AV blocks are one of those ECG topics that look intimidating until you realize the whole game is just two questions: (1) What’s the PR doing? and (2) Are P waves conducting to QRS complexes? This one-page cheat sheet is built for rapid recognition on USMLE Step 1/2—especially when you’re tired and the answer choices are trying to bait you with “Mobitz I vs II.”
The 10-second AV block algorithm (exam-proof)
Ask these in order:
- Is there a PR interval?
- Is the PR consistently prolonged?
- Are any QRS complexes dropped?
- If dropped, does PR lengthen before the drop?
- Are P waves and QRS complexes dissociated?
If you can answer those, you can diagnose every AV block on exams.
“Visual” mnemonic device: the PR Ladder
Think of the PR interval as a ladder you climb toward a fall:
- 1st degree: Long ladder, but you never fall (no dropped beats)
- Mobitz I: Ladder gets longer… then you fall (progressive PR → drop)
- Mobitz II: Ladder stays the same… you suddenly fall (fixed PR → drop)
- 3rd degree: Two separate ladders (P and QRS don’t talk)
One-page table: AV blocks at a glance
| Block | Key ECG pattern | One-liner | Typical level of block | QRS width | High-yield associations | Treatment (USMLE style) |
|---|---|---|---|---|---|---|
| 1st degree | PR > 200 ms, every P followed by QRS (1:1) | Slow conduction, but all impulses get through | AV node (usually) | Usually narrow | Increased vagal tone, beta blockers, non-DHP CCBs, digoxin; inferior MI | Usually none; address meds/ischemia |
| 2nd degree Mobitz I (Wenckebach) | PR progressively lengthens → dropped QRS (grouped beating) | Tired AV node: delays more, then skips | AV node | Usually narrow | Inferior MI, increased vagal tone | Often benign; atropine if symptomatic |
| 2nd degree Mobitz II | Constant PR with intermittent dropped QRS (often 2:1, 3:1) | All-or-nothing conduction failure | His-Purkinje (below AV node) | Often wide (bundle involvement) | Anterior MI, structural conduction disease | Pacemaker (high risk → complete heart block) |
| 3rd degree (complete heart block) | AV dissociation: P waves and QRS independent; ventricular escape rhythm | Atria and ventricles are marching to different drummers | AV node or infra-nodal | Narrow if junctional escape; wide if ventricular escape | MI (inferior or anterior), Lyme disease, degenerative fibrosis | Temporary pacing → permanent pacemaker; atropine may help if nodal |
The blocks in plain language (with exam triggers)
1st-degree AV block
ECG: PR interval > 200 ms, but every P wave conducts.
- USMLE clue: “Young athlete,” “increased vagal tone,” or medication list (beta blocker, verapamil/diltiazem, digoxin).
- Pearl: Usually asymptomatic; not an emergency.
Memory hook: “1st degree = 1:1, just slow.”
2nd-degree AV block, Mobitz I (Wenckebach)
ECG: PR interval gets longer and longer, then a QRS drops. Pattern repeats (“grouped beats”).
- Mechanism: AV node progressively fatigues.
- USMLE clue: Inferior MI (RCA supplies AV node), or high vagal tone.
- Prognosis: Often benign; can cause dizziness if slow.
Memory hook: “Wenckebach = WAXING PR.”
2nd-degree AV block, Mobitz II
ECG: PR interval is constant, but some P waves don’t conduct → sudden dropped QRS.
- Mechanism: Conduction failure below the AV node (His-Purkinje).
- USMLE clue: Anterior MI (LAD territory) or “wide QRS” with conduction disease.
- Big rule: Mobitz II is dangerous → can progress to complete heart block.
Memory hook: “Type II = Two problems: dropped beats + needs pacing.”
3rd-degree (complete) AV block
ECG: No relationship between P waves and QRS complexes (AV dissociation).
Atrial rate and ventricular rate are independent.
- Escape rhythms:
- Junctional escape (higher, faster) → narrow QRS, ~40–60 bpm
- Ventricular escape (lower, slower) → wide QRS, ~20–40 bpm
- USMLE clue: Syncope, hypotension, cannon A waves, severe bradycardia.
- Classic associations: MI, Lyme disease, degenerative fibrosis.
Memory hook: “3rd degree = 3 separate thoughts: P’s, QRS’s, and symptoms.”
Rapid-fire differentiators (what test writers love)
Mobitz I vs Mobitz II: the “PR question”
- PR changes?
- Mobitz I: PR progressively increases
- Mobitz II: PR constant
- Where is the block?
- Mobitz I: AV node
- Mobitz II: His-Purkinje
- Management implication:
- Mobitz I: usually observe
- Mobitz II: pacemaker
Mini-ECG “text sketches” you can visualize
-
1st degree:
P—(long PR)—QRS | P—(long PR)—QRS | P—(long PR)—QRS -
Mobitz I:
P—(longer)—QRS | P—(longer)—QRS | P—(longest)—(drop) | repeat -
Mobitz II:
P—(same)—QRS | P—(same)—QRS | P—(same)—(drop) | repeat -
3rd degree:
P…P…P…P (regular)
QRS…QRS…QRS (regular but slower)
(no consistent pairing)
High-yield clinical correlations (Step 1 + Step 2)
MI territory associations
- Inferior MI (RCA) → AV node ischemia → Mobitz I (or 1st degree)
- Anterior MI (LAD) → septal/His-Purkinje damage → Mobitz II/complete block
Drugs that slow AV nodal conduction (cause/worsen 1st degree or Mobitz I)
- Beta blockers
- Non-DHP calcium channel blockers (verapamil, diltiazem)
- Digoxin
- (Also: adenosine can transiently block AV node during SVT treatment)
Lyme disease
- Can cause fluctuating degrees of AV block (including complete heart block).
Board move: treat Lyme + manage unstable bradycardia with pacing as needed.
Exam-ready “if unstable” management snapshot
If the stem screams bradycardia + hypotension/AMS/chest pain/shock:
- Think atropine first if nodal block likely, but don’t bet the farm on it in Mobitz II/infra-nodal disease.
- Prepare for temporary pacing, then permanent pacemaker for Mobitz II and complete heart block.
Final takeaways (memorize these 4 lines)
- 1st degree: PR > 200 ms, no dropped beats.
- Mobitz I: PR gets longer → drop (AV node, often benign).
- Mobitz II: PR fixed → drop (His-Purkinje, needs pacemaker).
- 3rd degree: AV dissociation (pace them if symptomatic/unstable).