Last updated September 30, 2025
Introduction
First-degree AV block is not a true “block” but a delay in AV nodal conduction, defined by PR interval >200 ms with 1:1 AV conduction. Usually benign, but in select contexts it can indicate conduction system disease or predict future arrhythmias.
ECG Findings
Feature | Explanation |
PR interval >200 ms | Measured from P-wave onset to QRS onset; typically constant beat-to-beat. |
Normal P-wave axis & morphology | Confirms sinus origin. |
QRS narrow | (≤120 ms) if block at AV node; may widen if coexistent His-Purkinje disease. |
RR regular | All P waves conduct (no dropped beats). |
Pearl: If PR >300 ms (so-called “marked” first-degree), symptoms (fatigue, exertional dyspnea) can mimic pacemaker syndrome due to atrial contraction against closed AV valves.
ECG Example - PR prolongation (~260 ms)
ECG Example - Marked PR prolongation (>300 ms)
Electrophysiology
- Delay typically occurs within the AV node (slowed calcium channel conduction).
- Can occur at intra-Hisian or infra-Hisian levels (His-Purkinje disease), esp. if QRS is wide or with other conduction abnormalities (e.g., RBBB/LBBB).
- PR prolongation increases with vagal tone and AV nodal–blocking drugs (β-blockers, calcium channel blockers, digoxin).
ECG Example - wide QRS (RBBB)+ first-degree AV block
ECG Example - wide QRS (LBBB)+ first-degree AV block
Differential Diagnosis
- Physiologic: high vagal tone (athletes, nausea), sleep, well-conditioned young adults, progressive conduction system degeneration in older patients
- Medications: β-blockers, non-DHP calcium channel blockers, digoxin, antiarrhythmics (IA, IC, III).
- Pathologic: ischemia/infarction (esp. inferior MI), myocarditis, infiltrative disease (sarcoid, amyloid), Lyme carditis.
- Electrolyte/metabolic: hyperkalemia, hypothyroidism.
- Post-procedural: post-TAVR, post-surgical (esp. valve surgery).
ECG Example - marked 1st degree after cannabinoid ingestion
ECG Example - Inferior MI wi/ 1st degree av block
ECG example - 1st degree av block in hyperkalemia (K 7.7)
ECG Example - 1st degree av block in a patient taking digoxin
PR prolongation in ACS can herald AV nodal ischemia — watch for progression in inferior MI.
Key Points
- PR >200 ms = First-Degree AV Block, usually benign but not always meaningless.
- Red flags: PR >300 ms + symptoms, wide QRS, structural heart disease, acute MI, Lyme risk.
- ED role: recognize when it’s just incidental vs. harbinger of concerning pathology.
References
- Davila E. The ECG. Updated ed. 2024.
- Garcia TB. 12-Lead ECG: The Art of Interpretation. 2nd ed. Jones & Bartlett; 2013.
- Chou TC, Surawicz B. Chou’s Electrocardiography in Clinical Practice. 7th ed. Philadelphia: Elsevier/Saunders; 2018.
- Brady WJ, Mattu A, Tabas JA. Critical Decisions in Emergency and Acute Care Electrocardiography. Wiley-Blackwell; 2009.
- Podrid PJ. Podrid’s Real-World ECGs. Vol 6. Cardiotext Publishing; 2016.
- Burns E, Buttner R. First-degree AV block. Life in the Fast Lane ECG Library. Updated 2024.
- Smith SW. Dr. Smith’s ECG Blog – First-degree AV block posts.
- Cheng S, Keyes MJ, Larson MG, et al. Long-term outcomes in individuals with prolonged PR interval (Framingham Heart Study). Circulation. 2009;119:2131-2139.
- Magnani JW, et al. PR interval and risk of atrial fibrillation (ARIC). Heart Rhythm. 2013;10:1249-1256.
This post is for education and not medical advice.