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Arrhythmias · Atrioventricular conduction disease

First-degree AV block

Lesson 1 of 214 <2% of the exam 5 min 7 sources

What the exam asks

Mostly diagnosis and the decision not to pace. Expect an incidental long PR interval and a choice between reassurance, more tests and a pacemaker, sometimes in an athlete, after an inferior MI or with a wide QRS.

The one-minute version

  • First-degree AV block is a PR interval over 200 ms with every P wave conducted: a delay, not a block [1].
  • The AV node is the usual site of the delay; a long PR with a wide QRS raises the chance that the delay sits in the His-Purkinje system, where disease can progress [2].
  • It is not always harmless: in the Framingham cohort it carried about twice the risk of atrial fibrillation and three times the risk of a pacemaker [3].
  • Without symptoms it needs no pacemaker; pacing is reasonable only when a PR above 300 ms causes symptoms like pacemaker syndrome [2][4].
Evidence (5)
  • "The condition is more accurately defined as an AV delay because no P waves are blocked." [1] PMID 29083636
  • "The atrioventricular node is the most common site of conduction delay." [2] PMID 28846254
  • "Conduction delay within the His-Purkinje system is more often associated with underlying heart disease and may progress to higher-degree atrioventricular block." [2] PMID 28846254
  • "except those with a PR interval greater than 0.30 seconds and symptoms attributable to the conduction delay" [2] PMID 28846254
  • "Individuals with first-degree atrioventricular block had a 2-fold adjusted risk of AF" [3] PMID 19549974

Reading the ECG

The normal PR interval is 120 to 200 ms; first-degree AV block is a PR above 200 ms that stays constant, with a QRS after every P wave [1].

A PR above 300 ms is called marked first-degree AV block, and at that length the P wave can sit inside the preceding T wave, so look for it before calling the rhythm junctional [2].

Evidence (4)
  • "First-degree AV block is a 1:1 association between P waves and QRS (1:1 AV conduction) and a constant PR interval of >200 ms." [1] PMID 29083636
  • "Typically, a P wave precedes each QRS complex by a fixed PR interval of 120 to 200 milliseconds." [1] PMID 29083636
  • "A PR interval greater than 0.30 seconds indicates marked first-degree atrioventricular block." [2] PMID 28846254
  • "Significant PR prolongation may result in the P wave occurring within the preceding T wave." [2] PMID 28846254

Where the delay is, and why

The PR interval covers the atria, the AV node, the His bundle and the Purkinje fibres, so a long PR can come from any of them; most often it is the AV node [2].

The QRS is the clue to the level. A narrow QRS points to the node; a long PR with a bundle branch block makes a His-Purkinje delay more likely, and that kind is more often due to heart disease and more likely to progress [2].

In the young the usual cause is vagal tone (more than 10% of young athletes have it); in older people it is fibrosis of the conduction system [2].

Other causes to look for: AV nodal blocking drugs and antiarrhythmics, electrolyte disorders, ischaemia, infections such as Lyme disease and endocarditis, infiltrative disease such as sarcoidosis, and neuromuscular disease [2].

Evidence (6)
  • "PR interval prolongation can result from conduction delay in the atria, atrioventricular node, His bundle, or Purkinje system." [2] PMID 28846254
  • "Patients with conduction delay within the His-Purkinje system are more likely to have a prolonged QRS interval in addition to PR prolongation." [2] PMID 28846254
  • "Prevalence rates exceeding 10% have been observed among young athletes" [2] PMID 28846254
  • "Fibrotic changes in the cardiac conduction system appear to be a common etiology in older adults." [2] PMID 28846254
  • "class Ia, Ic, II, III, and IV antiarrhythmic agents and digoxin" [2] PMID 28846254
  • "infections (endocarditis, rheumatic fever, Chagas disease, Lyme disease, and diphtheria)" [2] PMID 28846254

What it means for the patient

In 7,575 Framingham participants followed for decades, a PR above 200 ms was linked to more atrial fibrillation, more pacemakers and higher mortality; each extra 20 ms of PR raised the adjusted risk of a pacemaker by 22% [3].

These are associations from observational data: whether the long PR causes harm or marks underlying heart disease is not settled [2].

Evidence (4)
  • "Prospective, community-based cohort including 7575 individuals from the Framingham Heart Study" [3] PMID 19549974
  • "1.22 (95% CI, 1.14-1.30; P < .001) for pacemaker implantation" [3] PMID 19549974
  • "Prolongation of the PR interval is associated with increased risks of AF, pacemaker implantation, and all-cause mortality." [3] PMID 19549974
  • "Whether these associations reflect the higher prevalence of underlying organic heart disease or an independent pathologic process is uncertain." [2] PMID 28846254

Management

Find and remove what can be reversed (drugs, electrolytes, ischaemia); antiarrhythmic drugs have no role [2].

