What is third degree heart block?

Understanding Third-Degree Heart Block

Third-degree heart block is a complete absence of electrical conduction from the atria to the ventricles, resulting in independent atrial and ventricular rhythms that can lead to sudden death if not promptly recognized and treated. 1

Core Pathophysiology

• The atria and ventricles beat completely independently of each other because no electrical impulses from the atria reach the ventricular conduction system 1, 2
• The ventricular rate depends entirely on where an escape pacemaker emerges within the heart's conduction system 1, 2
• The anatomic location of the block is the single most critical factor determining both prognosis and urgency of intervention 1

Two Critical Types Based on Location

Intranodal (AV Node Level) Block

• The escape rhythm originates at the AV node or high in the His-Purkinje system 1, 2
• The QRS complex appears narrow (normal width) on ECG 2
• The ventricular rate is typically 40-60 beats per minute 2
• This type is more stable with reliable junctional escape rhythms, though monitoring is still required 1

Infranodal (His-Purkinje System) Block

• The escape rhythm originates from the ventricles themselves 1, 2
• The QRS complex appears wide on ECG 2
• The ventricular rate is dangerously slow at 20-40 beats per minute 2
• This type is associated with sudden death and can progress rapidly and unpredictably, requiring continuous arrhythmia monitoring until pacemaker implantation 1

Clinical Presentation Spectrum

• Patients range from completely asymptomatic to experiencing life-threatening symptoms 1
• Symptomatic patients may present with syncope, hypotension, hemodynamic compromise, or cardiac arrest 1, 2
• If no escape rhythm generates, patients develop asystole and die 2
• The most common underlying cause is ischemic heart disease, with up to 8% of post-MI patients developing complete heart block 2

Common Etiologies

• Acute myocardial infarction (most common cause) 2
• Infiltrative cardiac diseases including sarcoidosis and amyloidosis 1
• Myocarditis and infectious endocarditis 2
• Congenital AV blocks (30% remain undiscovered until adulthood and may present during pregnancy) 1
• Electrolyte disturbances and drug side effects 2
• Trauma to the chest or head 3, 4

Key ECG Recognition Features

• P waves march through at their own regular rate 2
• QRS complexes occur at their own independent, slower rate 2
• There is no relationship between P waves and QRS complexes—they are completely dissociated 2
• The width of the QRS complex (narrow vs. wide) tells you the location and severity of the block 2

Critical Clinical Pitfall

One in 600 adults over age 65 develops a supraventricular conduction abnormality each year, making this an important diagnosis to recognize in elderly patients, especially after seemingly minor events like falls, head trauma, or even eye surgery 2, 3