Why does the type of block matter?

Mobitz II and 3rd-degree blocks are unstable — they can deteriorate to asystole. They warrant urgent clinician notification, often pacing pads on standby. Mobitz I is generally more benign.

How do I tell Mobitz I from Mobitz II?

Mobitz I lengthens the PR before dropping; Mobitz II keeps the PR fixed and just drops. The pattern matters.

Is 1st-degree AV block dangerous?

Usually not on its own, but it can be a marker of underlying conduction disease. Document and trend.

AV heart blocks — an educational nurse reference.

First, second-degree (Mobitz I and II), and third-degree AV blocks: how to recognize them and why the type matters.

Recognizing this rhythm at the bedside means matching pattern, intervals, and clinical context. This page covers the educational pattern; calipers and your clinical judgment carry it across the line.

What RN PocketPal does

Identifying traits

1st degree: PR > 0.20 sec, every P conducts.
2nd degree Mobitz I (Wenckebach): progressively longer PR until a QRS is dropped, then resets.
2nd degree Mobitz II: constant PR, intermittent dropped QRS (no PR lengthening).
3rd degree (complete): no relationship between Ps and QRSs; atrial and ventricular rates independent.

Verify with

Calipers: walk out PR intervals across multiple cycles.
Lead II + V1 if available.
Patient's symptoms: dizziness, syncope, hypotension.

Who it’s for

For Nurses and nursing students learning rhythm interpretation.

We publish a real ten-second recording on a rhythm reference only where we hold one under an open licence, rather than illustrating the pattern with a drawing. This rhythm does not have one yet — the identifying traits above are the reference until it does.

Rhythms this gets confused with

Known-answer practice — RN PocketPal does not interpret your patient's monitor. To work through strips with the answer revealed after you commit, open Rhythm Practice.

Frequently asked questions

Why does the type of block matter?: Mobitz II and 3rd-degree blocks are unstable — they can deteriorate to asystole. They warrant urgent clinician notification, often pacing pads on standby. Mobitz I is generally more benign.
How do I tell Mobitz I from Mobitz II?: Mobitz I lengthens the PR before dropping; Mobitz II keeps the PR fixed and just drops. The pattern matters.
Is 1st-degree AV block dangerous?: Usually not on its own, but it can be a marker of underlying conduction disease. Document and trend.

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Reviewed by RN PocketPal Clinical Team, RN. Last reviewed May 7, 2026.