Without symptoms: no pacemaker, routine follow-up for progression [2][4].

A PR above 300 ms can uncouple atrial and ventricular contraction and cause symptoms like pacemaker syndrome; when the symptoms clearly come from the long PR, permanent pacing is reasonable [2][4].

A long PR with neuromuscular disease or a wide QRS may also warrant considering a pacemaker [2].

Evidence (4)
  • "Antiarrhythmic medications have no role in treating first-degree atrioventricular block." [2] PMID 28846254
  • "Most patients with first-degree atrioventricular block require no treatment." [2] PMID 28846254
  • "A PR interval exceeding 0.30 seconds can impair atrioventricular synchrony, reduce ventricular preload, and produce symptoms resembling pacemaker syndrome." [2] PMID 28846254
  • "Individuals with first-degree atrioventricular block and coexisting neuromuscular disease or prolonged QRS intervals may also warrant consideration for permanent pacemaker placement." [2] PMID 28846254

Board traps

  • A pacemaker for an asymptomatic long PR is a wrong answer, however long the PR [2][4].
  • A new PR prolongation in aortic valve endocarditis is not a benign finding: it suggests the infection has spread into the septum or conduction system and points to urgent surgery [5].
  • First-degree block in Lyme carditis can turn into complete heart block within hours, so it is monitored, not dismissed [6].
  • A long PR plus a bundle branch block is not the same lesion as a long PR alone: the level of the delay, and the risk, may differ [2].
Evidence (4)
  • "Guidelines from the American Heart Association and American College of Cardiology do not recommend permanent pacemaker placement for patients with first-degree atrioventricular block" [2] PMID 28846254
  • "Patients with conduction delay within the His-Purkinje system are more likely to have a prolonged QRS interval in addition to PR prolongation." [2] PMID 28846254
  • "Prolongation of the PR interval or a new conduction block may indicate extension of infection into the interventricular septum or the conduction system and is generally considered a poor prognostic sign necessitating urgent intervention." [5] PMID 31335013
  • "The degree of the AVB can fluctuate rapidly, with two-thirds of patients progressing to complete AVB." [6] PMID 35631104

Europe differs

They agree. The ESC pacing guideline, like the ACC/AHA one, does not pace asymptomatic first-degree block and considers pacing only for persistent symptoms like pacemaker syndrome clearly due to a PR above 300 ms [4][7].

Not yet checked against the source's own words: the guideline's full text could not be fetched by our build. The citation is right; the wording check is pending.

Think it through

Case. A 24-year-old cyclist has a pre-participation ECG: sinus rhythm 52/min, PR 250 ms, narrow QRS, no other finding. He has no symptoms and no family history of heart disease.

Question. What do you do?

Show the answer

Answer. Reassure him and do not pace. A narrow QRS points to a nodal delay, vagal tone is the usual cause in young athletes, and asymptomatic first-degree block needs no pacemaker; follow-up looks for symptoms or progression [2].

  • "Researchers have attributed first-degree atrioventricular block in younger patients to increased vagal tone" [2] PMID 28846254
  • "Individuals without clinically significant symptoms require no treatment beyond surveillance for progression of atrioventricular block." [2] PMID 28846254

References

  1. 1. Ahmed I, Goyal A, Chhabra L. Atrioventricular Block. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2024. StatPearls PMID 29083636
  2. 2. Varghese A, Makaryus AN. First-Degree Heart Block. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2026. StatPearls PMID 28846254
  3. 3. Cheng S, Keyes MJ, Larson MG, et al. Long-term outcomes in individuals with prolonged PR interval or first-degree atrioventricular block. JAMA. 2009;301(24):2571-7. Article PMID 19549974 doi:10.1001/jama.2009.888
  4. 4. Kusumoto FM, Schoenfeld MH, Barrett C, et al. 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. Circulation. 2019;140(8):e382-e482. Guideline PMID 30586772 doi:10.1161/CIR.0000000000000628
  5. 5. Khalid N, Ahmad SA, Thapa N. Aortic Valve Endocarditis. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2026. StatPearls PMID 31335013
  6. 6. Radesich C, Del Mestre E, Medo K, et al. Lyme Carditis: From Pathophysiology to Clinical Management. Pathogens. 2022;11(5). Review PMID 35631104 doi:10.3390/pathogens11050582
  7. 7. Glikson M, Nielsen JC, Kronborg MB, et al. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. Eur Heart J. 2021;42(35):3427-3520. Guideline PMID 34455430 doi:10.1093/eurheartj/ehab364

After the lesson

Flashcards

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