First-Degree AV Block

First-degree AV block (more accurately termed first-degree AV delay) is defined as a PR interval >200 ms with 1:1 AV conduction preserved — no P waves are dropped. [1-2] It is generally considered a benign conduction abnormality, though emerging data associate it with increased risks of atrial fibrillation, heart failure, and mortality, particularly in older adults and those with structural heart disease. [3-5]

• Onset and duration of any symptoms: fatigue, exercise intolerance, lightheadedness, dyspnea on exertion, near-syncope, or syncope
• Temporal correlation of symptoms with bradycardia or pauses
• Medication history — specifically beta-blockers (including ophthalmic), non-dihydropyridine calcium channel blockers (verapamil, diltiazem), digoxin, antiarrhythmics (amiodarone, flecainide, sotalol, propafenone), lithium, donepezil, SSRIs, opioids, and fingolimod [1][6]
• Recent tick exposure, erythema migrans rash, travel to Lyme-endemic areas [1][7]
• History of cardiac surgery, catheter ablation, or TAVR [1]
• Athletic training level (high vagal tone is a common benign cause in young athletes) [8-9]
• Symptoms of hypothyroidism, sarcoidosis, or autoimmune disease

• PR interval ≥300 ms — can cause "pseudo-pacemaker syndrome" with hemodynamic compromise (loss of AV synchrony, decreased cardiac output, elevated pulmonary capillary wedge pressure) [1][10]
• Concurrent wide QRS or bundle branch block — suggests infranodal disease with risk of progression [1-2]
• Syncope or presyncope temporally correlated with bradycardia
• New first-degree AV block in the setting of acute MI (especially inferior) [2]
• New AV block with tick exposure — Lyme carditis can progress from first-degree to complete heart block within minutes to hours [7]
• First-degree AV block in patients with neuromuscular disease (myotonic dystrophy, Kearns-Sayre, Emery-Dreifuss) — unpredictable progression to high-grade block and sudden death [1-2]
• First-degree AV block in lamin A/C cardiomyopathy — associated with ventricular arrhythmias and sudden cardiac death [1-2]

• Common causative agents: [1][6]
  • Beta-blockers (including ophthalmic timolol)
  • Non-dihydropyridine CCBs (verapamil, diltiazem)
  • Digoxin
  • Antiarrhythmics: amiodarone, dronedarone, flecainide, propafenone, sotalol, procainamide
  • Others: lithium, donepezil, phenytoin, SSRIs/TCAs, fingolimod, ivabradine, clonidine
• Key pearl: Therapeutic doses of AV-nodal blocking agents are rarely the sole cause of new AV block — most patients presenting with new AV block on these medications ultimately require permanent pacing [1-2]
• First-degree AV block is not an absolute contraindication to AV-nodal blocking drugs, but the PR interval should be monitored to ensure no progression [6]
• For digoxin toxicity: anti-digoxin Fab antibody fragments for severe cases [1]
• Avoid combining multiple AV-nodal blocking agents when possible [6]

• No specific dietary triggers for first-degree AV block
• In the setting of hyperkalemia-related conduction delay, restrict potassium intake and correct the electrolyte abnormality urgently [1]
• Ensure adequate hydration, particularly in patients with vagally mediated bradycardia

• Cardiovascular: palpitations, chest pain, dyspnea on exertion, orthopnea, lower extremity edema, syncope/presyncope
• Neurologic: muscle weakness, myotonia, ptosis (neuromuscular disease screening)
• Infectious: fever, rash (erythema migrans), arthralgias, recent tick bite
• Endocrine: cold intolerance, weight gain, constipation, dry skin (hypothyroidism)
• Pulmonary: cough, dyspnea (sarcoidosis)
• Rheumatologic: joint pain, skin changes (SLE, scleroderma, RA) [1]

• Family history of sudden cardiac death, cardiomyopathy, or conduction disease
• Family history of neuromuscular disease (myotonic dystrophy, Emery-Dreifuss, limb-girdle dystrophy) [2][10]
• Family history of lamin A/C mutations — autosomal dominant with high risk of progressive AV block and ventricular arrhythmias [1-2]
• Maternal history of SLE — associated with congenital AV block [1]
• Social history: athletic conditioning level, cannabis use [1]

