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AV Blocks: Three-Question Path

12 min Cardiac & ECG Skip to quiz

Objective: Use three questions to separate first-degree, Mobitz I, Mobitz II, and complete heart block. Check a CardioStrip example for each. Then treat perfusion, not the label alone.

Why this matters

AV block means atrioventricular block. All of these strips look like something is wrong between the P wave and the QRS. Under stress they are easy to mix up. Use three questions. Four memorized pictures will not sort them out on scene.

Name the strip, then check the patient. Feel the pulse, look at perfusion, and ask about symptoms. Follow your bradycardia protocol. Mobitz II and complete block need more urgency. Do not treat the label while the patient is crashing.

The 3-Question Path

  1. Does every P wave get a QRS? If yes, decide only whether the PR interval is long or normal. A long PR is first-degree block. If no, some beats drop, or the P waves do not conduct. Go to question 2.
  2. On beats that conduct, does the PR get longer or stay fixed? If the PR gets longer and then a QRS drops, that is Mobitz I, also called Wenckebach. If the PR stays the same and then a QRS drops suddenly, that is Mobitz II. If nothing conducts in a steady way, go to question 3.
  3. Do the atria and the ventricles run on separate rhythms? The P waves are regular and the QRS complexes are regular, but there is no real PR relationship. P waves march through the QRS and the T wave. That is third-degree, or complete, heart block.

Clinical sequence

  1. Print or freeze a clean strip. Motion on Lead II is not a block.
  2. Run the three questions. Does every P wave get a QRS? Does the PR get longer, or stay fixed? Are there two separate rhythms?
  3. Check the pulse, perfusion, and symptoms. Do not treat the degree of block by itself.
  4. If this is Mobitz II or complete block with poor perfusion, put the pads on and get ready to pace. Atropine often fails when the block is below the AV node. Do not wait on more atropine if the patient is unstable.

1. First-Degree AV Block

Every beat still gets through. It is just late. The PR is longer than normal, and it stays long on every beat.

CardioStrip example: first-degree AV block
CardioStrip example · 1° AV block. Teaching strip (education only).
Look for
  • A P wave comes before every QRS. The ratio is 1 to 1.
  • The PR is longer than about one large box. The teaching range is above 200 ms. It stays the same on every beat.
  • No QRS is dropped.

How to say it: “Sinus rhythm with first-degree AV block.” Name the base rhythm and the delay.

2. Mobitz I (Wenckebach)

The PR gets longer, then longer, then a P wave has no QRS. You often see groups of beats and a pause.

CardioStrip example: Mobitz I
CardioStrip example · Mobitz I. Watch the PR get longer before the pause.
Look for
  • Some P waves are followed by a dropped QRS.
  • The PR gets longer on the beats that conduct.
  • Beats that come in groups are a common clue.

Say what you see. The PR gets longer, then a QRS is missing.

3. Mobitz II

The beats that conduct share the same PR. Then a QRS disappears without warning. The PR does not get longer first.

CardioStrip example: Mobitz II
CardioStrip example · Mobitz II. Constant PR, sudden drop.
Look for
  • A QRS drops.
  • The PR is the same on every beat that conducts.
  • The QRS may be wide. Still use the PR pattern to tell this from Mobitz I.

The PR stays fixed, then a QRS is missing. Treat this as more likely to worsen than classic Wenckebach. Match care to the patient and the protocol.

When every other P wave conducts, you may not see the PR get longer. That is 2:1 block. Label it carefully and use the clinical picture. Do not force a Mobitz I or Mobitz II name you cannot see.

4. Third-Degree (Complete) Block

The atria run on one rhythm. The ventricles run on a slow backup rhythm. P waves march through the QRS and the T waves.

CardioStrip example: third-degree block
CardioStrip example · 3° block. No true PR link.
Look for
  • The P-to-P interval is regular. The R-to-R interval is often regular too, and the two intervals are unrelated.
  • The PR intervals look random, because there is no true conduction.
  • The escape QRS may be narrow, from the junction, or wide, from the ventricle.

You are seeing two rhythms, and they are not related.

Prehospital Priorities

  • Patient first: check the pulse and perfusion on every slow or blocked strip. First-degree block in a talking, well-perfused patient is not the same as complete block with low blood pressure.
  • Support the airway, breathing, and circulation. Give oxygen if the sat is low. Get IV access, keep the monitor on, and get a 12-lead when you can.
  • Symptomatic bradycardia: use atropine, get TCP ready, and use other drugs only if your protocol allows them. TCP means transcutaneous pacing. Atropine often fails when the block is below the AV node. That is the teaching for Mobitz II and complete block. Get pacing ready early. Do not wait on more atropine if the patient is unstable.
  • Higher-grade blocks: for Mobitz II and third-degree block, get pacing ready and choose the hospital even if the patient looks okay right now.
  • Unstable complete block: do not delay pacing to debate the strip. If the patient is awake, give sedation and pain control for pacing only if protocol allows it.
  • Look for the cause while you treat: ischemia, an inferior heart attack, beta-blockers, calcium-channel blockers, digoxin, electrolytes, and a recent arrest. Tell the ED the history.

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 about 30, hypotensive — pacing now.”

Common Mix-Ups

  • Sinus arrhythmia versus Wenckebach: you need a lengthening PR plus a dropped QRS. A changing rate alone is not enough.
  • Mobitz I versus II: a PR that gets longer is Mobitz I. A fixed PR plus a sudden drop is Mobitz II. That difference matters most.
  • Complete block versus “lots of P waves”: map several cycles. True dissociation has a PR that looks random and never settles.
  • 2:1 second-degree block: it is hard to call Mobitz I versus II because you never see two PR intervals in a row. Treat the patient. Write “2:1 AV block.” Get ready for a higher-grade block.
  • Slow wide escape versus VT: VT means ventricular tachycardia. Rate and the clinical picture matter. A slow wide escape in complete block is not fast monomorphic VT.

Common errors

  • Staring at the strip while the patient is very slow and hypotensive, and not getting TCP ready.
  • Assuming every second-degree block is harmless Wenckebach.
  • Missing complete block because a few P waves look related by chance.
  • Forgetting that patches and pad placement can create artifact that looks like dropped beats. Move them or write it down.

Practice

One-minute check

In CardioStrip Learn mode, open each block type and run the three questions out loud. End with one sentence. Say whether the patient is stable or unstable, and whether you are monitoring or getting pacing ready, per protocol.

Related: Symptomatic Bradycardia, Systematic ECG Approach.

Sources & Further Study

Named guidelines for further study. They are not clinical orders and do not replace your protocol or medical direction.

Education only. Apply learning within your protocols and medical direction. This is not clinical care guidance. Strips are simulated teaching examples, not patient recordings.

Check Your Understanding

Questions drawn only from this lesson. After you check, the key is highlighted. Education practice — not a certification exam.

1. First-degree AV block is best described as:
2. Mobitz I (Wenckebach) signature is:
3. Mobitz II is suggested when:
4. Third-degree (complete) heart block means:
5. In the 3-question path, if every P gets a QRS, you next ask about:
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