What are the characteristics and treatment options for each degree of Atrioventricular (AV) block?

Atrioventricular (AV) Block: Characteristics and Management

First-Degree AV Block

First-degree AV block is defined as a PR interval >200 ms and is generally benign, requiring no treatment in asymptomatic patients with PR <300 ms. 1, 2

Key Characteristics

• Represents delayed conduction through the AV node, not true "block" since all atrial impulses conduct to ventricles 1, 3
• PR interval prolonged beyond 200 ms but all P waves conduct 1, 3
• Usually occurs at the AV nodal level with narrow QRS complexes 3

Management Algorithm

For PR interval 200-300 ms (asymptomatic):

• No treatment required; permanent pacemaker is NOT indicated (Class III recommendation) 1, 2
• Regular ECG monitoring and clinical follow-up only 1
• Athletes can participate in all competitive sports unless structural heart disease present 1

For PR interval ≥300 ms (marked first-degree block):

• Assess for symptoms resembling "pacemaker syndrome" (fatigue, exercise intolerance, dyspnea) due to loss of AV synchrony 1, 2, 4
• Consider echocardiogram if QRS abnormal or signs of structural disease 2
• Exercise stress testing reasonable (Class IIa) to assess if PR shortens appropriately with exercise 1, 2
• Permanent pacemaker implantation is reasonable (Class IIa) if symptoms clearly attributable to AV block or hemodynamic compromise present 1, 2

Critical Pitfalls to Avoid

• Do NOT implant pacemakers for isolated, asymptomatic first-degree AV block—this is explicitly contraindicated (Class III) 1, 2
• Exercise caution with AV nodal blocking agents (beta-blockers, diltiazem, verapamil, digoxin, amiodarone) in patients with pre-existing first-degree block 2, 5
• Recognize high-risk scenarios requiring closer monitoring: coexisting bifascicular block, neuromuscular diseases (myotonic dystrophy, Kearns-Sayre syndrome), structural heart disease 1, 2

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Second-Degree AV Block

Second-degree AV block represents intermittent failure of atrial impulses to conduct to the ventricles and is subdivided into two distinct types with different anatomic locations, prognoses, and management strategies.

Mobitz Type I (Wenckebach)

Mobitz Type I is characterized by progressive PR interval prolongation until a P wave fails to conduct, typically occurs at the AV nodal level, and is often benign requiring only monitoring unless symptomatic. 3, 6

Key Characteristics

• Progressive PR prolongation before dropped beat (classic Wenckebach pattern) 3, 7
• Usually associated with narrow QRS complexes (<120 ms) 3, 6
• Block occurs at the AV nodal level (supra-Hisian) in most cases 6, 7
• Often transient and asymptomatic 3
• May be vagally mediated, especially during sleep or in athletes 8

Management

• Asymptomatic patients require observation only; permanent pacing NOT indicated 8
• Identify and correct reversible causes (medications, electrolyte abnormalities, Lyme disease, sleep apnea) 8
• For symptomatic bradycardia: atropine 0.5 mg IV every 3-5 minutes (maximum 3 mg) as first-line treatment 3, 9
• Permanent pacemaker indicated only if symptomatic and not reversible 8

Mobitz Type II

Mobitz Type II is characterized by constant PR intervals with sudden dropped beats, almost always occurs below the AV node (infranodal), carries high risk of progression to complete heart block, and requires permanent pacemaker implantation. 3, 6, 7

Key Characteristics

• Constant PR intervals before and after blocked P waves—no progressive prolongation 3, 6, 7
• Usually associated with wide QRS complexes (≥120 ms) indicating infranodal disease 3, 6, 7
• Block occurs in the His-Purkinje system (infranodal) 6, 7
• High risk of progression to complete heart block and sudden death 6, 7
• Often symptomatic with presyncope or syncope 3

Management

• Permanent pacemaker implantation is indicated (Class I) regardless of symptoms 8
• For acute symptomatic bradycardia: transcutaneous pacing preferred over atropine (atropine unreliable for infranodal block) 3, 9
• Transvenous temporary pacing bridge to permanent pacemaker if unstable 3

2:1 AV Block (Special Consideration)

2:1 AV block cannot be classified as Type I or Type II based on surface ECG alone, but QRS width and clinical context help determine anatomic location and urgency. 6, 7

