Heart Block: Types, Causes, Diagnosis, Management, Treatment and Prevention

Heart Block: Types, Causes, Diagnosis and Management

~Introduction

The human heart functions as a finely tuned electrical pump. Every heartbeat originates from an electrical impulse that travels through a specialized conduction system, ensuring coordinated contraction of the atria and ventricles. Any disruption in this electrical pathway can interfere with the normal rhythm and rate of the heart.

Heart block, also known as atrioventricular (AV) block), refers to a delay or complete interruption in the transmission of electrical impulses from the atria (upper chambers) to the ventricles (lower chambers). Depending on its severity, a heart block may cause bradycardia (slow heart rate), fainting spells, or even cardiac arrest.

Heart block is not the same as a coronary artery blockage; it refers to an electrical conduction problem, not a mechanical obstruction. Understanding its pathophysiology, causes, and clinical implications is essential for timely diagnosis and management.

~Anatomy and Physiology of Cardiac Conduction System

To appreciate heart block, one must understand the normal conduction pathway of the heart:

1. Sinoatrial (SA) Node: Located in the right atrium; the natural pacemaker that initiates each heartbeat.

2. Atrioventricular (AV) Node: Located at the junction between atria and ventricles; it delays the electrical impulse slightly to allow the ventricles to fill properly.

3. Bundle of His: Transmits impulses from the AV node to the ventricles.

4. Right and Left Bundle Branches: Conduct impulses to the respective ventricles.

5. Purkinje Fibers: Distribute the impulse throughout the ventricular walls, causing coordinated contraction.

A heart block occurs when there is impairment or failure in conduction through any part of this pathway—most commonly at the AV node, bundle of His, or bundle branches.

~Classification of Heart Block

Heart block is broadly categorized into three degrees based on the severity of conduction delay.

1. First-Degree Heart Block

Definition

In first-degree heart block, there is a prolonged delay in the conduction of electrical impulses from the atria to the ventricles, but each impulse still reaches the ventricles. It is the mildest form and often asymptomatic.

ECG Finding

• Prolonged PR interval (> 0.20 seconds or >200 milliseconds) with every P wave followed by a QRS complex.

Causes

• Increased vagal tone (seen in athletes)

• Medications: beta-blockers, calcium channel blockers, digoxin

• Ischemic heart disease

• Myocarditis or degenerative changes in the conduction system

• Electrolyte disturbances (e.g., hyperkalemia)

Clinical Features

• Usually asymptomatic

• May be detected incidentally during routine ECG

Management

• No specific treatment required unless associated with symptoms or drug toxicity.

• Monitoring and correction of reversible causes.

2. Second-Degree Heart Block

In this type, some atrial impulses fail to conduct to the ventricles, resulting in dropped beats. It is further subdivided into Mobitz Type I (Wenckebach) and Mobitz Type II.

A. Mobitz Type I (Wenckebach Phenomenon)

Definition

There is progressive prolongation of the PR interval until one atrial impulse fails to conduct, leading to a dropped QRS complex. After the dropped beat, the cycle repeats.

ECG Features

• Progressive lengthening of PR interval

• Dropped QRS after every few beats

• Regularly irregular rhythm

Causes

• Often benign and transient

• Increased vagal tone

• Drugs (digoxin, beta-blockers, calcium channel blockers)

• Myocardial ischemia, particularly of the right coronary artery

Clinical Features

• Usually asymptomatic, but may cause mild dizziness or palpitations.

Management

• Usually requires no specific therapy unless symptomatic.

• Treat underlying cause.

• If severe bradycardia or hypotension occurs, atropine may be administered.

B. Mobitz Type II

Definition

In Mobitz Type II block, some impulses are suddenly blocked without progressive prolongation of the PR interval. It represents more serious conduction system disease, usually below the AV node (in the His-Purkinje system).

ECG Features

• Constant PR interval for conducted beats

• Intermittent dropped QRS complexes (e.g., 2:1 or 3:1 block)

• QRS complexes may be widened if associated with bundle branch block.

