Heart Block Made Easy: First, Second & Third Degree AV Block

Heart Block: Definition, Types, ECG Features, Symptoms and Treatment

Heart block, also called atrioventricular block or AV block, is a cardiac conduction disorder where electrical impulses are delayed or blocked between the atria and ventricles. It may cause slow heart rate, dizziness, syncope, fatigue, chest pain, shortness of breath, or cardiac arrest in severe cases.

Cardiac Conduction System

The normal electrical pathway of the heart is:

• SA Node: Natural pacemaker of the heart
• AV Node: Delays impulse before ventricular contraction
• Bundle of His
• Right and Left Bundle Branches
• Purkinje Fibres: Stimulate ventricular contraction

Normal conduction pathway: SA Node → AV Node → Bundle of His → Bundle Branches → Purkinje Fibres

Types of AV Block

Type	ECG Features	Symptoms	Treatment
First-Degree AV Block	PR interval > 0.20 seconds; every P wave followed by QRS	Usually asymptomatic	Observation and treat cause
Second-Degree Type I / Mobitz I / Wenckebach	Progressive PR prolongation followed by dropped QRS	Dizziness, mild fatigue, bradycardia	Monitor; atropine if symptomatic
Second-Degree Type II / Mobitz II	Constant PR interval with sudden dropped QRS complexes	Syncope, dizziness, fatigue, bradycardia	Temporary pacing; permanent pacemaker often required
Third-Degree AV Block / Complete Heart Block	P waves and QRS complexes independent; AV dissociation	Severe bradycardia, hypotension, syncope, chest pain, heart failure	Emergency pacing and permanent pacemaker

First-Degree Heart Block

In first-degree AV block, all impulses from the atria reach the ventricles, but conduction is delayed.

ECG Features

• PR interval greater than 0.20 seconds
• Every P wave has a QRS complex
• Usually regular rhythm

Treatment

Usually no treatment is needed. Monitor ECG and review medications such as beta-blockers, calcium channel blockers, digoxin, or amiodarone.

Second-Degree Heart Block

Mobitz Type I / Wenckebach

This is usually less serious. The PR interval becomes longer until one QRS complex is dropped.

Mobitz Type II

This is more serious because it can progress to complete heart block. PR interval remains constant, but QRS complexes are suddenly dropped.

Third-Degree Heart Block

Third-degree heart block is also called complete heart block. No atrial impulses conduct to the ventricles. The atria and ventricles beat independently.

Clinical Features

• Severe bradycardia
• Dizziness or fainting
• Hypotension
• Chest pain
• Shortness of breath
• Heart failure symptoms
• Risk of cardiac arrest

Causes of Heart Block

• Myocardial infarction
• Coronary artery disease
• Myocarditis
• Cardiomyopathy
• Age-related degeneration of conduction system
• Beta-blockers
• Calcium channel blockers
• Digoxin toxicity
• Amiodarone
• Hyperkalaemia
• Cardiac surgery
• Congenital heart disease

Complications of Heart Block

• Severe bradycardia
• Reduced cardiac output
• Syncope and falls
• Heart failure
• Hypotension
• Ventricular arrhythmias
• Sudden cardiac death

Treatment and Nursing Management

• Assess airway, breathing, circulation
• Monitor heart rate, blood pressure, oxygen saturation
• Obtain 12-lead ECG
• Start continuous cardiac monitoring
• Check electrolytes, especially potassium
• Review medications causing bradycardia
• Administer atropine if prescribed for symptomatic bradycardia
• Prepare for temporary pacing if unstable
• Educate patient about permanent pacemaker care if required

Pacemaker Indications

A pacemaker may be required in symptomatic Mobitz II AV block, complete heart block, or persistent symptomatic bradycardia.

Quick Memory Tip for Nurses

First degree: PR interval is prolonged.

Mobitz I: Longer, longer, longer, drop.

Mobitz II: Sudden dropped QRS.

Third degree: P waves and QRS complexes are divorced.

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Meta Description

Learn heart block in simple nursing notes: definition, cardiac conduction system, first-degree, second-degree and third-degree AV block, ECG features, symptoms, causes, complications and pacemaker treatment.

Disclaimer

This article is for nursing education and exam preparation only. It is not a substitute for medical diagnosis or treatment. Always follow local clinical guidelines and senior medical advice.