ECG Parameters for Atrioventricular Block
The key ECG parameters for assessing AV block are: PR interval (>200 ms defines first-degree), P wave-to-QRS relationship (determines second-degree subtypes), number of consecutive non-conducted P waves (≥2 defines high-grade), presence or absence of any AV conduction (defines third-degree), QRS duration (narrow vs wide localizes block level), and ventricular rate during complete block (<50 bpm and regular suggests complete AV block in atrial fibrillation). 1
First-Degree AV Block (AV Delay)
- PR interval >200 ms with 1:1 AV conduction
- Every P wave conducts to the ventricles, just with delay
- When PR interval exceeds 300 ms, consider "pseudo-pacemaker syndrome" with loss of AV synchrony 1
Second-Degree AV Block
Mobitz Type I (Wenckebach)
- Progressive PR prolongation before a dropped QRS
- P waves at constant rate (<100 bpm)
- Creates "group beating" pattern on ECG
- Inconstant PR intervals in conducted beats 2
Mobitz Type II
- Constant PR intervals in all conducted beats
- Periodic single non-conducted P wave without preceding PR prolongation
- Excludes 2:1 block (cannot be classified as Type I or II) 2
- Almost always indicates infranodal disease 3
2:1 AV Block
- Every other P wave conducts
- Cannot be classified as Mobitz I or II from surface ECG alone
- Wide QRS + normal PR suggests infranodal disease
- Narrow QRS + prolonged PR suggests AV nodal disease 4
- May show ventriculophasic sinus arrhythmia 2
High-Grade (Advanced) AV Block
- ≥2 consecutive non-conducted P waves at physiologic atrial rate
- Some evidence of AV conduction still present (distinguishes from complete block)
- Generally indicates intra- or infra-Hisian disease requiring pacing 1
Third-Degree (Complete) AV Block
- No AV conduction whatsoever
- Complete AV dissociation with independent atrial and ventricular rhythms
- Associated with junctional (40-60 bpm) or ventricular (<40 bpm) escape rhythm 5
- In atrial fibrillation: slow (<50 bpm) and regular ventricular response suggests complete block 1
Critical Localizing Parameters
QRS Duration
- Narrow QRS (<120 ms): Block typically at AV node level
- Wide QRS (≥120 ms): Block typically infranodal (His bundle or below) 4
- Infranodal block has worse prognosis with unpredictable ventricular escape 1
Escape Rhythm Characteristics
- 40-60 bpm with narrow QRS matching pre-block morphology: AV junctional origin
- <40 bpm or wide QRS with different morphology: Ventricular origin 5
Important Diagnostic Pitfalls
Careful ECG evaluation is mandatory to avoid misdiagnosis 1:
- Isorhythmic dissociation: Atrial and ventricular rates similar but not related—mimics 1:1 conduction
- Atrial bigeminy: Premature atrial contractions may be blocked, mimicking second-degree block
- Accelerated junctional rhythm with sinus bradycardia: Ventricular rate exceeds atrial rate—not true block
- Pseudo-AV block: ECG artifact from signal saturation can mimic conduction abnormalities 6
Additional Predictive Parameters
Recent research identifies PR segment duration, QRS duration, and age as key predictors of progression to higher-grade block in patients with first-degree AV block 7. The RR interval, P duration, and QT interval provide supplementary risk stratification data.
Vagally-Mediated Block
During tilt-table testing or neurocardiogenic syncope, AV block shows distinctive features 8:
- Preceded by sinus slowing with unstable, slow sinus rhythm
- PR interval prolongation before block onset
- Multiple AV block forms may coexist (Mobitz I, 2:1, advanced, complete)
- Junctional escape rhythm present in 61% of cases
The anatomic level of block determines prognosis and treatment urgency: AV nodal block responds to atropine and has reliable junctional escape, while infranodal block requires pacing due to unreliable ventricular escape and risk of sudden progression to complete block 1.