebach): Gradual PR lengthening until a beat drops. Typically AV nodal. Often transient (e.g., sleep, vagal tone, inferior MI, medications).
o Mobitz II: Sudden dropped beats with a fixed PR. Usually infranodal (His–Purkinje). Higher risk and more likely to progress to complete block; pacemaker often needed.
• Third degree (complete) AV block: No atrial impulses conduct to the ventricles. An escape rhythm maintains life but is slow and unreliable; urgent pacing is common.
Bundle branch and fascicular blocks
• Right bundle branch block (RBBB): Electrical delay to the right ventricle. Can be incidental, but may occur with pulmonary embolism or congenital heart disease.
• Left bundle branch block (LBBB): Delay to the left ventricle. Often indicates underlying heart disease (hypertension, coronary disease, cardiomyopathy, aortic stenosis).
• Left anterior fascicular block (LAFB) and left posterior fascicular block (LPFB): Partial blocks in branches of the left bundle. May combine with RBBB (bifascicular block) and sometimes with PR prolongation (so called trifascicular pattern).
What Leads to Each Type: A Cause by Cause Map
Here’s what you need to know:
First degree AV block: Common and often benign
• Causes: High vagal tone (e.g., athletes, sleep), AV nodal slowing drugs (beta blockers, non dihydropyridine calcium channel blockers, digoxin), inferior MI (transient), myocarditis, and age related fibrosis.
• What it means: Usually no treatment if asymptomatic; review medications and electrolytes.
Mobitz I (Wenckebach): Often AV nodal and transient
• Causes: Inferior MI, increased vagal tone, sleep, medications that slow the AV node, sometimes in young, healthy people during rest.
• What it means: Often resolves with treating the trigger. Persistent symptomatic Wenckebach may require pacing, but that’s uncommon.
Mobitz II: Usually infranodal and higher risk
• Causes: Anterior MI, degenerative disease of the conduction system, cardiac surgery, infiltrative disease (amyloidosis, sarcoidosis), or severe hyperkalaemia.
• What it means: Higher risk of progression to complete heart block. Pacemaker is frequently indicated.
Complete (third degree) AV block: An emergency pattern
• Causes: Extensive anterior MI, advanced conduction disease with ageing, severe Lyme carditis, sarcoidosis, post operative injury, or drug toxicity.
• What it means: Often requires temporary pacing acutely and permanent pacemaker unless a reversible cause is identified and resolves.
Bundle branch blocks (RBBB, LBBB) and fascicular blocks
• RBBB: May be benign; also seen with right heart strain, congenital heart disease, or after procedures.
• LBBB: Often indicates underlying structural disease (hypertension, coronary disease, aortic stenosis, cardiomyopathy), or occurs after transcatheter aortic valve implantation.
• Bifascicular/trifascicular patterns: Suggest widespread conduction system disease; higher risk of progression, especially with syncope or Mobitz II.
Heart Attack Patterns: Inferior vs Anterior MI
Here’s what you need to know:
Heart attacks can disrupt the conduction system in different places, producing different heart block patterns—with very different outlooks.
Inferior MI
• Tends to affect the AV node itself (AV nodal ischemia).
• Often leads to first degree AV block or Mobitz I. High grade block can occur but is frequently transient.
• Prognosis: AV nodal block with inferior MI often resolves as the infarct stabilises. Temporary pacing may be needed if haemodynamically unstable, but a permanent pacemaker is less commonly required than in anterior MI.
Anterior MI
• More likely to injure the His–Purkinje system (below the AV node).
• More often causes Mobitz II or complete heart block with a wide QRS.
• Prognosis: Worse. Infranodal block signals larger myocardial damage, carries higher mortality, and often necessitates permanent pacing.
Infections and Inflammatory Causes
Here’s what you need to know:
Lyme carditis
• What it is: A complication of Lyme disease where the bacteria affect cardiac conduction tissue.
• What it leads to: Fluctuating levels of AV block, often high grade or complete; PR interval can swing rapidly over hours to days.
• Symptoms: Lightheadedness, palpitations, chest pain, shortness of breath, fainting—often in younger, outdoors exposed individuals in endemic areas.
• Outlook: With antibiotics, AV conduction typically recovers, often within days to a few weeks; temporary pacing may be needed in severe cases.
Sarcoidosis, myocarditis, infiltrative diseases
• Sarcoidosis can infiltrate the conduction system, causing any degree of AV block, often in younger adults without traditional risk factors. Cardiac MRI or PET may show inflammation; steroids and immunosuppression can help, and pacing/defibrillation may be indicated.
