How should a second-degree atrioventricular (AV) block on an ECG be evaluated and managed?

Second-Degree AV Block: ECG Evaluation and Management

Second-degree AV block requires immediate classification into Mobitz Type I (Wenckebach) versus Mobitz Type II based on PR interval behavior, QRS width, and clinical context, because Type II block is almost always infranodal and mandates pacemaker implantation regardless of symptoms, while Type I with narrow QRS is typically benign AV nodal block. 1

ECG Classification Algorithm

Mobitz Type I (Wenckebach) Block

• Progressive PR interval prolongation before the blocked P wave is the defining characteristic 1
• The PR interval shortens after the dropped beat, then the cycle repeats 2
• With narrow QRS complexes (<120 ms), the block is almost always at the AV node level and carries a benign prognosis 1
• With wide QRS complexes (bundle branch block), the block may be infranodal in 60-70% of cases and requires further evaluation 3, 2

Mobitz Type II Block

• Constant PR intervals before and after the blocked P wave is the sine qua non for diagnosis 3, 4
• An unchanged PR interval after the block is mandatory—if the first post-block PR shortens, Type II is excluded 3, 2
• Type II block is invariably infranodal (His-Purkinje system) and requires permanent pacemaker implantation even without symptoms 3, 4, 2
• Most commonly presents with bundle branch block pattern 1

2:1 AV Block

• Cannot be classified as Type I or Type II because there is no opportunity to observe PR interval behavior 1
• Requires assessment of QRS width, clinical context, and often electrophysiology study to determine the site of block 3, 2

Advanced (High-Grade) AV Block

• Multiple consecutive P waves are blocked, but complete AV dissociation is not present 1
• Generally indicates infranodal disease requiring pacemaker 1

Critical Diagnostic Pitfalls to Avoid

Pseudo-AV Block

• Concealed His bundle or ventricular extrasystoles can mimic both Type I and Type II block without true conduction disease 3, 2
• These concealed beats remain confined to the specialized conduction system without myocardial penetration 2
• Electrophysiology study may be needed to exclude pseudo-block 1

Vagal Surge Mimicking Type II

• Simultaneous sinus slowing with AV nodal block can superficially resemble Type II block but is benign 3, 4, 2
• Absence of sinus slowing is an important criterion for diagnosing true Type II block 3, 4
• Type II block has never been reported in inferior MI or young athletes, where vagal Type I block may be misinterpreted 3

Atypical Wenckebach

• A series of constant PR intervals before a block can occur in atypical Wenckebach and be misdiagnosed as Type II 4
• If a pattern resembling narrow QRS Type II coexists with obvious Type I structure on the same recording (e.g., Holter), Type II is effectively ruled out because coexistence is exceedingly rare 3, 2

Anatomic Localization and Prognosis

Narrow QRS Complex (<120 ms)

• Type I block with narrow QRS is almost always AV nodal and has a benign prognosis 1, 2
• The clinical course depends on underlying heart disease severity rather than the conduction abnormality itself 1
• Type II block with narrow QRS is exceedingly rare—always consider pseudo-block or atypical Wenckebach 3, 2

Wide QRS Complex (≥120 ms)

• Type I block with bundle branch block is infranodal in 60-70% of cases (except in acute MI where it may be nodal) 3, 2
• Type II block is usually infranodal and most often presents with bundle branch block 1
• Untreated chronic infranodal second-degree AV block has poor prognosis—patients frequently progress to complete heart block and syncope 1

Intra-Hisian Block

• Prognosis is uncertain but patients frequently manifest congestive heart failure and syncope 1
• Requires electrophysiology study for definitive diagnosis 1

Indications for Electrophysiology Study

Class I (Strongly Indicated)

• Symptomatic patients in whom His-Purkinje block is suspected but not established 1
• Patients with pacemaker who remain symptomatic and another arrhythmia is suspected 1

Class II (Reasonable)

• Patients with second-degree AV block in whom knowledge of the site of block would direct therapy or assess prognosis 1
• Suspected pseudo-AV block from concealed junctional depolarizations 1

Class III (Not Indicated)

• Symptomatic patients in whom symptoms and AV block are already correlated by ECG 1

Management Algorithm

Immediate Assessment

• Evaluate for reversible causes: medications (beta-blockers, calcium channel blockers, digoxin, antiarrhythmics), electrolyte abnormalities, Lyme disease, acute MI, myocarditis 5
• Assess hemodynamic stability: hypotension, altered mental status, chest pain, heart failure 5
• Determine symptom correlation: syncope, presyncope, fatigue, dyspnea 1

Type I (Wenckebach) with Narrow QRS

• Asymptomatic patients require no specific treatment—observation and monitoring for progression 1
• Treat reversible causes if identified 5
• Permanent pacing is NOT indicated unless symptoms develop 1

Type I with Wide QRS (Bundle Branch Block)

• Consider electrophysiology study to determine if block is infranodal 1
• If infranodal, permanent pacemaker is indicated regardless of symptoms 3, 2
• If nodal, manage as narrow QRS Type I 2

Type II Block (Any QRS Width)

• Permanent pacemaker implantation is mandatory even without symptoms because Type II is invariably infranodal with high risk of progression to complete heart block 3, 4, 2
• Do not delay pacing while awaiting symptoms—sudden progression to complete block can cause Stokes-Adams attacks 6

2:1 AV Block

• Assess QRS width: narrow QRS suggests nodal (better prognosis), wide QRS suggests infranodal (worse prognosis) 1
• Consider electrophysiology study to definitively localize the site of block 1
• If infranodal, proceed with permanent pacemaker 2

Advanced/High-Grade Block

• Generally requires permanent pacemaker due to high likelihood of infranodal disease 1

Special Clinical Contexts

Acute Myocardial Infarction

• Inferior MI: AV block is usually vagally mediated, AV nodal, and may be transient 6, 3
• Anterior MI: AV block suggests extensive septal necrosis, is usually infranodal, and has poor prognosis 6
• Type II block has not been reported in inferior MI—apparent Type II in this setting is likely vagal Type I 3

Sleep and Sleep Apnea

• AV block during sleep apnea is reversible and does not require pacing unless symptomatic when awake 5
• Asymptomatic sinus bradyarrhythmias or pauses during sleep are benign 1

Drug-Related Block

• If block is clearly drug-related and the medication can be discontinued, permanent pacing is not indicated 5
• Reassess conduction after drug washout 5

Neuromuscular Diseases

• Patients with myotonic dystrophy, Kearns-Sayre syndrome, or similar conditions have unpredictable progression risk 5
• Consider prophylactic pacing even for asymptomatic second-degree block in this population 5