What are the different types of heart blocks?

Types of Heart Blocks

Heart blocks are classified into three main categories: first-degree, second-degree (with multiple subtypes), and third-degree (complete) atrioventricular block, each with distinct ECG patterns, anatomic locations, and clinical implications for progression and mortality. 1

First-Degree AV Block

First-degree AV block represents a conduction delay rather than true block, characterized by a PR interval >200 ms with 1:1 AV conduction where every P wave conducts to the ventricles. 1

Key Features:

• All P waves conduct, just with prolonged PR interval 1
• Conduction delay typically occurs at the AV node level when QRS is narrow 1
• If QRS is wide, delay may be in AV node or His-Purkinje system—only His bundle electrogram can localize precisely 1

Clinical Significance:

• Generally benign, but extreme forms (PR >300 ms) can cause pacemaker-like syndrome with hemodynamic compromise due to mistimed atrial-ventricular contractions 2
• Pacemaker implantation is reasonable only when symptoms similar to pacemaker syndrome or hemodynamic compromise are present 2
• Does not typically require pacing unless symptomatic 1

Second-Degree AV Block

Second-degree AV block occurs when some, but not all, atrial impulses conduct to the ventricles (P wave rate <100 bpm with non-1:1 conduction). 1

Mobitz Type I (Wenckebach)

Characterized by progressive PR interval prolongation before a nonconducted P wave, with inconstant PR intervals before and after the blocked beat. 1

Key Features:

• PR interval lengthens progressively until a P wave fails to conduct 1
• The increase may be subtle in the last cycles before the block 1
• Block typically occurs at the AV node level 1
• Deterioration to higher-degree block is uncommon 1

Clinical Management:

• Pacing indications are controversial unless block occurs below the AV node or symptoms are present 1
• Some evidence suggests pacemaker implantation improves survival even in asymptomatic elderly patients, especially with diurnal occurrence 1

Mobitz Type II

Defined by constant PR intervals before and after a nonconducted P wave, with periodic single blocked P waves (excluding 2:1 block). 1, 3

Critical Distinguishing Features:

• PR intervals remain constant—this is the key differentiator from Type I 1, 3
• Block typically occurs in the His-Purkinje system, especially with wide QRS 1, 4
• High risk of progression to complete heart block 1, 4

Management Algorithm:

• Place transcutaneous pacing pads immediately due to high progression risk 3
• Obtain transthoracic echocardiography to assess structural heart disease 3
• Check electrolytes to rule out reversible causes 3
• Class I indication for permanent pacemaker implantation, even in asymptomatic patients 1, 3
• Atropine 0.5 mg IV every 3-5 minutes (max 3 mg) for temporary symptomatic management, with caution in acute coronary ischemia 3
• Arrange urgent transvenous temporary pacing for hemodynamically unstable patients 3

2:1 AV Block

Every other P wave conducts to the ventricles with constant (or near-constant due to ventriculophasic sinus arrhythmia) P wave rate <100 bpm. 1

• Cannot be classified as Type I or Type II based on surface ECG alone 4
• Requires clinical context (age, QRS width, clinical setting) to determine anatomic location and prognosis 4

Advanced (High-Grade) AV Block

Two or more consecutive P waves at constant physiologic rate fail to conduct, but some evidence of AV conduction remains. 1

• Indicates severe conduction system disease 1
• High risk for progression to complete block 1

Third-Degree (Complete) AV Block

No atrial impulses conduct to the ventricles—complete dissociation between atrial and ventricular activity. 1, 5

Key Features:

• Ventricles depend entirely on escape rhythm 6
• Escape rhythm location determines QRS morphology and rate: 6
  • AV nodal or high His-Purkinje escape: narrow QRS, rate 40-60 bpm
  • Ventricular escape: wide QRS, rate 20-40 bpm

Clinical Significance:

• Cardiovascular emergency requiring prompt recognition 6
• Seen in 8% of post-MI patients 6
• Ventricular escape rhythms can destabilize; absence of escape leads to asystole 6

Management:

• Class I indication for permanent pacing, especially with symptomatic bradycardia, documented asystole >3 seconds, escape rate <40 bpm, confusional states, congestive heart failure, or ectopic rhythms requiring suppressive drugs 1
• Non-randomized studies demonstrate permanent pacing improves survival, particularly with syncope 1, 3
• Immediate transcutaneous pacing for unstable patients 1
• Transvenous temporary pacing bridge to permanent device 1

Special Anatomic Considerations

Infranodal Block

Conduction block occurring distal to the AV node (in His-Purkinje system), identified by clinical or electrophysiologic evidence. 1

• More ominous prognosis than AV nodal block 1
• Higher risk of progression to complete block 1
• May manifest during exercise—if not due to ischemia, indicates His-Purkinje damage with poor prognosis 1

Vagally Mediated AV Block

Any type of AV block caused by heightened parasympathetic tone. 1

• Often reversible with atropine 3
• May not require permanent pacing if clearly vagal in origin 1

Common Pitfalls

Do not confuse nonconducted premature atrial contractions or atrial tachycardia with block for true second-degree AV block—these represent "causes of pauses" rather than conduction system disease. 4

In 2:1 block, avoid labeling as Type I or Type II without additional information—use QRS width, patient age, and clinical context to determine anatomic location and risk stratification. 4

Wide QRS with first-degree block requires His bundle electrogram to definitively localize the delay, as it could be nodal or infranodal with vastly different prognoses. 1