Causes

• Anterior wall myocardial infarction

• Fibrosis of the conduction system

• Myocarditis or infiltrative diseases (sarcoidosis, amyloidosis)

• Post-cardiac surgery

Clinical Features

• Dizziness, fatigue, presyncope or syncope (Stokes-Adams attacks)

• Palpitations

• Bradycardia

Management

• Immediate hospitalization is warranted.

• Temporary pacing may be required.

• Permanent pacemaker implantation is indicated as the risk of complete heart block is high.

3. Third-Degree (Complete) Heart Block

Definition

There is a complete failure of conduction between the atria and ventricles. The atria and ventricles beat independently—the atrial rhythm is governed by the SA node, while the ventricles rely on an escape pacemaker (either junctional or ventricular).

ECG Features

• P waves and QRS complexes are present but show no relationship (AV dissociation).

• Atrial rate faster than ventricular rate.

• QRS complexes may be narrow (if junctional escape) or wide (if ventricular escape).

Causes

• Acute myocardial infarction (especially inferior or anterior wall)

• Congenital heart block (maternal lupus antibodies)

• Degenerative conduction disease (Lev’s or Lenègre’s disease)

• Cardiac surgery or catheter manipulation

• Drugs (digoxin toxicity, beta-blockers)

• Infiltrative disorders (sarcoidosis, Lyme disease)

Clinical Features

• Severe bradycardia

• Fatigue, dizziness, syncope (Stokes-Adams attacks)

• Heart failure symptoms

• In severe cases, sudden cardiac arrest

Management

• Emergency treatment includes:

  • Atropine (if the block is at the AV nodal level)

  • Temporary transcutaneous or transvenous pacing

• Definitive treatment: Permanent pacemaker implantation

• Treat underlying cause if reversible (e.g., drug toxicity, ischemia).

Bundle Branch Blocks (Intraventricular Blocks)

Apart from AV blocks, conduction may also be delayed or interrupted in the bundle branches, leading to intraventricular conduction blocks.

1. Right Bundle Branch Block (RBBB)

• Delay in right ventricular activation.

• ECG: QRS >120 ms, rsR’ pattern in V1 (“rabbit ears”), and wide S wave in leads I and V6.

• May be seen in pulmonary embolism, right ventricular hypertrophy, or normal individuals.

2. Left Bundle Branch Block (LBBB)

• Delay in left ventricular activation.

• ECG: QRS >120 ms, broad notched R wave in leads I, aVL, V5–V6, and absent Q wave in these leads.

• Often indicates underlying cardiac disease (hypertension, coronary artery disease, cardiomyopathy).

~Etiology (Causes of Heart Block)

1. Congenital Causes

• Congenital AV block (may occur with maternal autoimmune conditions such as lupus)

• Structural heart defects

2. Acquired Causes

Ischemic Heart Disease

• Myocardial infarction (especially inferior wall)

• Chronic coronary artery disease

Degenerative Changes

• Age-related fibrosis of the conduction system (Lev’s or Lenègre’s disease)

Inflammatory and Infectious Conditions

• Myocarditis

• Rheumatic fever

• Lyme disease

• Sarcoidosis, amyloidosis

Iatrogenic Causes

• Post-cardiac surgery (especially valve replacement)

• Catheter ablation or transcatheter aortic valve implantation (TAVI)

Drug-Induced

• Digitalis toxicity

• Beta-blockers

• Non-dihydropyridine calcium channel blockers (verapamil, diltiazem)

• Antiarrhythmic agents (amiodarone, sotalol)

Metabolic and Electrolyte Disturbances

• Hyperkalemia

• Hypoxia

• Hypothyroidism

~Clinical Manifestations

The symptoms of heart block depend on the degree and type of conduction delay:

Degree of Block	Heart Rate	Symptoms
First-degree	Normal	Usually asymptomatic
Second-degree (Mobitz I)	Slightly slow	Mild dizziness, fatigue
Second-degree (Mobitz II)	Slow	Dizziness, fainting, presyncope
Third-degree	Very slow (30–40 bpm)	Syncope, heart failure, sudden death

Stokes-Adams attacks (sudden fainting due to transient asystole or severe bradycardia) are classic in advanced blocks.

