Chart — Cardiac
AV Block Comparison Chart
A side-by-side comparison of all four degrees of atrioventricular (AV) block — from the benign first-degree delay to complete dissociation. Use this chart to quickly identify the pattern and understand the clinical implications.
Educational use only. Always correlate ECG findings with clinical presentation, hemodynamic status, and symptoms. Notify the provider for any high-degree block or new symptomatic bradycardia. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.
AV Block Comparison
| Feature | 1st Degree | 2nd Degree Mobitz I | 2nd Degree Mobitz II | 3rd Degree |
|---|---|---|---|---|
| PR Interval | > 0.20 sec, constant | Progressively lengthens | Constant (any length) | Variable — no relationship |
| Dropped QRS? | No | Yes — cyclical | Yes — random, no warning | Complete dissociation |
| P:QRS Ratio | 1:1 | > 1:1 (cyclical groups) | > 1:1 (random drops) | P rate ≠ QRS rate; independent |
| QRS Width | Narrow | Usually narrow | Often wide (infranodal) | Wide (ventricular escape) or narrow (junctional escape) |
| Block Location | AV node | AV node | Bundle of His / bundle branches | AV node or infranodal |
| Escape Rate | None needed | None (transient pause) | None (pauses) | 20–60 bpm (ventricular/junctional) |
| Atropine Effective? | Usually | Usually | No (may worsen) | No |
| Clinical Urgency | Monitor | Low–moderate | High — pacemaker likely | Emergency |
Pattern Recognition Tips
1st Degree — “Long but consistent”
Every P wave conducts. The PR is consistently long (> 0.20 sec) but never changes and no beats are dropped. The rhythm looks like NSR with a delay. Often discovered incidentally on a 12-lead ECG.
Mobitz I — “Longer, longer, longer, drop”
The classic Wenckebach pattern: PR progressively lengthens until a QRS is dropped, then resets and repeats. The grouped beating creates a recognizable visual rhythm on the strip. The R-R interval shortens just before the dropped beat.
Mobitz II — “Constant PR, then suddenly dropped”
No warning — the PR stays exactly the same with each conducted beat, then a QRS simply does not appear after a P wave. There is no Wenckebach pattern. Wide QRS is a red flag for infranodal location. Unpredictability makes this far more dangerous than Mobitz I.
Third-Degree — “Two independent rhythms on one strip”
P waves and QRS complexes march through at their own independent rates, with no relationship. Confirm by “walking out” P waves (they are regular at their own rate) and QRS complexes (regular at their own slower rate). The PR interval varies with each beat because it is entirely random.
Common Causes by Block Degree
| Block | Common Causes |
|---|---|
| First-Degree | Increased vagal tone, beta-blockers, calcium channel blockers, digoxin, inferior MI, electrolyte imbalance, aging |
| Mobitz I | Inferior MI (RCA territory), increased vagal tone, beta-blockers, digoxin toxicity |
| Mobitz II | Anterior MI, structural heart disease, cardiomyopathy, Lyme disease, post-cardiac surgery |
| Third-Degree | Inferior or anterior MI, Lev's/Lenegre's disease (degenerative), Lyme disease, digoxin toxicity, hyperkalemia, post-cardiac surgery |
Related Resources
Standards & sources
Fact-checked Jun 20, 2026This page is written to align with AHA / ACC Cardiology Guidelines. It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →