High-grade AV block on an ECG strip

Two or more P waves in a row aren't conducted (3:1, 4:1), but the ones that do conduct keep the same PR. Very slow, and one step from complete block.

What to look for

Two or more P waves in a row fail to conduct, so the ventricular rate is a third or a quarter of the atrial rate, often 20-40/min. What keeps it out of complete heart block is that some P waves still get through, and when they do, the PR is the same every time. In complete block no P conducts and the PR is all over the place. The QRS is usually wide, because the block is usually below the AV node. Expect atropine to fail and go to pacing.

Examples

Each strip is 6 seconds of lead II, drawn to scale (25 mm a second, 10 mm a millivolt), so you can count boxes the way you would on paper. Each one is a different example, measured below it.

Don't confuse it with

• Third-degree (complete) AV block

  P waves and QRS complexes each regular but unrelated: no P is conducted, so the PR is all over the place.

• 2:1 AV block

  Two P waves for every QRS: every other P isn't conducted, and the conducted beats all have the same PR. From 2:1 alone you can't call it type I or type II.

• Second-degree AV block, type II

  A constant PR, then a P wave that suddenly isn't conducted. The QRS is often wide.

• Idioventricular rhythm

  Regular wide complexes at 20-40/min, with no P waves.

Where the criteria come from

• LITFL: AV block 2nd-degree high grade AV block
• LITFL: AV block 2nd-degree fixed ratio blocks
• LITFL: AV block 3rd-degree complete heart block

Others in the AV blocks group

• First-degree AV block
• Second-degree AV block, type I (Wenckebach)
• Second-degree AV block, type II
• 2:1 AV block
• Third-degree (complete) AV block
• All ECG rhythm practice