Atrioventricular Blocks /Heart Blocks

Definition

AV block refers to delay or failure of conduction of atrial impulses to the ventricles at the level of:

• AV node
• His bundle
• Bundle branches

Normal AV Conduction (Recap)

• SA node → atria → AV node (physiological delay) → His-Purkinje system → ventricles
• Normal PR interval: 120–200 ms (3–5 small squares)

Classification of AV Blocks

1° AV Block (First-Degree AV Block)

Definition

• PR interval > 200 ms
• Every P wave followed by a QRS

📌 There is delay, not block

ECG Features

• Regular rhythm
• Prolonged but constant PR interval
• QRS usually narrow (unless associated bundle branch disease)

Site of Block

• Most commonly AV node

Causes

Common

• Increased vagal tone (athletes, sleep)
• Inferior wall MI
• Aging / fibrosis

Drugs (very common in ICU)

• β-blockers
• Calcium channel blockers (verapamil, diltiazem)
• Digoxin
• Amiodarone

Structural

• Ischemic heart disease
• Cardiomyopathy

Clinical Significance

• Usually benign
• Often asymptomatic
• Marker of conduction system disease if PR >300 ms

📌 Very prolonged PR (>300 ms) → may cause:

• Pseudo-pacemaker syndrome
• Reduced ventricular filling

Management

• No treatment required
• Correct reversible causes
• Avoid AV-blocking drugs if PR very long

2° AV Block (Second-Degree AV Block)

👉 Some P waves fail to conduct

Two Types:

1. Mobitz Type I (Wenckebach)
2. Mobitz Type II

2° AV Block – Mobitz Type I (Wenckebach)

Definition

• Progressive PR prolongation
• Eventually a dropped QRS
• Cycle then repeats

ECG Features

• PR interval increases beat-to-beat
• RR interval shortens
• Dropped beat occurs
• Grouped beating (e.g., 3:2, 4:3)

Site of Block

• AV node

Causes

• Increased vagal tone
• Inferior wall MI
• Drugs:
• β-blockers
• Digoxin
• Verapamil

Clinical Features

• Usually asymptomatic
• May cause mild dizziness

Prognosis

• Benign
• Rarely progresses to complete heart block

Management

• Observation
• Atropine if symptomatic
• Remove offending drugs

📌 Wenckebach improves with atropine

2° AV Block – Mobitz Type II

Definition

• Sudden dropped QRS without prior PR prolongation
• PR interval remains constant in conducted beats

ECG Features

• Fixed PR interval
• Intermittent non-conducted P waves
• Often wide QRS

Site of Block

• Below AV node
  (His bundle or bundle branches)

📌 This is a malignant block

Causes

• Anterior wall MI
• Degenerative His-Purkinje disease
• Cardiac surgery
• Infiltrative cardiomyopathy

Clinical Features

• Syncope
• Bradycardia
• Hemodynamic instability

Prognosis

• High risk of:
• Progression to complete heart block
• Sudden cardiac death

Management (VERY IMPORTANT FOR EXAMS)

• Permanent pacemaker indicated even if asymptomatic
• Temporary pacing if unstable
• Atropine often ineffective

📌 Mobitz II = Pacemaker

2:1 AV Block (Special Situation)

• Every alternate P wave blocked
• Cannot distinguish Mobitz I vs II by ECG alone

Clues:

3° AV Block (Complete Heart Block)

Definition

• Complete dissociation between atria and ventricles
• No atrial impulses conduct to ventricles

ECG Features

• P waves and QRS complexes independent
• Atrial rate > ventricular rate
• Ventricular escape rhythm:
• Junctional (40–60 bpm, narrow QRS)
• Ventricular (20–40 bpm, wide QRS)

Site of Block

• AV node
• His bundle
• Infra-Hisian (most dangerous)

Causes

Acute

• Inferior wall MI (usually transient)
• Anterior wall MI (extensive damage)

Chronic

• Degenerative fibrosis (Lenègre disease)
• Congenital complete heart block
• Post cardiac surgery
• Drugs

Clinical Features

• Severe bradycardia
• Syncope (Stokes–Adams attack)
• Hypotension
• Heart failure
• Sudden cardiac arrest

Management (ICU EMERGENCY)

Acute Management

• Immediate temporary pacing
• Atropine (only if nodal block)
• Isoprenaline (bridging)

Definitive Management

• Permanent pacemaker (Class I indication)

📌 All symptomatic complete heart blocks require pacing

Comparison Table (EXAM FAVORITE)

AV Block in ICU — High-Yield Points

• Inferior MI → Wenckebach, transient
• Anterior MI → Mobitz II / CHB, poor prognosis
• Digoxin toxicity → Any AV block
• Hyperkalemia → PR prolongation → CHB
• Post-cardiac surgery → Temporary AV block