• Myocarditis (viral/autoimmune) may cause AV block or bundle branch block; severity ranges from mild to life threatening.
• Chagas disease (in endemic areas) and amyloidosis can also damage conduction tissue.
Medicines and Toxins That Slow Conduction
Here’s what you need to know:
AV nodal blockers
• Beta blockers, non dihydropyridine calcium channel blockers (diltiazem, verapamil), digoxin, and adenosine all slow AV conduction. Overdose, drug interactions, dehydration, or kidney dysfunction can amplify their effect and cause first degree block, Mobitz I, or rarely high grade block in predisposed patients.
Other agents
• Antiarrhythmics (e.g., amiodarone, flecainide, sotalol) can depress conduction below the AV node, predisposing to Mobitz II or worsening bundle branch disease.
• Opioids, sedatives, and certain recreational drugs (including cannabis in high doses) can augment vagal tone, occasionally tipping susceptible individuals into AV nodal block.
• Electrolyte active medications (e.g., potassium sparing diuretics) can contribute to hyperkalaemia related conduction delays.
Practical tip: If a new heart block appears after starting or changing a medication, bring all medicines (including over the counter and herbal supplements) to your doctor. Adjusting the regimen may reverse the heart block without further intervention. If your condition does not improve after trying these changes, book a physical visit to a doctor with Apollo 24|7.
Procedures and Surgery
Here’s what you need to know:
Transcatheter aortic valve implantation (TAVI/TAVR) and surgical aortic valve replacement
• These procedures sit close to the heart’s conduction tissues. New LBBB, RBBB, or complete heart block can occur after valve implantation; a notable proportion of TAVI patients require a permanent pacemaker (rates vary by valve type and anatomy, commonly around 10–25%).
• Risk factors include pre existing RBBB, deep valve implantation, heavy calcification, and certain valve designs.
Catheter ablation and other interventions
• Ablation near the AV node (e.g., treating certain arrhythmias) can rarely cause AV block; centres use mapping and safety margins to minimise this risk.
• Cardiac surgery (e.g., septal myectomy, congenital repairs) can lead to temporary or permanent conduction disturbances needing pacing.
Metabolic, Endocrine, and Autonomic Factors
Here’s what you need to know:
Electrolytes
• Hyperkalaemia is a well known cause of conduction slowing; severe elevations can produce wide QRS, bundle branch blocks, and high grade AV block. Correction often reverses the heart block.
• Severe hypokalaemia or hypomagnesaemia may predispose to arrhythmias and conduction abnormalities.
Endocrine and autonomic
• Hypothyroidism can slow heart rate and contribute to AV block; treating the thyroid disorder may improve conduction.
• High vagal tone (athletes, sleep, pain, vomiting) can induce AV nodal block (first degree or Mobitz I), often transient.
• Sleep apnoea is associated with bradyarrhythmias and nocturnal AV block; treating apnoea can reduce events.
Note: If lab tests are needed (e.g., thyroid function, electrolytes), Apollo 24|7 offers convenient home collection for tests, which can speed up evaluation.
Congenital and Pregnancy Related Heart Block
Here’s what you need to know:
Maternal antibodies and fetal AV block
• Mothers with anti Ro/SSA and anti La/SSB antibodies can passively transfer them to the fetus, occasionally causing inflammation and scarring of the fetal AV node. This can produce congenital complete heart block, often detected on prenatal ultrasound by a very slow fetal heart rate.
• Risk is low overall but higher if a prior child was affected. Multidisciplinary care is essential.
Postoperative pediatric heart block
• Children undergoing surgery for congenital heart disease (e.g., ventricular septal defect repair) can develop transient or permanent AV block due to proximity of surgical repair to the conduction system.
Symptoms and When to Seek Care
Here’s what you need to know:
Symptoms vary with the degree of heart block and the underlying cause:
• Mild block (first degree) usually has no symptoms.
• Intermittent block (Mobitz I/II) can cause palpitations, skipped beats, fatigue, lightheadedness, or near fainting.
• High grade or complete block can cause severe fatigue, shortness of breath, confusion, chest pain, or fainting (syncope).
Seek urgent care immediately if you have:
• Fainting, near fainting, or new severe dizziness.
• Chest pain, shortness of breath, or signs of a heart attack.
