OVERVIEW

How does the conduction system conduct impulses in heart block?

The cardiac conduction system consists of specialized myocardial cells capable of generating and conducting impulses, including the sinoatrial (SA) node, internodal tracts, atrioventricular (AV) node, bundle of His, right bundle branch, and Purkinje fibers.

The SA node serves as the natural pacemaker of the heart. The internodal tracts act as conduction pathways between the SA and AV nodes. The AV node, located beneath the right endocardium of the interatrial septum, extends downward to form the bundle of His. Together, the AV node and bundle of His constitute the AV junction, which further divides into the left and right bundle branches at the lower end of the membranous interventricular septum. The left bundle branch originates on the left side of the interventricular septum and splits into anterior and superior fascicles, while the right bundle branch descends along the right side of the septum, branching into Purkinje fibers near the apex. The right bundle branch connects with the Purkinje fiber network beneath the endocardium, ultimately linking to the ventricular myocardium.

What is heart block?

Heart block refers to the slowing or interruption of impulse conduction at any site within the cardiac conduction system. It can occur at any level, with sinoatrial block, atrioventricular block, and intraventricular block being the most clinically common.

• Sinoatrial block occurs when impulses generated by the SA node partially or completely fail to reach the atria, resulting in single or consecutive pauses of atrial and ventricular activity. It is a relatively rare arrhythmia.

• Atrioventricular block involves abnormal delay or failure of impulses to travel from the atria to the ventricles. It is classified into first-degree (prolonged conduction time), second-degree (intermittent conduction failure), and third-degree (complete conduction failure). First- and second-degree blocks are incomplete, while third-degree is complete. Blockage may occur in the AV node, bundle of His, or left/right bundle branches.

• Intraventricular block refers to conduction disturbances below the bifurcation of the bundle of His. The intraventricular conduction system includes the left and right bundle branches, left anterior fascicle, and left posterior fascicle. Right bundle branch block is the most common single-branch block, followed by left anterior fascicular block. Multibranch blocks (two or three branches) often indicate severe cardiac pathology.

Is heart block common?

Heart block is a common arrhythmia, particularly among the elderly, with incidence increasing significantly with age.

Who is at higher risk of heart block?

Patients with cardiovascular diseases or those who have undergone cardiac surgery.

SYMPTOMS

What are the common manifestations of patients with heart block?

• Sinoatrial block: Often asymptomatic, but may present with mild palpitations, fatigue, and "skipped beats." Cardiac auscultation may reveal arrhythmia, bradycardia, or long pauses. If recurrent, continuous missed beats may occur without escape rhythms, leading to dizziness, syncope, coma, or Adams-Stokes syndrome.

• Atrioventricular (AV) block:

• First-degree AV block is usually asymptomatic.

• Second-degree AV block may cause skipped beats, leading to palpitations. It can be further classified into Type I and Type II based on ECG findings.

• Third-degree AV block: Symptom severity depends on ventricular rate. Common symptoms include fatigue, dizziness, syncope, chest pain, and heart failure. Severe cases may lead to cerebral ischemia, transient loss of consciousness, dizziness, or even Adams-Stokes attacks, which can be fatal.

• Intraventricular block: Single or bifascicular blocks are often asymptomatic, though splitting of the first or second heart sound may occasionally be heard. Trifascicular block presents similarly to third-degree AV block.

What complications may arise from heart block?

In severe cases, cerebral ischemia may occur, leading to transient loss of consciousness, dizziness, or even Adams-Stokes attacks (sudden, life-threatening bradyarrhythmias or tachyarrhythmias), which can result in sudden death.

What conditions should be differentiated from heart block?

This condition should be distinguished from other arrhythmias such as sinus arrhythmia, sinus pause, sinus premature beats, and slow atrial fibrillation or atrial flutter.

CAUSES

What are the causes of heart block?

The specific causes vary depending on the location of the block, as detailed below:

• Causes of atrioventricular (AV) block:

• Increased vagal tone, often occurring at night.

• Congenital AV block: Congenital AV block due to cardiac malformations, Kearnes-Sayre syndrome (caused by mitochondrial DNA mutations).

• Primary AV block: Calcification and sclerosis of the cardiac fibrous skeleton, primary sclerodegenerative diseases of the conduction system.

• Secondary AV block: Acute myocardial infarction, coronary artery spasm, myocarditis, cardiomyopathy, endocarditis, calcific aortic stenosis, ablation injury, electrolyte imbalances, drug effects (e.g., digitalis), etc.

• Causes of intraventricular block:

• Right bundle branch block: Rheumatic mitral stenosis, atrial septal defect, acute or chronic cor pulmonale, also seen in hypertensive heart disease, coronary artery disease, cardiomyopathy, congenital heart disease, etc.

