What are the ECG criteria for the different types of atrioventricular (AV) block and how should each be initially managed?

AV Blocks: ECG Detection and Initial Management

First-Degree AV Block

First-degree AV block is defined by a PR interval >200 ms with 1:1 AV conduction and generally requires no treatment in asymptomatic patients. 1

ECG Criteria

• PR interval >200 ms (>0.20 seconds) 1
• Every P wave conducts to the ventricles (1:1 conduction) 1
• Narrow QRS suggests AV nodal delay; wide QRS may indicate His-Purkinje involvement 1

Initial Management

• No treatment required for asymptomatic patients with PR <300 ms 2
• Identify reversible causes: β-blockers, calcium-channel blockers, digoxin, antiarrhythmics, electrolyte abnormalities, Lyme disease, myocarditis, acute MI 3, 2
• Permanent pacemaker is reasonable (Class IIa) when PR ≥300 ms causes symptoms resembling pacemaker syndrome (fatigue, exercise intolerance, dyspnea) or hemodynamic compromise 2
• Do not pace asymptomatic patients with PR <300 ms—this is potentially harmful (Class III) 2

Critical Pitfalls

• Wide QRS with first-degree AV block suggests infranodal disease with worse prognosis and warrants further evaluation 1, 2
• Exercise-induced progression to second-degree block indicates His-Purkinje disease requiring permanent pacing 2
• Neuromuscular diseases (myotonic dystrophy, Kearns-Sayre syndrome) carry unpredictable progression risk; consider prophylactic pacing (Class IIb) 2

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Second-Degree AV Block

Mobitz Type I (Wenckebach)

Mobitz type I with narrow QRS is usually benign AV nodal block; wide QRS mandates electrophysiology study to exclude dangerous infranodal disease. 4

ECG Criteria

• Progressive PR interval prolongation before a blocked P wave 1
• PR interval is longest immediately before the block and shortest immediately after 1
• Constant P-wave rate <100 bpm 1
• Inconstant PR intervals distinguish this from Mobitz type II 1

Initial Management Based on QRS Width

Narrow QRS (<120 ms):

• Usually AV nodal level block with benign prognosis 1, 4
• Observation without pacing for asymptomatic patients 4
• Exclude reversible causes: drugs, electrolytes, vagal tone, sleep apnea 4
• Permanent pacing indicated if symptomatic (syncope, presyncope, dizziness, exertional fatigue) 4

Wide QRS (≥120 ms):

• Raises suspicion for intra-Hisian or infra-Hisian block 1, 4
• Electrophysiology study mandatory to localize block site 4
• Permanent pacing required if infranodal block confirmed, even if asymptomatic 4

Special Scenarios

• Asymptomatic elderly with daytime Wenckebach: Permanent pacing improves survival 1, 4
• Isolated nocturnal Wenckebach: Observation appropriate; no pacing needed 4
• Drug-induced block: Discontinue offending agent; observe for resolution 4

Mobitz Type II

Mobitz type II is almost always infranodal (His-Purkinje) block and requires permanent pacing due to high risk of progression to complete heart block. 1, 5

ECG Criteria

• Constant PR intervals before and after blocked P waves 1
• Periodic single nonconducted P wave 1
• Usually associated with wide QRS (bundle branch block) 1, 5
• Constant P-wave rate <100 bpm 1

Initial Management

• Permanent pacemaker indicated regardless of symptoms 1
• Block is within or below His bundle in >90% of cases 1
• Untreated chronic Mobitz type II has poor prognosis with frequent progression to higher-grade block and syncope 1
• Do not delay pacing while awaiting symptoms—progression is unpredictable 1, 5

2:1 AV Block

2:1 AV block cannot be classified as type I or type II; QRS width and clinical context determine management. 1

ECG Criteria

• Every other P wave conducts to ventricles 1
• Constant or near-constant P-wave rate <100 bpm 1

Initial Management

• Narrow QRS: Likely AV nodal; manage as Mobitz type I 1
• Wide QRS: Likely infranodal; manage as Mobitz type II with permanent pacing 1
• Electrophysiology study may be needed to definitively localize block 1

Advanced (High-Grade) AV Block

ECG Criteria

• ≥2 consecutive P waves blocked at constant physiologic rate 1
• Evidence of some AV conduction present (distinguishes from third-degree) 1

Initial Management

• Permanent pacemaker indicated in most cases 1
• High risk of progression to complete heart block 1
• Treat as third-degree block if hemodynamically unstable 3

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Third-Degree (Complete) AV Block

Third-degree AV block is a cardiovascular emergency requiring immediate stabilization and permanent pacemaker implantation after reversible causes are excluded. 3, 6

