Third-Degree Atrioventricular (AV) Block: Understanding ECG Findings

Introduction

Third-degree AV block, also known as complete heart block, is a serious cardiac condition where electrical signals from the atria do not travel to the ventricles. This leads to independent beating of the atria and ventricles, causing bradycardia and potentially life-threatening complications.

ECG Characteristics of Third-Degree AV Block

On an electrocardiogram (ECG), third-degree AV block is diagnosed based on the following key features:

1. Complete dissociation of P waves and QRS complexes:

   • Atrial (P wave) activity is regular but has no relationship with the ventricular rhythm.
   • Ventricular rhythm (QRS complexes) is also regular but independent.

2. Escape rhythm present:

   • If the AV node or His bundle takes over, there will be a junctional escape rhythm with narrow QRS complexes (rate of 40-60 bpm).
   • If the escape rhythm originates below the His bundle, ventricular escape rhythms result in wide QRS complexes (rate of 20-40 bpm).

3. Constant but unrelated P wave rate:

   • P waves march through the ECG strip consistently but do not trigger QRS complexes.

Causes and Risk Factors

• Ischemic heart disease (e.g., myocardial infarction)
• Degenerative changes in the conduction system (e.g., fibrosis of AV node)
• Inflammatory and infectious conditions (e.g., myocarditis, Lyme disease)
• Medications (e.g., beta-blockers, calcium channel blockers, digoxin toxicity)

Clinical Symptoms

• Severe bradycardia
• Fatigue, dizziness, or syncope (fainting)
• Hypotension and heart failure symptoms in severe cases

Treatment Approach

• Acute management: If the patient is hemodynamically unstable, immediate temporary pacing is required.
• Definitive treatment: Permanent pacemaker implantation is the treatment of choice for persistent third-degree AV block.
• Medication review: Discontinuation of drugs that may contribute to AV block.

Conclusion

Third-degree AV block is a life-threatening cardiac conduction disorder that requires prompt recognition and intervention. ECG remains the key diagnostic tool, and pacemaker therapy is often necessary.

Source recommendations

1. American Heart Association 2023 Guidelines on the Management of Bradycardia

1. https://cpr.heart.org/en/resuscitation-science/cpr-and-ecc-guidelines/algorithms
2. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000628
3. https://cpr.heart.org/-/media/cpr-files/cpr-guidelines-files/algorithms/algorithmacls_bradycardia_200612.pdf
4. https://pubmed.ncbi.nlm.nih.gov/30412710/
5. https://www.ahajournals.org/doi/10.1161/CIR.0000000000001194

2. European Society of Cardiology Guidelines for the Management of Patients with Cardiac Arrhythmias

1. https://academic.oup.com/eurheartj/article/43/40/3997/6675633
2. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines/Ventricular-Arrhythmias-and-the-Prevention-of-Sudden-Cardiac-Death
3. https://pubmed.ncbi.nlm.nih.gov/36017572/
4. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines/Supraventricular-Tachycardia
5. https://pubmed.ncbi.nlm.nih.gov/14557344/