AV Block Cheat Sheet: 1st, Mobitz I vs II, 2:1 & 3rd Degree

AV block is delayed or failed conduction from the atria to the ventricles, graded by two questions: what is the PR interval doing, and does every P wave reach the ventricles? First-degree block is a PR over 200 ms with every P wave conducted; Mobitz I (Wenckebach) stretches the PR until a QRS drops and usually sits in the AV node; Mobitz II drops a QRS without warning after constant PR intervals and sits below the node (His-Purkinje); in third-degree block no P wave conducts and an escape rhythm runs the ventricles. The 2018 ACC/AHA/HRS bradycardia guideline, still the current US guideline, recommends a permanent pacemaker for Mobitz II, high-grade and third-degree AV block not caused by a reversible or physiologic problem, regardless of symptoms.

The one-page table

Read any strip in five questions

1. Is every P wave followed by a QRS? If yes and the PR is longer than one large box (5 small boxes, 200 ms at standard paper speed), it is first-degree block. Normal values are in the normal ECG intervals breakdown.
2. Some P waves have no QRS? Compare the PR just before and just after the pause. Longest before, shortest after: Mobitz I. Identical: Mobitz II.
3. Only 2:1 conduction? With no two conducted beats in a row, the type can't be named; use QRS width and other strips.
4. Two or more P waves in a row blocked, but some still conduct? That is high-grade AV block.
5. Nothing conducts? PR intervals vary at random, P waves outnumber QRS complexes: third-degree block. Then judge the escape rhythm by its QRS width and rate.

Level of block decides the prognosis

Each block in brief: what the table can't hold

First-degree AV block

• No P wave is actually blocked, which is why the guideline prefers "first-degree AV delay." Common: about 1-1.5% of people until age 60, 6% after, and more than 10% of young athletes (StatPearls). Vagal tone explains it in the young and conduction-system fibrosis in the old; AV nodal blockers (beta blockers, verapamil, diltiazem, digoxin, amiodarone) are the classic drug cause.
• A PR over 300 ms ("marked") can cost AV synchrony and cause exertional dyspnea and fatigue (pseudo-pacemaker syndrome). Pacing is reasonable only when symptoms are clearly due to the block.

Mobitz I and Mobitz II

• Mobitz I with a narrow QRS is nearly always in the AV node. With a wide QRS (over 120 ms), 60-70% are in the His-Purkinje system (Merck), which changes management.
• Mobitz I is often physiologic in young, athletic people and during sleep, and rarely progresses suddenly. Pacing an asymptomatic nodal Mobitz I is a Class III (harm) recommendation.
• Mobitz II sits in the His bundle in 25% and the bundle branches in the rest (Merck), so its QRS is usually wide, often with a bundle branch block. In patients aged 18-60 with new Mobitz II or complete block and no obvious structural disease, up to one-third have cardiac sarcoidosis (Merck).

2:1 and high-grade block

• The guideline is explicit that 2:1 block "cannot be classified as Mobitz I or II." A narrow QRS, Wenckebach cycles elsewhere on the strip and better conduction with exercise or atropine point to the node. A wide QRS or bundle branch block (though 15-20% of these are still nodal) and worse conduction with exercise point below it. An electrophysiology study can settle the level.
• High-grade block (2 or more consecutive blocked P waves at a normal rate, such as 3:1) is generally within or below the His bundle and is paced.

Third-degree (complete) AV block

• Exam signs of AV dissociation: cannon a waves in the jugular veins, and beat-to-beat changes in S1 loudness and blood pressure.
• Causes beyond the table: amyloidosis, valve surgery, TAVR, and congenital block from maternal anti-Ro/SSA and anti-La/SSB antibodies (maternal lupus; see SLE).

MI and AV block: inferior vs anterior

Guideline rules after MI: wait before deciding on a permanent pacemaker; then pace persistent Mobitz II, high-grade, alternating bundle branch or third-degree block (persistent or infranodal); and do not pace transient block that resolves. ECG localization of the infarct is in the STEMI patterns post.

Reversible causes: fix them before a pacemaker

• Lyme carditis (about 1 in 100 Lyme cases, CDC): usually nodal block that can vary in degree and change rapidly. Severe carditis (symptoms, PR ≥300 ms, or second- or third-degree block) means admission and IV ceftriaxone 2 g daily, then oral therapy once high-grade block resolves, 14-21 days in total. About 40% need temporary pacing; permanent block is rare. More in the Lyme disease breakdown.
• Drug toxicity: digoxin (anti-digoxin Fab if severe), beta blocker overdose (glucagon may help), calcium channel blockers. Block on stable therapeutic doses usually does not reverse when the drug is stopped.
• Others: hyperkalemia, severe hypothyroidism, high vagal tone (sleep apnea, athletes).