• Left bundle branch block: Mostly seen in patients with left ventricular involvement, such as congestive heart failure, acute myocardial infarction, hypertensive heart disease, also seen in acute infections, quinidine toxicity, rheumatic heart disease, coronary artery disease, and syphilitic heart disease.

• Additionally, congenital heart disease, cardiac surgery, and hyperkalemia can also cause intraventricular block.

• Causes of sinoatrial (SA) block:

• Organic heart disease: Coronary artery disease, hypertensive heart disease, rheumatic heart disease, congenital heart disease, and sinoatrial node or perinodal lesions.

• Drugs: Mostly temporary, such as digitalis, quinidine, verapamil, disopyramide, amiodarone, beta-blocker toxicity, and high-dose propafenone.

• In rare cases, the cause is unknown, and some may be familial.

DIAGNOSIS

How to diagnose heart block?

The diagnosis can be easily made based on typical ECG or Holter monitor findings combined with clinical manifestations.

What tests do patients with heart block need?

ECG or Holter monitoring: Surface ECG can only record characteristic manifestations during disease episodes, while Holter monitoring allows continuous recording of electrical heart activity.

TREATMENT

Which department should I visit for heart block?

Cardiology.

Does heart block require hospitalization?

Single bundle branch or fascicular block usually only shows ECG abnormalities without obvious clinical symptoms and generally requires follow-up observation. Type II second-degree and third-degree atrioventricular block often require hospitalization for pacemaker implantation.

How is heart block treated?

• Atrioventricular block:

• First-degree and type I second-degree atrioventricular block usually require no specific treatment.

• Type II second-degree and third-degree atrioventricular block with slow ventricular rate or ventricular arrest may require temporary cardiac pacing in emergencies. Medications like atropine (0.5–2.0 mg, IV) or isoproterenol (1–4 μg/min IV infusion) can also be used to increase ventricular rate.

• Simple bundle branch block requires no specific treatment; the focus is on treating the underlying cause. Left posterior fascicular block often indicates extensive and severe myocardial damage and is frequently combined with varying degrees of right bundle branch block and left anterior fascicular block, which may progress to complete atrioventricular block, requiring clinical monitoring.

• Sinoatrial block: The primary treatment is addressing the underlying disease.

• For transient and asymptomatic cases, close observation is sufficient, and most patients recover spontaneously.

• For frequent, recurrent, persistent, or symptomatic cases, oral atropine (0.3–0.6 mg, 3 times/day) or IV/subcutaneous atropine (0.5–1 mg) may be used. Oral ephedrine (25 mg, 3 times/day) can also be administered.

• Severe cases may require slow IV infusion of isoproterenol (1 mg in 5% glucose 200 ml or 100 ml).

• Patients with syncope, Adams-Stokes syndrome, or ineffective drug therapy should promptly receive an implantable pacemaker.

What is the prognosis for heart block patients after treatment?

• Most first-degree and type I second-degree atrioventricular block cases have a good prognosis. However, patients with distal His bundle block have a poorer prognosis and require close follow-up.

• Type II second-degree and third-degree atrioventricular block often occur in the distal His bundle due to extensive and irreversible lesions, resulting in a poorer prognosis. Active pacemaker therapy is recommended.

• The prognosis of intraventricular block depends on the severity of the underlying heart disease. Patients without organic heart disease generally have a good prognosis.

DIET & LIFESTYLE

What should patients with heart block pay attention to in their diet?

No specific dietary restrictions are required. However, if there are pre-existing heart conditions such as coronary artery disease, a low-salt and low-fat diet is recommended, avoiding overly salty and fried foods. For patients with diabetes, starch and high-sugar intake should be avoided.

What should patients with heart block pay attention to in daily life?

• Maintain a regular routine, avoid excessive fatigue, and engage in appropriate exercise. Prevent infections and avoid stressful situations like emotional agitation.

• Quit smoking and limit alcohol consumption for a healthy lifestyle.

• Actively manage underlying conditions such as coronary artery disease, hypertension, or diabetes through standardized treatment, strictly controlling blood pressure and blood sugar levels.

• Treat myocarditis, acute myocardial infarction, and myocardial ischemia promptly, and restore electrolyte balance. Eliminate triggers of the condition, adhere to prescribed medications after illness, consolidate treatment effects, and avoid harmful stimuli.

PREVENTION

Can Heart Block Be Prevented?

Actively identify and address the underlying causes to prevent further progression of the disease. Most type II and III atrioventricular blocks occur in the distal His bundle, often caused by extensive irreversible lesions, resulting in poor prognosis. Prompt artificial cardiac pacing therapy should be implemented. Actively treat the primary condition and maintain a regular lifestyle.