~Diagnosis

1. Electrocardiogram (ECG)

The main diagnostic tool:

• PR interval analysis

• Relationship between P waves and QRS complexes

• QRS morphology and duration

2. Holter Monitoring (24–48 hours)

Detects intermittent or transient blocks not captured on standard ECG.

3. Event Recorder / Loop Recorder

Used in cases of unexplained syncope or intermittent symptoms.

4. Electrophysiological (EP) Studies

Identify exact site and mechanism of conduction block, useful in complex or unexplained cases.

5. Echocardiography

Assesses structural abnormalities and ventricular function.

6. Laboratory Investigations

Help identify underlying causes:

• Electrolyte levels (especially potassium)

• Drug levels (e.g., digoxin)

• Thyroid function tests

• Markers of infection or inflammation

~Management and Treatment

1. General Principles

• Assess hemodynamic stability.

• Identify and treat reversible causes (drug toxicity, electrolyte imbalance, ischemia).

• Determine need for pacing.

2. Acute Management

A. Stable Patients

• Observation and monitoring.

• Discontinue offending medications.

B. Unstable Patients (symptomatic bradycardia, hypotension, syncope)

• Atropine 0.5 mg IV every 3–5 minutes (maximum 3 mg)

• If ineffective → Temporary pacing (transcutaneous or transvenous)

• Dopamine or epinephrine infusions as bridge to pacing in emergency settings.

3. Permanent Pacemaker Indications

Class I (Definite) Indications:

• Symptomatic second-degree (Mobitz II) or third-degree AV block

• Asymptomatic complete heart block with ventricular rate <40 bpm

• Congenital complete heart block with symptoms or ventricular dysfunction

• Advanced AV block after myocardial infarction not resolving within days

• Post-surgical or post-ablation complete heart block

Class II (Consider) Indications:

• Asymptomatic Mobitz II block with wide QRS

• Chronic bifascicular block with intermittent symptoms

4. Pharmacologic Support

While pacing is definitive, certain drugs can provide temporary improvement:

• Isoproterenol infusion (stimulates heart rate)

• Atropine (useful in AV nodal blocks)
  However, these are bridge therapies until pacing is established.

5. Management of Bundle Branch Blocks

• RBBB: Often benign; treat underlying disease if present.

• LBBB: May indicate significant heart disease; echocardiographic evaluation and possible pacemaker if symptomatic.

~Complications

If untreated or unrecognized, heart block may lead to:

• Syncope and falls

• Heart failure (due to reduced cardiac output)

• Sudden cardiac death

• Ventricular arrhythmias

• Pacemaker dependency (in those treated)

~Prognosis

The outlook depends on the type and cause of the block:

• First-degree and Mobitz I blocks: Excellent prognosis.

• Mobitz II and complete heart block: Poor without pacing; excellent once pacemaker is implanted.

• Congenital complete heart block: Variable; some remain stable for years, others require pacing early.

Modern pacemaker therapy has significantly improved survival and quality of life in affected individuals.

~Prevention

• Regular ECG monitoring in patients on rate-limiting medications.

• Prompt treatment of myocardial infarction and myocarditis.

• Avoiding drug overdoses (especially digitalis).

• Managing chronic diseases (hypertension, diabetes) that predispose to conduction abnormalities.

~Recent Advances

• Leadless pacemakers (e.g., Micra device) reduce complications of traditional pacing.

• Dual-chamber and biventricular pacemakers improve synchrony and cardiac output.

• Remote pacemaker monitoring allows early detection of malfunctions or arrhythmias.

• Gene therapy research aims to develop biological pacemakers using stem cells or gene editing.

~Conclusion

Heart block represents a spectrum of conduction disturbances, ranging from benign delays to life-threatening arrhythmias. While mild forms require observation, advanced blocks often necessitate urgent pacing therapy. Early recognition, identification of reversible causes, and appropriate use of pacemaker technology remain the cornerstones of management.

With advances in cardiac electrophysiology and pacemaker design, most patients with heart block can lead normal, active lives with minimal limitations.