Third-Degree Atrioventricular (AV) Block

Introduction

Third-degree atrioventricular (AV) block, also known as complete heart block, is a serious cardiac condition in which electrical signals from the atria do not reach the ventricles at all. This results in the atria and ventricles beating independently, which can cause bradycardia (slow heart rate), dizziness, fainting, and, in severe cases, sudden cardiac arrest.

ECG Findings of Third-Degree AV Block

Key ECG Characteristics:

1. P waves and QRS complexes are completely dissociated – The atrial and ventricular rhythms are independent.
2. Regular P-P intervals and R-R intervals – Both the atria and ventricles maintain their own steady rhythms, but they do not coordinate with each other.
3. Ventricular bradycardia – The ventricular rate is much slower than the atrial rate, often around 30-40 beats per minute if the rhythm originates from the His-Purkinje system, or less than 20 beats per minute if originating from a lower ventricular pacemaker.
4. Wide or narrow QRS complexes, depending on the escape rhythm origin – If the blockage occurs at the AV node, the QRS complexes may be narrow; if it occurs lower in the conduction system, they may be wide.

Causes of Third-Degree AV Block

• Degeneration of the conduction system (e.g., fibrotic changes, aging)
• Ischemic heart disease (e.g., myocardial infarction affecting conduction pathways)
• Electrolyte imbalances (e.g., hyperkalemia)
• Drug toxicity (e.g., beta-blockers, calcium channel blockers, digoxin)
• Endocarditis or myocarditis
• Congenital heart defects (in younger patients)

Clinical Symptoms

• Fatigue
• Dizziness or lightheadedness
• Syncope (fainting)
• Severe bradycardia
• Shortness of breath
• Chest pain (if myocardial ischemia is present)

Treatment Approaches

Emergency Management:

• Atropine (may be ineffective if the block is at or below the His bundle)
• Temporary pacing (via transcutaneous or transvenous pacing)
• Treatment of reversible causes (e.g., correcting electrolyte imbalances or stopping offending medications)

Long-Term Management:

• Permanent Pacemaker (PPM) – The definitive treatment for symptomatic third-degree AV block.

Prognosis

Without treatment, third-degree AV block can lead to severe complications such as cardiac arrest or heart failure. However, patients who receive a pacemaker generally have an excellent prognosis with restored normal heart function.

Source recommendations

1. American Heart Association – 2023 Guidelines for 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.0000000000001161
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 – 2021 Guidelines on Cardiac Pacing and Cardiac Resynchronization Therapy

1. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines/Cardiac-Pacing-and-Cardiac-Resynchronization-Therapy
2. https://pubmed.ncbi.nlm.nih.gov/34455430/
3. https://academic.oup.com/eurheartj/article/42/35/3427/6358547
4. https://www.acc.org/Latest-in-Cardiology/Articles/2021/11/23/19/45/ESC-Guidelines-on-Cardiac-Pacing-and-CRT-esc-2021
5. https://academic.oup.com/europace/article/24/1/71/6358627