Third-Degree Heart Block (Complete Heart Block) on ECG

Introduction

Third-degree heart block, also known as complete heart block, is a serious cardiac condition in which the electrical signals from the atria do not reach the ventricles. As a result, the atria and ventricles beat independently of each other, which can lead to severe bradycardia (slow heart rate) and potentially life-threatening complications.

ECG Findings in Third-Degree Heart Block

1. Complete dissociation between P waves and QRS complexes:
   • The P waves (representing atrial activity) occur at a regular rate, independent of the QRS complexes (ventricular activity).
2. Regular but separate atrial and ventricular rhythms:
   • The atria follow their intrinsic pacemaker (e.g., the sinoatrial node), while the ventricles rely on an escape rhythm, likely from the AV junction or the ventricles.
3. Ventricular rate is slower than atrial rate:
   • The ventricular rate is usually 30-40 bpm if the escape rhythm comes from the AV junction, or 20-30 bpm if the escape rhythm is from the ventricles.
4. QRS morphology depends on the origin of the escape rhythm:
   • If the escape rhythm originates from the AV junction, the QRS complexes will be narrow (<120 ms).
   • If the escape rhythm originates from the ventricles, the QRS complexes will be wide (>120 ms), resembling a bundle branch block.

Symptoms and Clinical Significance

• Mild symptoms: Fatigue, dizziness
• Severe symptoms: Syncope (fainting), heart failure, cardiac arrest
• Risk of sudden cardiac death: If untreated, complete heart block may lead to life-threatening arrhythmias.

Causes of Third-Degree Heart Block

• Idiopathic degeneration of the conduction system (e.g., Lev’s or Lenegre’s disease)
• Myocardial infarction (especially inferior or anterior MI)
• Electrolyte imbalances (e.g., hyperkalemia)
• Medications (e.g., beta-blockers, calcium channel blockers, digoxin toxicity)
• Cardiac infections (e.g., Lyme disease, myocarditis)

Treatment

Emergency Management:

• Atropine (ineffective in most cases of complete heart block)
• Temporary pacing (transcutaneous or transvenous pacing) in unstable patients
• Identify and treat reversible causes (e.g., correct electrolyte imbalances, stop medications)

Long-Term Treatment:

• Permanent pacemaker implantation is usually required to restore normal heart function and prevent complications.

Conclusion

Third-degree heart block is a severe electrical conduction disorder of the heart requiring urgent medical attention. If you or someone you know experiences symptoms like dizziness, fainting, or extreme fatigue, seek medical help immediately to prevent life-threatening outcomes.

Source recommendations

1. American Heart Association Guidelines on Management of Bradycardia

1. https://www.heart.org/en/health-topics/arrhythmia/about-arrhythmia/bradycardia--slow-heart-rate
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/30586772/
5. https://cpr.heart.org/en/resuscitation-science/cpr-and-ecc-guidelines/algorithms

2. European Society of Cardiology Guidelines for 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/