• Age — prevalence follows a J-shaped curve: common in young athletes (vagal tone), then increases from the 5th–6th decade onward, peaking in the 9th–10th decade due to degenerative fibrosis [9]
• Chronic hypertension and diabetes mellitus — associated with degenerative conduction system disease [2]
• Structural heart disease — ischemic or nonischemic cardiomyopathy [2]
• AV-nodal blocking medications [1][6]
• High vagal tone — athletes, sleep, obstructive sleep apnea [1-2]
• Infiltrative/inflammatory disease — sarcoidosis, amyloidosis, myocarditis [1]
• Post-cardiac surgery or TAVR [1]

• Medication effect — most common iatrogenic cause; review all AV-nodal blocking agents [1]
• High vagal tone — benign in young, athletic patients; resolves with exercise [8]
• Lyme carditis — cannot-miss diagnosis in endemic areas; can rapidly progress to complete heart block [7]
• Acute inferior MI — AV block due to right coronary artery supply to AV node; usually transient [2]
• Cardiac sarcoidosis — may present with AV block as initial manifestation [2]
• Hypothyroidism — reversible cause, though treatment often does not eliminate need for pacing if otherwise indicated [1-2]
• Hyperkalemia — reversible with correction [1]
• Infiltrative disease — amyloidosis (AL or ATTR), especially with low voltage on ECG [2]
• Neuromuscular disease — myotonic dystrophy, Kearns-Sayre syndrome [2][10]
• Degenerative fibrosis (Lev's/Lenegre's disease) — most common cause in elderly [1-2]

• Prior episodes of AV block or documented PR prolongation
• History of myocardial infarction (especially inferior)
• Congenital heart disease (e.g., congenitally corrected transposition of the great arteries) [8]
• Prior cardiac surgery or catheter ablation [1]
• Known cardiomyopathy, valvular disease
• Neuromuscular disorders
• Autoimmune disease (SLE, sarcoidosis)
• Prior Lyme disease

• Vital signs: Heart rate (may be normal), blood pressure (assess for hemodynamic compromise)
• Cardiac auscultation: Soft S1 (due to prolonged PR allowing partial mitral valve closure before ventricular systole), cannon A waves in JVP if atrial contraction occurs against a closed tricuspid valve, murmurs suggesting structural disease
• Signs of heart failure: JVD, peripheral edema, pulmonary crackles
• Skin: Erythema migrans (Lyme), malar rash (SLE), skin thickening (scleroderma)
• Neurologic: Muscle weakness, myotonia, ptosis (neuromuscular disease)
• Thyroid exam: Goiter, signs of hypothyroidism

• Recommended based on clinical suspicion: [1]
  • BMP/CMP: Potassium (hyperkalemia), calcium, magnesium, renal function
  • TSH: Rule out hypothyroidism
  • Digoxin level: If on digoxin therapy
  • Lyme serologies (ELISA + Western blot): In endemic areas or with suggestive history [1]
  • Troponin: If acute coronary syndrome suspected
  • ESR/CRP, ACE level: If sarcoidosis suspected
  • ANA: If autoimmune etiology suspected
• No systematic studies have validated routine lab panels specifically for isolated first-degree AV block [1]

• Echocardiogram: Indicated if structural heart disease is suspected (abnormal exam, abnormal ECG beyond PR prolongation, symptoms of heart failure) [8]
  • [8]
• Cardiac MRI with gadolinium: If sarcoidosis, myocarditis, or lamin A/C cardiomyopathy suspected — can identify myocardial fibrosis and inflammation [2]
• Chest X-ray: If heart failure or sarcoidosis suspected

• Ambulatory ECG monitoring (Holter or event monitor): Reasonable in patients with symptoms of unclear etiology and first-degree AV block to establish symptom-rhythm correlation (Class IIa) [2]
• Exercise treadmill test: Reasonable for exertional symptoms — improvement in AV conduction with exercise suggests AV nodal block (benign); worsening suggests infranodal disease (concerning) [2][8]
• Electrophysiology study (EPS): Rarely needed; may be considered if infranodal block is suspected (wide QRS, exercise-induced worsening) to measure HV interval [2][8]
• Carotid sinus massage/pharmacologic challenge: May be considered in selected cases to determine level of block [2]