ECG Criteria

• Complete AV dissociation: no atrial impulses conduct to ventricles 1
• Independent atrial and ventricular rhythms 6
• Narrow QRS escape (40-60 bpm): AV nodal or high His-Purkinje origin 3, 6
• Wide QRS escape (20-40 bpm): Ventricular origin; more unstable 3, 6

Immediate Stabilization

Hemodynamically Unstable (hypotension, altered mental status, syncope, chest pain, heart failure):

1. Apply transcutaneous pacing pads immediately while completing assessment 3
2. Initiate transcutaneous pacing without delay for wide-QRS escape or unstable patients 3
3. Do not postpone pacing to give atropine in unstable patients 3

Narrow-QRS Escape (AV Nodal Block):

• Atropine 0.5-1 mg IV bolus, repeat every 3-5 minutes up to total 3 mg 3
• Avoid doses <0.5 mg—may paradoxically worsen block via central vagal stimulation 3
• Atropine is ineffective for wide-QRS (infranodal) blocks—do not waste time 3

Refractory Bradycardia:

• β-adrenergic agonists (dopamine 5-10 µg/kg/min, dobutamine, epinephrine, isoproterenol) only when coronary ischemia unlikely and pacing unavailable 3
• Temporary transvenous pacing for persistent symptoms despite medical therapy 3
• Aminophylline IV may be considered in acute inferior MI 3

Exclude Reversible Causes Before Permanent Pacing

Mandatory evaluation for:

• Acute myocardial infarction 3
• Drug toxicity: β-blockers, calcium-channel blockers, digoxin 3
• Electrolyte disturbances (hyperkalemia, hypomagnesemia) 3
• Lyme carditis 3
• Myocarditis 3
• Thyroid disorders 3
• Infiltrative diseases (sarcoidosis, amyloidosis) 3

If reversible cause identified:

• Treat underlying condition 3
• Provide temporary pacing (transcutaneous or transvenous) as bridge 3
• Do not implant permanent pacemaker if block resolves (Class III—Harm) 3

Indications for Permanent Pacemaker (Class I)

Permanent pacing is mandatory for:

• Third-degree AV block at any anatomic level after reversible causes excluded 3
• Any symptomatic bradycardia (syncope, heart failure, ventricular arrhythmias) at any heart rate 3
• Asymptomatic patients with high-risk features: 3
  • Documented asystole ≥3 seconds
  • Escape rate <40 bpm
  • Escape rhythm below AV node (wide QRS)
  • Atrial fibrillation with pauses ≥5 seconds
• Third-degree AV block requiring medications that cause symptomatic bradycardia 3
• Persistent third-degree AV block post-MI after observation period 3

Post-Cardiac Arrest Considerations

• Mandatory observation period before permanent pacing decision because block may be transient 3
• Temporary transvenous pacing as bridge while observing 3
• Permanent pacing required if second-degree Mobitz II, high-grade, or third-degree block persists after observation 3

Critical Pitfalls

• Do not discharge asymptomatic patients with escape rate <40 bpm, ventricular escape rhythm, or pauses ≥3 seconds without pacemaker 3
• Do not assume benignity based on age alone—definitive evaluation required regardless of patient age 3
• Do not rely on atropine for infranodal blocks—effect limited to AV nodal conduction 3
• Do not implant pacemaker for asymptomatic vagally mediated AV block (Class III—Harm) 3
• Infranodal blocks progress rapidly and unpredictably—continuous monitoring until pacemaker implanted 3

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Summary Algorithm for AV Block Management

AV Block Type	QRS Width	Symptoms	Management
First-degree	Any	None, PR <300 ms	Observation; no pacing [2]
First-degree	Any	Symptomatic, PR ≥300 ms	Permanent pacing (Class IIa) [2]
Mobitz I	Narrow	None	Observation [4]
Mobitz I	Narrow	Symptomatic	Permanent pacing [4]
Mobitz I	Wide	Any	Electrophysiology study → pacing if infranodal [4]
Mobitz II	Usually wide	Any	Permanent pacing [1,5]
2:1 block	Narrow	None	Observation or EP study [1]
2:1 block	Wide	Any	Permanent pacing [1]
Third-degree	Narrow escape	Unstable	Atropine → transcutaneous pacing → permanent pacing [3]
Third-degree	Wide escape	Any	Immediate transcutaneous pacing → permanent pacing [3]
Third-degree	Any	Stable, reversible cause	Treat cause, temporary pacing, observe [3]
Third-degree	Any	Stable, no reversible cause	Permanent pacing (Class I) [3]