• A known infection risk (e.g., tick exposure) with rapid heart rate changes or presyncope.
If you have persistent symptoms for more than two weeks, or if your wearable/device flags bradycardia or pauses, consult a doctor online with Apollo 24|7 for timely guidance and possible tests.
Diagnosis: How Doctors Find the Cause
Here’s what you need to know:
ECG first
• The 12 lead ECG identifies the type of heart block, its location (AV nodal vs infranodal clues), QRS width, and associated patterns (e.g., bundle branch blocks).
Monitoring
• Holter monitor (24–48 hours) or event/patch recorders capture intermittent blocks and correlate symptoms with rhythms.
• Exercise testing may help reveal infranodal disease if conduction worsens with exertion.
Imaging and labs
• Echocardiogram looks for structural disease.
• Cardiac MRI or PET may identify sarcoidosis or myocarditis.
• Labs: electrolytes, kidney function, thyroid tests, troponin for suspected MI; Lyme serology if clinically indicated. Apollo 24|7 offers home collection for many of these tests.
Electrophysiology (EP) study
• In selected cases, invasive testing pinpoints where the block occurs (AV node vs His–Purkinje) and guides pacemaker decisions.
Treatment: Matching Therapy to Type and Cause
Here’s what you need to know:
Fix reversible causes
• Hold or adjust AV nodal slowing medications.
• Correct electrolytes and thyroid disorders.
• Treat infections (e.g., antibiotics for Lyme carditis), myocarditis, or sarcoidosis (immunosuppression where appropriate).
Acute stabilisation
• Atropine may help AV nodal block but is less effective in infranodal block.
• Temporary pacing (transcutaneous or transvenous) supports patients with unstable high grade block, especially during MI or Lyme carditis.
Permanent pacing
• Generally indicated for symptomatic Mobitz II, high grade or complete AV block not due to a transient reversible cause, and in certain post procedure scenarios.
• Device type (single vs dual chamber; conduction system pacing) is tailored to rhythm needs and underlying heart function.
Beyond pacing
• If LBBB suggests cardiomyopathy and heart failure with low ejection fraction, cardiac resynchronisation therapy (CRT) can improve symptoms and outcomes.
• In cardiac sarcoidosis at risk for ventricular arrhythmias, an implantable cardioverter defibrillator (ICD) may be considered.
In Lyme carditis and some inflammatory causes, clinicians often delay permanent pacemaker decisions while treating the underlying process, because conduction can recover.
Living Well: Prognosis, Prevention, and Follow up
Here’s what you need to know:
Prognosis depends on type and cause:
• First degree and Mobitz I often have an excellent outlook, especially if drug or vagal related.
• Mobitz II and complete heart block carry higher risk but are well managed with pacing and cause directed therapy.
• LBBB points to structural heart disease—treating the underlying condition is key.
Prevention and monitoring
• Review medications for AV nodal effects.
• Control cardiovascular risk factors (blood pressure, diabetes, cholesterol).
• Prevent tick bites in endemic areas; seek care early for Lyme symptoms.
• Treat sleep apnoea to reduce nocturnal bradyarrhythmias.
• After TAVR or cardiac surgery, adhere to rhythm monitoring and follow up plans.
Life with a pacemaker
• Most people return to normal activities with minimal restrictions.
• Routine device checks ensure proper function; modern devices are MRI conditional in many cases.
• Keep a medication and device card; inform healthcare providers before procedures.
Conclusion
“What leads to what heart block” largely depends on where the heart’s wiring is affected. AV nodal influences—like high vagal tone, certain medications, and inferior heart attacks—usually lead to first degree AV block or Mobitz I, which are often transient and lower risk. Infranodal injuries—like anterior heart attacks, age related scarring, infiltrative or inflammatory diseases, and some procedures—tend to produce Mobitz II, bundle branch blocks, bifascicular patterns, or complete heart block and may require pacing. Special scenarios matter: Lyme carditis can cause dramatic but reversible AV block; pregnancy related antibody transfer can cause congenital heart block; and TAVR commonly alters conduction around the aortic valve. The good news is that modern diagnostics and therapies—from Holter monitors to pacemakers and targeted treatments—allow most people to do very well. If your symptoms persist beyond two weeks, or you’re worried about your ECG findings or medications, consult a doctor online with Apollo 24|7. And if severe symptoms strike—such as fainting or chest pain—seek emergency care immediately.