Guideline rule: treat a reversible cause first, with temporary transvenous pacing if needed, and do not implant a permanent pacemaker after complete resolution.

Unstable bradycardia from AV block: the acute ladder

1. Atropine 1 mg IV every 3-5 minutes, up to 3 mg (2025 AHA bradycardia algorithm). It acts on the AV node, so it helps nodal block, is unlikely to help infranodal block (use it cautiously when the QRS is wide), and is ineffective in a transplanted heart without vagal reinnervation.
2. If atropine fails: transcutaneous pacing and/or a dopamine or epinephrine infusion (isoproterenol is another option); the 2018 guideline reserves beta-agonists for patients unlikely to have coronary ischemia.
3. Temporary transvenous pacing for refractory block, then decide on a permanent device.

How USMLE tests it

• 2:1 can't be classified, so choose the answer that localizes it.
• Blocked premature atrial complex mimics Mobitz II, but its P wave comes early (often hidden in the T wave). Mobitz II needs P waves at a steady rate.
• AV dissociation is not always complete block. In complete block the atria beat faster than the ventricles; dissociation with a faster junctional or ventricular rhythm is a different problem.
• Atropine for a wide-QRS infranodal block is the wrong answer when pacing is available.
• Young patient, summer, erythema migrans, fluctuating block: Lyme carditis, treated with antibiotics and, if needed, temporary pacing, not an immediate permanent pacemaker.
• Asymptomatic Mobitz I in an athlete: reassurance. Pacing is harm.

Practice question

A 74-year-old man has had two episodes of near-syncope in the past week. He takes no medications and has no tick exposure. He is alert, with pulse 54/min and blood pressure 132/74 mm Hg. ECG shows sinus P waves at a regular rate of 78/min. Most P waves conduct with a constant PR interval of 0.17 seconds, but every third or fourth P wave is suddenly not followed by a QRS complex; the PR interval is identical before and after each blocked P wave. QRS duration is 0.14 seconds with a right bundle branch block pattern. Potassium and TSH are normal. Which is the most appropriate next step?

A. Reassurance and a repeat ECG in 6 months B. 30-day ambulatory ECG monitoring before deciding on therapy C. IV atropine, with discharge if the heart rate improves D. Permanent pacemaker implantation E. Oral theophylline

Answer: D. A constant PR with suddenly blocked P waves at a steady atrial rate, plus a wide QRS, is Mobitz II: infranodal block that can progress without warning to complete heart block with an unreliable ventricular escape. With no reversible cause, permanent pacing is a Class I recommendation, with or without symptoms.

• A: reassurance suits first-degree block or asymptomatic nodal Mobitz I, not infranodal block.
• B: long-term monitoring helps link intermittent symptoms to nodal block. Mobitz II already meets the pacing indication, so waiting only adds risk.
• C: atropine works on the AV node and is unlikely to improve infranodal block; even a brief response would not remove the need for a pacemaker.
• E: aminophylline and theophylline have a limited role in AV nodal block during acute inferior MI. They do not treat degenerative His-Purkinje disease.

Key takeaways

• 1st degree: PR >200 ms with 1:1 conduction; usually benign and untreated.
• Mobitz I: PR lengthens until a beat drops; AV node; physiologic in athletes and sleep; inferior MI. No pacemaker unless symptoms are clearly caused by the block.
• Mobitz II: constant PR, sudden drop, usually wide QRS; His-Purkinje; pacemaker regardless of symptoms.
• High-grade and third-degree block (AV dissociation, atrial rate > ventricular rate, cannon a waves) get a pacemaker unless the cause is reversible.
• Reversible causes first: Lyme carditis, drug toxicity, hyperkalemia and transient post-MI block get treatment and, if needed, temporary pacing.
• Atropine for nodal block, pacing for infranodal block. The full ECG approach is in How to Read an ECG.

Sources

• Atrioventricular Block — Merck Manual Professional Edition (2024)
• Atrioventricular Block — StatPearls, NCBI Bookshelf (2024)
• First-Degree Heart Block — StatPearls, NCBI Bookshelf (2023)
• Second-Degree Atrioventricular Block — StatPearls, NCBI Bookshelf (2023)
• Third-Degree Atrioventricular Block — StatPearls, NCBI Bookshelf (2023)
• Atrioventricular Node — StatPearls, NCBI Bookshelf (2026)
• 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay, full text — Heart Rhythm (2019), copy hosted by STS
• 2018 ACC/AHA/HRS Guideline on Bradycardia and Cardiac Conduction Delay: Key Points — American College of Cardiology (2018)
• Clinical Care and Treatment of Lyme Carditis — CDC (2026)
• Atropine — StatPearls, NCBI Bookshelf (2025)
• Adult Bradycardia With a Pulse Algorithm — American Heart Association (2025)