• Defining feature: PR interval >200 ms with every P wave followed by a QRS complex (1:1 conduction) [1-2][10]
• QRS is typically narrow — a wide QRS raises concern for concomitant infranodal disease [2]
• Look for:
  • PR ≥300 ms — "marked" first-degree AV block with hemodynamic implications [1][10]
  • Bundle branch block pattern — RBBB, LBBB, or bifascicular block suggesting infranodal conduction disease [1]
  • Alternating bundle branch block — high risk for sudden complete heart block [1]
  • ST changes suggesting ischemia
  • Low voltage suggesting infiltrative disease
• The following figure illustrates the ECG patterns of different degrees of AV block:

• First-degree AV block is typically benign and does not progress suddenly to complete heart block when the block is at the AV nodal level [1-2]
• However, a prolonged PR interval is not entirely benign — meta-analysis data show associations with a 1.45× increased risk of AF, 1.39× increased risk of heart failure, and 1.24× increased risk of mortality. The Framingham Heart Study demonstrated a 2-fold adjusted risk of AF and 3-fold adjusted risk of pacemaker implantation in individuals with first-degree AV block [3-4]
• In patients with stable CAD, first-degree AV block is associated with a 2.3× increased risk of HF hospitalization and 1.6× increased risk of mortality [5]
• In patients ≥85 years, first-degree AV block as a single ECG finding was the strongest predictor of mortality [9]
• Severity stratification:
  • Mild: PR 200–300 ms, narrow QRS, asymptomatic → benign
  • Marked: PR ≥300 ms → risk of pseudo-pacemaker syndrome [1][10]
  • With conduction disease: Wide QRS or bundle branch block → risk of progression [1]

• Asymptomatic, isolated first-degree AV block: No treatment required. Permanent pacing is not indicated (Class III — Harm per ACC/AHA/HRS) [2][10]
• Identify and treat reversible causes:
  • Discontinue or reduce offending medications if clinically feasible [1][6]
  • Treat Lyme disease with antibiotics (AV block typically resolves in 1–2 weeks) [1]
  • Correct hyperkalemia, hypothyroidism [1-2]
• Symptomatic first-degree AV block (PR ≥300 ms with pseudo-pacemaker syndrome — fatigue, exertional intolerance, hemodynamic compromise):
  • reasonable[1][10][12]
• Neuromuscular disease with any degree of AV block:
  • may be considered[2][10][12]
• Lamin A/C cardiomyopathy with first-degree AV block:
  • pacing + defibrillator capability[1-2]
• Acute hemodynamic instability (rare with isolated first-degree AV block):
  • Atropine 0.5 mg IV (effective for AV nodal block) [6]
  • Isoproterenol, dopamine, or temporary pacing if refractory [6]

The following algorithm from the 2018 ACC/AHA/HRS guidelines outlines the management approach for chronic AV block:

• Discharge: The vast majority of patients with isolated, asymptomatic first-degree AV block with PR <300 ms and narrow QRS can be safely discharged with outpatient follow-up [2]
• Observation/Admission criteria:
  • New first-degree AV block with suspected Lyme carditis (risk of rapid progression) [7]
  • Symptomatic bradycardia or hemodynamic compromise
  • PR ≥300 ms with symptoms of pseudo-pacemaker syndrome
  • Concurrent high-degree AV block, wide QRS, or alternating bundle branch block
  • Acute MI with new conduction abnormality
  • Drug toxicity (digoxin, beta-blocker, CCB overdose) requiring monitoring
• Cardiology consultation: For symptomatic patients, marked PR prolongation, wide QRS/bundle branch block, suspected neuromuscular or infiltrative disease, or consideration of pacemaker implantation