• Narrow QRS + 2:1 block: Usually AV nodal (Type I physiology), may respond to atropine 6, 7
• Wide QRS + 2:1 block: Likely infranodal (Type II physiology), requires permanent pacing 6, 7
• Exercise testing or electrophysiology study may help localize block if management unclear 7

Critical Pitfalls

• Do NOT rely on atropine for Type II or wide-complex second-degree block—these are infranodal and will not respond 9, 6
• Distinguish true Type II block from pseudo-AV block caused by concealed His bundle extrasystoles 7
• Vagal surges can cause simultaneous sinus slowing and AV nodal block that superficially resembles Type II—look for sinus rate changes 7
• Coexistence of Type I and Type II patterns in same patient with narrow QRS effectively rules out true Type II block 7

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Third-Degree (Complete) AV Block

Third-degree AV block represents complete absence of AV conduction with total dissociation between atrial and ventricular activity, and permanent pacemaker implantation is indicated in virtually all cases. 8, 3

Key Characteristics

• Complete dissociation between P waves and QRS complexes with no relationship 3
• Escape rhythm determines ventricular rate and symptoms 8
• Can occur at any anatomic level (AV nodal, His bundle, or infranodal) 8, 3
• Junctional escape (narrow QRS, rate 40-60 bpm) suggests AV nodal level block 3
• Ventricular escape (wide QRS, rate <40 bpm) suggests infranodal block with worse prognosis 3

Management—Permanent Pacing Indications (Class I)

Permanent pacemaker implantation is indicated for third-degree AV block in the following scenarios:

1. Any symptomatic third-degree AV block (including heart failure, presyncope, syncope, or ventricular arrhythmias presumed due to block) 8

2. Asymptomatic third-degree AV block with any of the following:

   • Documented asystole ≥3.0 seconds while awake 8
   • Escape rate <40 bpm while awake 8
   • Escape rhythm below the AV node (wide QRS) 8
   • Atrial fibrillation with pauses ≥5 seconds 8

3. Third-degree AV block requiring medications that cause symptomatic bradycardia 8

4. Third-degree AV block after catheter ablation of AV junction 8

5. Postoperative third-degree AV block not expected to resolve 8

Acute Management of Symptomatic Third-Degree Block

• Atropine 0.5 mg IV every 3-5 minutes (maximum 3 mg) for AV nodal level block (narrow QRS escape) 3, 9
• Transcutaneous pacing immediately for unstable patients or wide QRS escape rhythms unresponsive to atropine 3, 9
• Transvenous temporary pacing as bridge to permanent pacemaker 3
• Atropine has NO effect in patients with transplanted hearts 9

Special Clinical Contexts

Acute Myocardial Infarction:

• Inferior MI: Third-degree block usually at AV nodal level, often transient, may respond to atropine 3
• Anterior MI: Third-degree block usually infranodal, worse prognosis, often requires permanent pacing 3
• Revascularization should be considered in patients with AV block who have not received reperfusion therapy 2

Exercise-Induced AV Block:

• If not due to ischemia, indicates His-Purkinje disease with poor prognosis—permanent pacing indicated 8, 2

Sleep Apnea-Related AV Block:

• Reversible in absence of symptoms; does NOT require pacing 8, 2
• If symptomatic, treat as other indications 8

Neuromuscular Diseases:

• Permanent pacing may be considered (Class IIb) for ANY degree of AV block (including first-degree) in myotonic dystrophy, Kearns-Sayre syndrome, Erb dystrophy, or peroneal muscular atrophy due to unpredictable progression 8, 2, 3

Critical Pitfalls

• Always exclude reversible causes before permanent pacing: electrolyte abnormalities, medications, Lyme disease, hypothermia, perioperative inflammation 8
• Conditions requiring pacing despite potential reversibility: sarcoidosis, amyloidosis (due to disease progression risk) 8
• Atropine doses <0.5 mg may paradoxically worsen bradycardia 2
• In acute MI setting, use atropine cautiously as increased heart rate may worsen ischemia 2

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Advanced Second-Degree AV Block

Advanced second-degree AV block refers to block of two or more consecutive P waves but with occasional conducted beats (distinguishing it from complete third-degree block). Management is identical to third-degree AV block—permanent pacemaker indicated in same clinical scenarios. 8