Second-Degree Heart Block on ECG
Second-degree AV block is identified on ECG by intermittent failure of P waves to conduct to the ventricles and is classified into two critical types: Mobitz Type I (Wenckebach) shows progressive PR interval prolongation before a dropped QRS, while Mobitz Type II shows constant PR intervals before the blocked P wave—this distinction is essential because Mobitz Type II requires permanent pacemaker implantation even in asymptomatic patients. 1, 2, 3
ECG Identification Criteria
Mobitz Type I (Wenckebach)
- Progressive PR interval lengthening until a P wave fails to conduct 1
- The PR increment is often subtle in the last cycles before the block—compare to the shortest PR interval (typically after the blocked P wave) 1
- Usually associated with narrow QRS complexes 4
- Creates a "group beating" pattern on ECG 3
Mobitz Type II
- Constant PR intervals before and after the blocked P wave 1, 5
- Requires stable sinus rate for diagnosis—vagal surges causing simultaneous sinus slowing can mimic Type II block 5, 6
- Usually associated with wide QRS complexes (bundle branch block) 4
- The diagnosis cannot be made if the first post-block P wave shows a shortened PR interval 6, 7
2:1 AV Block
- Every other P wave conducts to the ventricles 3, 8
- Cannot be classified as Type I or Type II based on ECG alone 9, 6
- Requires additional evaluation to determine the level of block
Advanced/High-Grade AV Block
- ≥2 consecutive P waves blocked at physiologic rates without complete AV dissociation 2, 3, 8
- Generally indicates infranodal disease requiring pacing 3
Anatomic Localization
AV Nodal (Proximal) Block
- Mobitz Type I with narrow QRS: almost always AV nodal 1, 7
- Generally benign prognosis 2, 10
- Responds to atropine 3
- Common in athletes and during sleep 2
Infranodal (His-Purkinje) Block
- Mobitz Type II: invariably infranodal 5, 6, 7
- Mobitz Type I with wide QRS: infranodal in 60-70% of cases (except acute MI) 6, 7
- Risk of rapid progression to complete heart block 2, 10
- Does not respond to atropine but may improve with catecholamines 3
Critical Diagnostic Pitfalls
Watch for these mimics that can lead to misdiagnosis:
- Concealed His bundle or ventricular extrasystoles can create pseudo-AV block patterns resembling both Type I and Type II 5, 6, 7
- Vagal surges causing simultaneous sinus slowing and AV nodal block can superficially resemble Mobitz Type II 5, 6
- Atypical Wenckebach with minimal PR changes may be misinterpreted as Type II 11, 5
- A short baseline PR interval with second-degree block suggests infranodal involvement 11
- Coexistence of obvious Type I pattern elsewhere in the same recording (e.g., Holter) effectively rules out Type II block 6, 7
Management Algorithm
Mobitz Type II or Infranodal Block
Permanent pacemaker implantation is indicated even in asymptomatic patients 4, 3
- Class IIa recommendation, particularly with fascicular block 4
- High risk of progression to complete heart block 1, 2
- Requires continuous monitoring until pacemaker placement 2
Mobitz Type I (Narrow QRS)
- Generally does not require pacing unless symptomatic 1
- Monitoring may be considered but generally not required 2
- Exception: If conduction delay occurs below the AV node (wide QRS), treat as infranodal block 1
- Some evidence suggests pacing improves survival in asymptomatic elderly patients with diurnal Type I block 1
2:1 AV Block
- Determine level of block through:
- Treat as infranodal if evidence suggests His-Purkinje involvement
Before Pacemaker Decision
Exclude reversible causes:
- Electrolyte abnormalities 4
- Medications (beta-blockers, calcium channel blockers, digoxin, antiarrhythmics) 3
- Acute myocardial infarction 3
- Lyme carditis, endocarditis 3
- Sleep-related or athletic conditioning (benign) 2, 10
The key principle: Infranodal block requires pacing regardless of symptoms or QRS duration 6, 7, while AV nodal Mobitz Type I generally has a benign prognosis and requires pacing only when symptomatic 1, 2.