• Outpatient follow-up with PCP or cardiology within 2–4 weeks for new findings; routine annual ECG monitoring is reasonable for stable, asymptomatic patients
• Return precautions — instruct patients to return for:
  • Syncope or near-syncope
  • New or worsening dizziness, lightheadedness
  • Exertional intolerance or unexplained fatigue
  • Palpitations or irregular heartbeat
  • Chest pain or dyspnea
• Expected course: Isolated first-degree AV block is typically stable and does not progress suddenly to higher-degree block when at the AV nodal level. Progression to more advanced heart block is infrequent [1-2][9]
• Long-term awareness: Although benign in isolation, first-degree AV block is associated with increased long-term risk of AF, heart failure, and mortality — particularly in patients with structural heart disease, CAD, or advanced age. This may warrant periodic reassessment of cardiac status [3-5]

1. 2018 ACC/­AHA/­HRS Guideline on The Evaluation and Management Of Patients With Bradycardia and Cardiac Conduction Delay: A Report of the American College of Cardiology/­American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. — Kusumoto FM, Schoenfeld MH, Barrett C, et al. Journal of the American College of Cardiology. 2019.

2. 2018 ACC/­AHA/­HRS Guideline on The evaluation and Management Of patients With Bradycardia and Cardiac conduction Delay: A Report of the American College of Cardiology/­American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. — Writing Committee Members, Kusumoto FM, Schoenfeld MH, et al. Heart Rhythm. 2019.

3. Prolonged PR Interval, First-Degree Heart Block and Adverse Cardiovascular Outcomes: A Systematic Review and Meta-Analysis. — Kwok CS, Rashid M, Beynon R, et al. Heart. 2016.

4. Long-Term Outcomes in Individuals With Prolonged PR Interval or First-Degree Atrioventricular Block. — Cheng S, Keyes MJ, Larson MG, et al. The Journal of the American Medical Association. 2009.

5. First-Degree Atrioventricular Block Is Associated With Heart Failure and Death in Persons With Stable Coronary Artery Disease: Data From the Heart and Soul Study. — Crisel RK, Farzaneh-Far R, Na B, Whooley MA. European Heart Journal. 2011.

6. Drug-Induced Arrhythmias: A Scientific Statement From the American Heart Association. — Tisdale JE, Chung MK, Campbell KB, et al. Circulation. 2020.

7. Diagnosis and Treatment of Lyme Carditis: JACC Review Topic of the Week. — Yeung C, Baranchuk A. Journal of the American College of Cardiology. 2019.

8. Eligibility and Disqualification Recommendations for Competitive Athletes With Cardiovascular Abnormalities: Task Force 9: Arrhythmias and Conduction Defects: A Scientific Statement From the American Heart Association and American College of Cardiology. — Zipes DP, Link MS, Ackerman MJ, et al. Circulation. 2015.

9. Arrhythmias in Patients ≥80 Years of Age: Pathophysiology, Management, and Outcomes. — Curtis AB, Karki R, Hattoum A, Sharma UC. Journal of the American College of Cardiology. 2018.

10. 2012 ACCF/­AHA/­HRS Focused Update Incorporated Into the ACCF/­AHA/­HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities: A Report of the American College of Cardiology Foundation/­American Heart Association Task Force on Practice Guidelines and the Heart Rhythm Society. — Epstein AE, DiMarco JP, Ellenbogen KA, et al. Journal of the American College of Cardiology. 2013.

11. Mechanisms, Classification, and Clinical Aspects of Arrhythmias. — Antoni Bayés De Luna, Miquel Fiol‐Sala, Antoni Bayés‐Genís, et al. Clinical Electrocardiography. 2021.

12. Cardiac Pacemakers: Function, Troubleshooting, and Management: Part 1 of a 2-Part Series. — Mulpuru SK, Madhavan M, McLeod CJ, Cha YM, Friedman PA. Journal of the American College of Cardiology. 2017.

13. 2018 ACC/­AHA/­HRS Guideline on The Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay: Executive Summary: A Report of the American College of Cardiology/­American Heart Association Task Force on Clinical Practice Guidelines, and the Heart Rhythm Society. — Kusumoto FM, Schoenfeld MH, Barrett C, et al. Journal of the American College of Cardiology. 2019.