Third-Degree Atrioventricular (AV) Block: ECG Characteristics and Clinical Significance

Introduction

Third-degree atrioventricular (AV) block, also known as complete heart block, is a severe conduction disorder in which no electrical impulses from the atria reach the ventricles. This leads to an independent atrial and ventricular activity, causing significant bradycardia and potential hemodynamic instability.

ECG Characteristics

On an ECG, a third-degree AV block can be identified by the following features:

• Complete dissociation between P waves and QRS complexes: Atrial and ventricular rhythms are independent of each other.
• Regular P-P intervals and R-R intervals: The atrial rate is generally faster than the ventricular rate.
• Wide or narrow QRS complexes: If the escape rhythm originates from the AV junction, the QRS is narrow. If it originates from the ventricles, the QRS is wide.

Clinical Symptoms

Symptoms depend on the ventricular rate and patient’s overall condition. They may include: - Severe fatigue - Dizziness or syncope - Chest pain - Dyspnea - Signs of heart failure in severe cases

Causes and Risk Factors

• Ischemic heart disease (e.g., myocardial infarction)
• Degenerative changes in the conduction system (e.g., age-related fibrosis)
• Medications (e.g., beta-blockers, calcium channel blockers, digoxin)
• Electrolyte imbalances
• Infections (e.g., Lyme disease, myocarditis)

Treatment and Management

Emergency Management

• Immediate pacing (transcutaneous or transvenous) for symptomatic patients
• Atropine is usually ineffective but may be attempted in certain cases

Long-Term Treatment

• Permanent pacemaker implantation is the definitive treatment
• Address underlying causes if reversible factors are present
• Medication adjustments if drug-induced

Conclusion

Third-degree AV block is a life-threatening condition requiring prompt recognition and often immediate intervention with pacing therapy. Long-term management typically includes pacemaker implantation. If you or someone you know has symptoms of AV block, seek immediate medical attention.

Source recommendations

1. American Heart Association Guidelines for the Management of Bradycardia and Conduction Disorders

1. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000628
2. https://www.heart.org/en/health-topics/arrhythmia/about-arrhythmia/conduction-disorders
3. https://www.jacc.org/guidelines/bradycardia
4. https://pubmed.ncbi.nlm.nih.gov/30586772/
5. https://www.hrsonline.org/guidance/clinical-resources/2018-accahahrs-guideline-evaluation-and-management-patients-bradycardia-and-cardiac-conduction

2. European Society of Cardiology Guidelines on Cardiac Pacing and Resynchronization Therapy

1. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines/Cardiac-Pacing-and-Cardiac-Resynchronization-Therapy
2. https://academic.oup.com/eurheartj/article/42/35/3427/6358547
3. https://pubmed.ncbi.nlm.nih.gov/34455430/
4. https://academic.oup.com/eurheartj/article/34/29/2281/401445
5. https://pubmed.ncbi.nlm.nih.gov/23801827/