Consult Top Specialists
Dr. Anand Ravi
General Physician
2 Years • MBBS
Bengaluru
PRESTIGE SHANTHINIKETAN - SOCIETY CLINIC, Bengaluru
Dr. Bhethala Sharan Prakash
General Physician/ Internal Medicine Specialist
5 Years • MBBS MD
Bengaluru
PRESTIGE SHANTHINIKETAN - SOCIETY CLINIC, Bengaluru
Dr. Sushith C
General Physician
2 Years • MBBS
Bengaluru
PRESTIGE SHANTHINIKETAN - SOCIETY CLINIC, Bengaluru
Dr. Sumanjita Bora
Cardiologist
9 Years • MBBS, PGDCC
Bengaluru
Apollo Clinic, Sarjapur Road, Bengaluru
Dr. Mahendranath Subramani Prasad
Cardiologist
16 Years • MBBS,MS ,DNB, M.Ch. Cardiovascular and Thoracic Surgery(Cardiology)
Bengaluru
Apollo Clinic, Sarjapur Road, Bengaluru
Consult Top Specialists
Dr. Anand Ravi
General Physician
2 Years • MBBS
Bengaluru
PRESTIGE SHANTHINIKETAN - SOCIETY CLINIC, Bengaluru
Dr. Bhethala Sharan Prakash
General Physician/ Internal Medicine Specialist
5 Years • MBBS MD
Bengaluru
PRESTIGE SHANTHINIKETAN - SOCIETY CLINIC, Bengaluru
Dr. Sushith C
General Physician
2 Years • MBBS
Bengaluru
PRESTIGE SHANTHINIKETAN - SOCIETY CLINIC, Bengaluru
Dr. Sumanjita Bora
Cardiologist
9 Years • MBBS, PGDCC
Bengaluru
Apollo Clinic, Sarjapur Road, Bengaluru
Dr. Mahendranath Subramani Prasad
Cardiologist
16 Years • MBBS,MS ,DNB, M.Ch. Cardiovascular and Thoracic Surgery(Cardiology)
Bengaluru
Apollo Clinic, Sarjapur Road, Bengaluru
Consult Top Specialists
Dr. Anand Ravi
General Physician
2 Years • MBBS
Bengaluru
PRESTIGE SHANTHINIKETAN - SOCIETY CLINIC, Bengaluru
Dr. Bhethala Sharan Prakash
General Physician/ Internal Medicine Specialist
5 Years • MBBS MD
Bengaluru
PRESTIGE SHANTHINIKETAN - SOCIETY CLINIC, Bengaluru
Dr. Sushith C
General Physician
2 Years • MBBS
Bengaluru
PRESTIGE SHANTHINIKETAN - SOCIETY CLINIC, Bengaluru
Dr. Sumanjita Bora
Cardiologist
9 Years • MBBS, PGDCC
Bengaluru
Apollo Clinic, Sarjapur Road, Bengaluru
Dr. Mahendranath Subramani Prasad
Cardiologist
16 Years • MBBS,MS ,DNB, M.Ch. Cardiovascular and Thoracic Surgery(Cardiology)
Bengaluru
Apollo Clinic, Sarjapur Road, Bengaluru
More articles from General Medical Consultation
Frequently Asked Questions
1) What causes first degree AV block and does it need treatment?
Common causes include high vagal tone, beta blockers, diltiazem/verapamil, and inferior MI. Most people need no treatment beyond checking medicines and electrolytes. This is one of the mildest forms of heart block.
2) What’s the difference between Mobitz I and Mobitz II causes?
Mobitz I (Wenckebach) is usually AV nodal (vagal tone, medications, inferior MI) and often transient. Mobitz II is usually infranodal (His–Purkinje) from degeneration, anterior MI, or infiltrative disease, and it more often requires a pacemaker.
3) Which conditions commonly cause complete heart block?
Anterior MI, advanced age related conduction disease, Lyme carditis, sarcoidosis, post operative injury, and severe metabolic or drug toxicity can all cause complete heart block.
4) What causes left bundle branch block, and is it serious?
LBBB often reflects underlying heart disease, such as hypertension, coronary artery disease, cardiomyopathy, or aortic stenosis, and can occur after TAVR. It warrants evaluation with imaging; seriousness depends on the underlying condition.
5) When do I need a pacemaker for heart block?
Pacemakers are typically recommended for symptomatic Mobitz II, high grade AV block, complete heart block not due to a transient reversible cause, or significant post procedure block. Your cardiologist will tailor the decision to your rhythm and overall health.