MedCourseFinder

Rhythm

First-degree AV block

Not medically reviewed · Content last updated · 1 min read ·

This is the date the draft was written, not a medical review date.

First-degree AV block is a delay in conduction through the AV node or His-Purkinje system, shown as a prolonged PR interval. Every P wave is still followed by a QRS, so no beats are dropped.

ECG criteria

  • PR interval longer than 200 ms, constant from beat to beat.
  • Every P wave is followed by a QRS complex (1:1 conduction).
  • Regular rhythm, with a rate that depends on the underlying sinus rate.
  • QRS usually narrow, unless there is a coexisting bundle branch block.
  • Marked prolongation can bring the P wave close to the preceding T wave.

Mechanism

Slowed conduction, most often within the AV node and less often in the His-Purkinje system, lengthens the interval between atrial and ventricular activation. All impulses are eventually conducted, which distinguishes it from higher-degree blocks.

Common causes

  • Increased vagal tone, as in athletes and during sleep; may be a normal variant.
  • Drugs: beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, amiodarone.
  • Inferior myocardial infarction, myocarditis, Lyme disease, rheumatic fever, infective endocarditis.
  • Degenerative conduction disease with age; electrolyte disturbances.

Management principles

Isolated first-degree block is usually benign and needs no specific treatment; review contributing drugs and reversible causes. Follow local guidelines for follow-up and for the uncommon patients with very long PR intervals or symptoms, in whom pacing may be considered.

High-level principles for study. Treatment decisions follow local guidelines and the clinical situation.

Pitfalls and mimics

  • Measure the PR interval from the start of the P wave to the start of the QRS in the lead where it is longest; a one-lead glance can miss it.
  • At fast rates the P wave may merge with the preceding T wave, and a long PR can be mistaken for a junctional rhythm.
  • A prolonged PR before dropped beats is Mobitz I; a constant long PR with sudden dropped P waves points to Mobitz II.

Practise in the ECG trainer — Recognise this pattern on generated tracings.

References

  1. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. American College of Cardiology / American Heart Association / Heart Rhythm Society (2018) — Tier 2 (guidelines / primary literature)
  2. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. European Society of Cardiology (2021) — Tier 2 (guidelines / primary literature)
  3. Wagner GS, Strauss DG. Marriott's Practical Electrocardiography. Wolters Kluwer, 12th edition (2013) — Tier 3 (textbook)

References are listed only where the article cites them. See the sources page for the full policy. How sources are chosen and credited