What are the types of heart block?

Heart Block Types

Heart blocks are classified into three main degrees—first-degree, second-degree (subdivided into Mobitz Type I and Type II), and third-degree (complete heart block)—each with distinct electrocardiographic patterns, anatomic locations, and clinical implications for progression and mortality. 1, 2

First-Degree AV Block

Definition and ECG Characteristics:

• PR interval prolonged beyond 200 milliseconds (0.20 seconds) with maintained 1:1 AV conduction where every P wave conducts to the ventricles 1, 2
• Represents a conduction delay rather than true "block" 2, 3

Anatomic Location:

• Conduction delay typically occurs at the AV node level when QRS is narrow 2
• If QRS is wide, delay may be in AV node or His-Purkinje system—only His bundle electrogram can localize precisely 2

Clinical Significance:

• Generally benign and does not typically require pacing unless symptomatic 2
• Extreme forms (PR interval >300 milliseconds) can cause symptoms similar to pacemaker syndrome due to inadequate timing of atrial and ventricular contractions 1, 3
• Little evidence suggests pacemakers improve survival in isolated first-degree AV block 1

Second-Degree AV Block

General Definition:

• Some, but not all, atrial impulses conduct to the ventricles (P wave rate <100 bpm with non-1:1 conduction) 2
• Further classified into Mobitz Type I and Mobitz Type II based on PR interval behavior 1

Mobitz Type I (Wenckebach)

ECG Characteristics:

• Progressive PR interval prolongation before a nonconducted P wave, with the PR interval gradually lengthening with each successive cardiac cycle 1, 2, 4
• Single non-conducted P wave that fails to generate a QRS complex after the progressive prolongation 4
• PR interval after the blocked beat is shorter compared to the PR interval immediately before the block 2, 4
• "Group beating" pattern on ECG due to repetitive cycles 4
• Inconstant PR intervals throughout the cycle distinguish this from Mobitz Type II 4

Anatomic Location:

• Block typically occurs at the AV node level, especially when QRS is narrow (<120 ms) 2, 4

Clinical Significance and Prognosis:

• Generally benign prognosis with slower progression to complete heart block 2, 4
• Associated with faster and more reliable junctional escape mechanism 4
• Responds to autonomic manipulation (atropine, isoproterenol, epinephrine) and is often reversible 4
• However, recent evidence challenges the "benign" reputation: chronic Mobitz Type I block has similar prognosis to Mobitz Type II block, with unpaced patients doing very badly (41% five-year survival) compared to paced patients (78% five-year survival) 5

Reversible Causes to Evaluate:

• Medication effects: beta-blockers, calcium channel blockers, digoxin, antiarrhythmic drugs 4
• Electrolyte abnormalities, particularly hyperkalemia 4
• Acute Lyme carditis 4

Management:

• Permanent pacemaker generally not indicated if asymptomatic and no structural heart disease 4
• Observation and reversible cause correction appropriate for most cases 4

Mobitz Type II

ECG Characteristics:

• Constant PR intervals before and after a nonconducted P wave, with periodic single blocked P waves (excluding 2:1 block) 2, 4
• No progressive PR prolongation 1, 4
• Usually associated with wide QRS complex 1

Anatomic Location:

• Block typically occurs in the His-Purkinje system, especially with wide QRS 2, 6
• Almost always infranodal (below the AV node) 4, 6

Clinical Significance:

• High risk of progression to complete heart block 2, 6
• More likely to progress to Stokes-Adams arrest 6
• Prognosis is compromised and progression to complete heart block is common 1

Management:

• Permanent pacing indicated due to high risk of progression 2

Advanced (High-Grade) Second-Degree AV Block

Definition:

• Block of two or more consecutive P waves with some conducted beats, indicating some preservation of AV conduction 1
• In atrial fibrillation, a prolonged pause (e.g., greater than 5 seconds) should be considered advanced second-degree AV block 1

Clinical Significance:

• Indicates severe conduction system disease 2
• High risk for progression to complete block 2

2:1 AV Block

Special Consideration:

• When AV conduction occurs in a 2:1 pattern, block cannot be classified unequivocally as Type I or Type II 1
• Width of QRS can be suggestive: narrow QRS suggests nodal (Type I-like), wide QRS suggests infranodal (Type II-like) 1

Third-Degree (Complete) AV Block

Definition and ECG Characteristics:

• No atrial impulses conduct to the ventricles—complete dissociation between atrial and ventricular activity 1, 2, 7
• Atrial impulses are not conducted to the ventricles at all 8

Escape Rhythm Characteristics:

• Heart rate dependent on location of block and functioning secondary pacemaker within conduction system 7
• AV nodal or high His-Purkinje escape: narrow QRS, ventricular rate typically 40-60 bpm 7
• Ventricular escape rhythm: wide QRS, rates of 20-40 bpm 7

Clinical Significance:

• Patients with ventricular escape rhythms can destabilize rapidly 7
• If no escape rhythm generates, patients develop asystole and cardiac arrest 7
• Non-randomized studies demonstrate permanent pacing improves survival, particularly with syncope 1, 2

Management:

• Class I indication for permanent pacing, especially with symptomatic bradycardia 1, 2
• Specific Class I indications include: symptomatic bradycardia, congestive heart failure, ectopic rhythms requiring drugs that suppress escape pacemakers, documented asystole ≥3.0 seconds or escape rate <40 bpm in symptom-free patients, confusional states that clear with temporary pacing, post-AV junction ablation 1

Anatomic Classification

Beyond degree classification, heart blocks are anatomically defined as: 1

• Supra-His (AV nodal): Generally better prognosis, more responsive to atropine 4
• Intra-His: Intermediate prognosis
• Infra-His (His-Purkinje): More ominous prognosis, higher risk of progression to complete block 2

Special Considerations

Vagally Mediated AV Block:

• Often reversible with atropine 2
• May not require permanent pacing if clearly vagal in origin 2

Post-Myocardial Infarction:

• Complete heart block occurs in 8% of patients post-MI 7
• Long-term prognosis related primarily to extent of myocardial injury and character of intraventricular conduction disturbances rather than AV block itself 1
• Requirement for temporary pacing in acute MI does not by itself constitute indication for permanent pacing 1