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.
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 .