Sinus Rhythm with First-Degree AV Block + 2:1 AV Block

First-Degree AV Block

• PR interval > 200 ms (constant)
• Every P wave followed by a QRS complex
• QRS is typically narrow unless intraventricular conduction disease coexists

• Normal healthy individuals, well-trained athletes (vagal tone)
• Inferior MI (AV nodal ischemia)
• Drug effects (beta-blockers, calcium channel blockers, digoxin, amiodarone)
• Inflammatory/infiltrative disease (myocarditis, sarcoidosis, Lyme disease)

"First-degree AV block is a misnomer because nothing is actually blocked; rather, there is delay, usually in the AV node, manifest by a prolonged PR interval." — Goldman-Cecil Medicine

2:1 AV Block

"When 2:1 AV block is caused by a block distal to the bundle of His (type II), it has a more serious prognosis than if it is caused by a block proximal to the bundle of His." — Braunwald's Heart Disease

Clinical Significance of the Combined Pattern

• Acute inferior MI (RCA supplies AV node in ~90% of people)
• Degenerative conduction system disease (Lev's or Lenègre's disease)
• Drug toxicity (digoxin, beta-blockers, CCBs)
• Hyperkalemia
• Inflammatory: myocarditis, Lyme carditis, sarcoidosis, rheumatic fever
• Post-cardiac surgery

ECG Recognition Tips

• Count 2 P waves per QRS — one conducted, one blocked
• Measure the PR interval of the conducted beat — prolonged = likely nodal; normal = likely infranodal
• Check QRS width — narrow = nodal; wide = infranodal until proven otherwise
• Compare with longer rhythm strips or Holter — catching 3:2 Wenckebach sequences would confirm Mobitz I

Management

• Asymptomatic + likely nodal: Monitor, treat underlying cause, avoid AV-blocking drugs
• Symptomatic (syncope, presyncope, hemodynamic compromise): Urgent evaluation; atropine (if nodal), temporary pacing if needed
• Infranodal 2:1 block: High risk of progression to complete heart block → permanent pacemaker indicated (Class I indication per guidelines)
• Acute inferior MI: Usually transient, resolves with reperfusion; temporary pacing if symptomatic
• Acute anterior MI with 2:1 block: More ominous — often infranodal, high risk of CHB

"When 2:1 AV block is associated with a bundle branch block, the block is more likely to be distal to the bundle of His, and a permanent pacemaker is often indicated." — Braunwald's Heart Disease

Approach & Treatment Strategy

Step 1 — Immediate Clinical Assessment

• Is the patient symptomatic? (syncope, presyncope, dizziness, fatigue, dyspnea, chest pain)
• What is the ventricular rate? (2:1 block halves the effective rate — if sinus rate is 80 bpm, ventricular rate is ~40 bpm → likely symptomatic)
• Blood pressure, perfusion, mental status

"Persistent 2:1 AV block can produce symptoms of chronic bradycardia. Complete AV block can be accompanied by signs and symptoms of reduced cardiac output, syncope or presyncope, angina, or palpitations." — Braunwald's Heart Disease

Step 2 — Localize the Level of Block (Critical Decision Point)

Step 3 — Identify and Treat Reversible Causes

"The treatment of AV blocks consists of first removing any medications that may precipitate dysfunction. Although some patients will recover normal conduction, the susceptibility to medications usually indicates an underlying conduction abnormality that may worsen over time." — Goldman-Cecil Medicine

Step 4 — Acute / Emergency Management

1. IV Atropine 0.5–1 mg (repeat to max 3 mg)
   • Effective for nodal block (increases AV node conduction)
   • May paradoxically worsen infranodal block (accelerates sinus rate → more P waves blocked)
2. IV Isoproterenol (1–2 μg/min infusion)
   • Increases conduction at any level
   • Use with extreme caution in acute MI (pro-arrhythmic)
3. Transcutaneous pacing — immediate bridge when drugs fail or infranodal block suspected
4. Temporary transvenous pacing — definitive bridge while awaiting permanent pacemaker or resolving reversible cause

Step 5 — Investigations

"In patients with presyncope or syncope, one should suspect intermittent infrahisian block in those with BBB or an intraventricular conduction defect. An EPS to evaluate AV conduction thoroughly may be warranted." — Braunwald's Heart Disease

Step 6 — Permanent Pacemaker Decision

Class I Indications (Pacemaker IS indicated):

• Symptomatic second-degree AV block at any level (syncope, presyncope, heart failure, bradycardia symptoms)
• Asymptomatic infranodal (His or below) 2:1 block
• Second-degree/advanced AV block with associated BBB (infranodal presumed)
• Second-degree AV block in the setting of neuromuscular disease (myotonic dystrophy, Kearns-Sayre)
• Post-operative AV block not expected to resolve
• Post-MI: persistent symptomatic block, or transient infranodal block with BBB

Class IIa Indications (Pacemaker is Reasonable):

• Asymptomatic Mobitz II with narrow QRS
• Asymptomatic infranodal second-degree AV block
• First- or second-degree AV block with symptoms resembling pacemaker syndrome (fatigue, dyspnea from loss of AV synchrony)

Pacemaker Mode Selection:

"There is growing evidence that some patients with AV block, especially those with preexisting LV dysfunction, may benefit from biventricular pacing rather than right ventricle-only pacing to prevent development or progression of heart failure." — Braunwald's Heart Disease

Summary Algorithm

Sinus rhythm + 1° AVB + predominant 2:1 AV block
           ↓
   Hemodynamically unstable?
   YES → Atropine → Transcutaneous pacing → Transvenous pacing
   NO  ↓
   Remove reversible causes (drugs, electrolytes, ischemia, infection)
           ↓
   Localize block: QRS width + PR of conducted beat
   ↙ Narrow QRS / long PR        ↘ Wide QRS / normal PR
   Likely nodal (Mobitz I)        Likely infranodal (Mobitz II)
   Monitor; treat cause            → EPS or proceed to PPM
           ↓
   Persistent block + symptoms → PERMANENT PACEMAKER (DDD)
   Asymptomatic infranodal block → PERMANENT PACEMAKER (Class I)
   Asymptomatic nodal block, reversible cause resolved → Monitor