What are the different types of heart blocks, classified as stable versus unstable?

Classification of Heart Blocks: Stable versus Unstable

Heart blocks should be classified by hemodynamic stability and anatomic location of block, with second-degree Mobitz type II, high-grade AV block, and complete heart block considered unstable or high-risk requiring urgent intervention, while first-degree and Mobitz type I (Wenckebach) blocks are generally stable unless symptomatic. 1, 2

Stable Heart Blocks

First-Degree AV Block

• Definition: PR interval >200 ms with 1:1 AV conduction (technically AV delay, not true block) 1
• Stability: Generally benign and asymptomatic 1, 3
• Exception: Extreme PR prolongation (>300 ms) can cause pacemaker syndrome-like symptoms due to AV dyssynchrony 3
• Management: No intervention needed for asymptomatic patients; permanent pacing reasonable only if hemodynamic compromise or pacemaker syndrome symptoms present 1, 2

Second-Degree Mobitz Type I (Wenckebach)

• Definition: Progressive PR interval prolongation before a blocked P wave, with inconstant PR intervals 1
• Anatomic location: Block typically occurs within the AV node (supra-His) 1, 2, 4
• QRS morphology: Usually narrow QRS complexes 2
• Stability: Benign prognosis; rarely progresses to complete heart block 2, 4
• Common in: Well-trained athletes, especially during sleep 2
• Management for asymptomatic patients: No treatment required; can participate in all competitive sports if no structural heart disease 2
• Management for symptomatic patients:
  • Atropine 0.5 mg IV, repeated every 5 minutes up to 2 mg total 2, 5
  • Caveat: Doses <0.5 mg may paradoxically worsen block via central vagal stimulation 2, 5
  • Temporary transvenous pacing if hemodynamically unstable or unresponsive to atropine 2

Unstable or High-Risk Heart Blocks

Second-Degree Mobitz Type II

• Definition: Periodic blocked P waves with constant PR intervals in conducted beats 1, 6
• Anatomic location: Almost always infranodal (within or below His bundle) 1, 6
• QRS morphology: Usually wide QRS complexes with bundle branch block pattern 1, 6
• Stability: UNSTABLE - high risk for progression to complete heart block and Stokes-Adams arrest 6, 4
• Management: Generally considered indication for permanent pacemaker implantation 2, 6
• Critical distinction: Must differentiate from 2:1 Wenckebach using stress testing (Wenckebach improves with exercise, Mobitz II worsens) 2

2:1 AV Block

• Definition: Every other P wave conducts to ventricles with constant rate 1
• Classification challenge: Cannot be definitively classified as type I or type II from surface ECG alone 1
• Risk stratification depends on:
  • QRS width (narrow suggests nodal/stable, wide suggests infranodal/unstable) 1, 6
  • Clinical context (age, structural heart disease, acute MI) 1, 6
  • Response to exercise or atropine (improvement suggests nodal block) 2
• Stability: Treat as potentially unstable until proven otherwise 1

Advanced (High-Grade) AV Block

• Definition: ≥2 consecutive P waves blocked at physiologic rate with evidence of some AV conduction 1
• Stability: UNSTABLE - high risk for progression to complete block 1
• Management: Urgent evaluation for temporary pacing and permanent pacemaker 1

Third-Degree (Complete) Heart Block

• Definition: No atrial impulses conduct to ventricles; complete AV dissociation 1, 7
• Escape rhythm location determines urgency:
  • Junctional escape (40-60 bpm, narrow QRS): Block at AV node level, relatively more stable 7
  • Ventricular escape (20-40 bpm, wide QRS): Infranodal block, HIGHLY UNSTABLE with risk of asystole 7
• Stability: ALWAYS UNSTABLE - cardiovascular emergency requiring immediate intervention 7
• Management:
  • Atropine 0.5 mg IV (may accelerate junctional escape but unreliable for ventricular escape) 5, 7
  • Immediate transcutaneous pacing 7
  • Emergent transvenous pacemaker placement 7
  • ICU admission with cardiology consultation 7

Special Clinical Contexts

Acute Myocardial Infarction

• Inferior MI with AV block: Usually nodal (Mobitz I or complete with narrow escape), often responds to atropine, may be transient 1, 2
• Anterior MI with AV block: Usually infranodal with wide escape rhythm, indicates extensive septal necrosis, poor prognosis (65% one-year mortality), requires permanent pacing 1, 8
• Temporary pacing in acute MI: Does not automatically indicate need for permanent pacemaker unless block persists 1

Bifascicular/Trifascicular Block

• Right bundle branch block + left axis deviation: 4% annual risk of progression to complete heart block when associated with heart disease 8
• Left bundle branch block + first-degree AV block: Highest frequency of underlying heart disease (78%) and cardiovascular morbidity (55%) 8
• Asymptomatic bifascicular block: Does not require pacing unless symptomatic or progresses to higher-grade block 1

Key Pitfalls to Avoid

• Do not assume all second-degree blocks are benign: Mobitz II requires urgent intervention unlike Mobitz I 4, 6
• Do not use low-dose atropine (<0.5 mg): May paradoxically worsen block 2, 5
• Do not delay pacing in complete heart block with wide escape: Risk of sudden asystole 7
• Do not miss 2:1 block masquerading as sinus bradycardia: Look carefully for non-conducted P waves 1
• Do not overlook drug causes: Digoxin, beta-blockers, calcium channel blockers, antiarrhythmics can cause reversible AV block 1