AV Blocks: Three-Question Path

Why This Matters on the Truck

AV blocks all look like “something wrong between P and QRS.” Under stress, that makes them easy to mix up. You need a short checklist — not four memorized textbook photos only.

Street rule: name the strip carefully, then assess the patient (pulse, perfusion, symptoms) and follow your bradycardia / block protocol. Higher-grade blocks (Mobitz II, complete block) deserve more urgency thinking in standard education frameworks.

The 3-Question Path

1. Does every P get a QRS? Yes → only decide if the PR is long (1° block) or normal. No → some beats drop or none of the P waves own the QRS — go to question 2.
2. On conducted beats, does the PR walk out or stay fixed? Walks out, then a drop → Mobitz I (Wenckebach). Stays the same, then a sudden drop → Mobitz II. If nothing consistently conducts, go to question 3.
3. Do atria and ventricles run on separate clocks? P regular, QRS regular, but no real PR relationship (P waves walk through QRS/T) → third-degree (complete) heart block.

1. First-Degree AV Block

Plain English: Every beat still gets through — it is just late. PR is longer than normal and stays long on every beat.

How to say it: “Sinus rhythm with first-degree AV block” (base rhythm + the delay).

2. Mobitz I (Wenckebach)

Plain English: PR gets longer… longer… then a P with no QRS. Often groups of beats with a pause.

Memory hook: walking PR → then a miss.

3. Mobitz II

Plain English: Conducted beats share the same PR. Then a QRS disappears without warning. No progressive lengthening.

Memory hook: fixed PR → sudden miss. Educationally higher concern for progression than classic Wenckebach — match care to the patient and protocol.

2:1 note: When every other P conducts, you may not see PR “walking.” Label carefully and use clinical context.

4. Third-Degree (Complete) Block

Plain English: Atria on one clock, ventricles on a slow backup clock. P waves march through the QRS and T waves.

Memory hook: two clocks, zero relationship.

Prehospital Priorities

• Patient first: assess pulse and perfusion with every slow or blocked strip. A first-degree block in a talking, well-perfused patient is not the same as complete block with hypotension.
• Support ABCs — oxygen as indicated, IV access, continuous monitoring, 12-lead when feasible.
• Symptomatic bradycardia pathway — atropine, TCP (transcutaneous pacing) readiness, and other agents only as your protocol authorizes. Know when atropine is less likely to help (for example, high-degree infranodal blocks in teaching frameworks) and when pacing should be prepared early.
• Higher-grade concern: Mobitz II and third-degree blocks warrant earlier pacing readiness and destination thinking even if the patient “looks okay right now.”
• Unstable complete block / severe symptomatic bradycardia: do not delay TCP for endless strip debates. Sedation/pain control for pacing per protocol when the patient is conscious.
• Cause hunting while treating: ischemia, inferior MI context, meds (beta-blockers, calcium-channel blockers, digoxin), electrolytes, post-arrest — communicate history to the ED.

What to Say on the Radio / to Your Partner

• “Sinus with first-degree AV block, stable, transporting for evaluation.”
• “Second-degree Mobitz I, rate in the 50s, well perfused — monitoring.”
• “Mobitz II with intermittent drops, preparing TCP pads, contacting medical control per protocol.”
• “Third-degree block, ventricular escape ~30, hypotensive — pacing now.”

Common Mix-Ups

• Sinus arrhythmia vs Wenckebach: need lengthening PR + dropped QRS, not rate variation alone.
• Mobitz I vs II: walking PR = I; fixed PR + sudden drop = II. This is the highest-yield differentiator.
• Complete block vs “lots of P waves”: map several cycles for true AV dissociation — random-looking PR that never settles.
• 2:1 second-degree block: hard to classify as I vs II because you never see two consecutive PR intervals to prove walking. Treat the patient; document “2:1 AV block” carefully; prepare for higher-grade behavior.
• Idioventricular escape vs VT: rate and clinical context matter — slow wide escape in complete block is not the same as fast monomorphic VT.

Field Pitfalls

• Staring at the strip while the patient is profoundly bradycardic and hypotensive without starting TCP readiness.
• Assuming all second-degree blocks are “benign Wenckebach.”
• Missing complete block because occasional P waves “look related” by chance.
• Forgetting to remove or note patches/pads placement issues that create artifact mimicking dropped beats.

Practice

Open CardioStrip and run the three questions out loud on each block type. Then add one sentence: stable or unstable, and what you would prepare next per your protocol.

Sources & Further Study

• CardioStrip teaching strips (simulated).
• Paramedic / ACLS AV block and bradycardia education; your pacing and atropine protocols.