How do you recognize 1st degree AV block?

Every P wave is followed by a QRS, but the PR interval is prolonged (> 200 ms / 5 small boxes). Usually benign, no treatment.

What is the difference between Mobitz I and Mobitz II?

Mobitz I (Wenckebach): PR progressively lengthens until a QRS is dropped. Usually AV nodal, benign. Mobitz II: PR is constant, then a QRS suddenly drops. Infranodal, unstable, often needs pacing.

What defines 3rd degree AV block?

Complete AV dissociation — P waves and QRS complexes march independently. Ventricular escape rhythm (usually 20–40 bpm). Requires pacing.

Which blocks need transcutaneous pacing?

Symptomatic Mobitz II and any 3rd degree block. Atropine is unlikely to help infranodal blocks and may make them worse — go straight to pacing.

ECG reference

AV blocks.

First-degree to complete heart block, mapped to what actually kills patients.

1st degree

ECG: Every P → QRS. PR > 200 ms, constant.

Level of block: AV node

EMS treatment: None. Reassess and monitor.

2nd degree, Mobitz I (Wenckebach)

ECG: PR progressively lengthens, then a QRS drops. Cycle repeats.

Level of block: AV node — often benign

EMS treatment: Usually observation. Atropine if symptomatic bradycardia.

2nd degree, Mobitz II

ECG: PR is constant. Occasional dropped QRS with no warning. Often wide QRS.

Level of block: Infranodal — unstable

EMS treatment: Transcutaneous pacing. Prepare for progression to complete block.

2:1 AV block

ECG: Every other P conducts. Can be Mobitz I or II — treat as II if unstable.

Level of block: Nodal or infranodal

EMS treatment: Pacing if symptomatic.

3rd degree (complete)

ECG: P waves and QRS march independently. Escape rhythm 20–40 (ventricular) or 40–60 (junctional).

Level of block: Infranodal — always unstable

EMS treatment: Transcutaneous pacing immediately. Epi or dopamine drip as bridge.

Key concept

Atropine works at the AV node, not below it. That's why atropine helps Mobitz I but often fails (or worsens) Mobitz II and 3rd degree — those are infranodal. When you see a wide-QRS escape and a stable P-wave rate, skip atropine and pace.

Bradycardia algorithm order (ACLS)

Identify signs of poor perfusion (AMS, ischemic chest pain, hypotension, shock).
Atropine 1 mg IV (up to 3 mg total) — first line for symptomatic bradycardia.
If atropine ineffective: transcutaneous pacing.
Consider epinephrine 2–10 mcg/min or dopamine 5–20 mcg/kg/min drip.
Prepare for transvenous pacing (in-hospital).

Frequently asked

How do you recognize 1st degree AV block?: Every P wave is followed by a QRS, but the PR interval is prolonged (> 200 ms / 5 small boxes). Usually benign, no treatment.
What is the difference between Mobitz I and Mobitz II?: Mobitz I (Wenckebach): PR progressively lengthens until a QRS is dropped. Usually AV nodal, benign. Mobitz II: PR is constant, then a QRS suddenly drops. Infranodal, unstable, often needs pacing.
What defines 3rd degree AV block?: Complete AV dissociation — P waves and QRS complexes march independently. Ventricular escape rhythm (usually 20–40 bpm). Requires pacing.
Which blocks need transcutaneous pacing?: Symptomatic Mobitz II and any 3rd degree block. Atropine is unlikely to help infranodal blocks and may make them worse — go straight to pacing.

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