Skip to content

1st-degree AV block

Also called first-degree heart block.

Every P conducts, but slowly: the PR interval is longer than 0.20 s and the same length on every beat. No QRS is ever dropped. A finding more than a rhythm.

A 6-second EKG rhythm strip showing 1st-degree AV block.
1st-degree AV block, 6 seconds of strip. Each small box is 0.04 seconds and each large box is 0.20 seconds.
Try the 60-second rhythm challenge →

How to identify 1st-degree AV block

Rate
That of the underlying rhythm
Rhythm
Regular
P waves
Upright, one before every QRS
PR interval
Over 0.20 s, the same on every beat
QRS
Under 0.12 s
Clincher
Every P conducts, just late. Nothing drops.

Rhythms it gets mistaken for

  • 1st-degree AV block vs Normal sinus rhythm

    1st-degree AV block has a PR interval over 0.20 s that is the same on every beat, with no dropped beats. With normal sinus rhythm you would see a normal PR interval of 0.12-0.20 s. How to spot normal sinus rhythm: Every step of the six-step read is normal.

  • 1st-degree AV block vs 2nd-degree type I (Wenckebach)

    1st-degree AV block has one upright P wave before every QRS, all the same shape. With Wenckebach you would see regular P waves, with more P waves than QRS complexes. It also has a PR interval over 0.20 s that is the same on every beat, with no dropped beats, while Wenckebach would show a PR interval that gets longer beat after beat until a QRS drops. It also has a regular rhythm, while Wenckebach would show pauses where a QRS drops. How to spot Wenckebach: Longer, longer, longer, drop. Usually nodal and usually benign.

  • 1st-degree AV block vs Sinus bradycardia

    1st-degree AV block has a PR interval over 0.20 s that is the same on every beat, with no dropped beats. With sinus bradycardia you would see a normal PR interval of 0.12-0.20 s. How to spot sinus bradycardia: A normal sinus complex, just slow.

Treatment by certification level

Open your level. Treat the patient, not the strip.

Paramedic

A finding more than a rhythm.

  • No rhythm treatment. Treat the patient and watch for a higher-grade block.
AEMT

Pulse present and stable.

  • Oxygen only if hypoxic or short of breath, IV access, monitoring.
  • Acquire and transmit a 12-lead ECG. Interpreting it is a Paramedic skill.
  • Transport and reassess. Request a paramedic if the patient worsens.
EMT

Pulse present: assess and support.

  • Treat the patient, not the rhythm name: airway, breathing, circulation, vital signs.
  • Oxygen if the SpO2 is under 94%, the patient is short of breath, or shows signs of shock.
  • Request ALS and transport promptly. Reassess on the way.

An AED goes only on a patient who is unresponsive, not breathing and pulseless. It cannot feel a pulse, so on a patient with a pulse it can advise a shock that would do harm.

Could you pick out 1st-degree AV block on a fresh strip? See how many rhythms you can identify in one minute.

Try the 60-second rhythm challenge