Understanding Heart Block on ECG

Introduction

Heart block is a condition in which the electrical signals that control the heartbeat are slowed or completely blocked. This abnormal conduction can be detected using an electrocardiogram (ECG).

Types of Heart Block

Heart block is classified into three main degrees:

First-Degree Heart Block

• The electrical impulse is delayed but still reaches the ventricles.
• ECG Findings: Prolonged PR interval (>200 ms), but all P waves are followed by QRS complexes.
• Symptoms: Usually asymptomatic and often an incidental finding.

Second-Degree Heart Block

This type is further divided into two types:

Type I (Mobitz I or Wenckebach)

• Progressive PR interval prolongation until a QRS complex is dropped.
• ECG Findings: PR interval lengthens before a beat is missed.
• Symptoms: Mild dizziness or no symptoms at all.

Type II (Mobitz II)

• Sudden, unpredictable failure to conduct atrial impulses to ventricles without PR prolongation.
• ECG Findings: Constant PR interval, but some P waves are not followed by QRS complexes.
• Symptoms: More severe symptoms like dizziness, fainting, or fatigue as it may progress to complete heart block.

Third-Degree (Complete) Heart Block

• There is no communication between the atria and ventricles.
• ECG Findings: P waves and QRS complexes are present, but they are independent of each other.
• Symptoms: Severe bradycardia, dizziness, fainting, and even heart failure.

Causes of Heart Block

• Aging-related degeneration of the conduction system.
• Heart conditions such as myocardial infarction.
• Electrolyte imbalances (e.g., high potassium levels).
• Side effects of certain medications like beta-blockers or calcium channel blockers.
• Congenital heart disease.

Diagnosis and Treatment

• Diagnosis: ECG is the key tool. Sometimes further testing with Holter monitoring or an electrophysiological study is required.
• Treatment: Depends on severity:
  • First-degree: Usually no treatment needed.
  • Second-degree Type I: Monitoring, stopping medications that worsen conduction.
  • Second-degree Type II and third-degree: Often require a pacemaker to restore normal heart rhythm.

Conclusion

Identifying heart block on an ECG is crucial for diagnosing and managing conduction abnormalities. While mild forms often don't need treatment, advanced types require timely intervention to prevent complications like sudden cardiac arrest.

Source recommendations

1. American Heart Association Guidelines on Bradycardia and Conduction Disorders

1. https://www.heart.org/en/health-topics/arrhythmia/about-arrhythmia/conduction-disorders
2. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000628
3. https://www.heart.org/en/health-topics/arrhythmia/about-arrhythmia/bradycardia--slow-heart-rate
4. https://pubmed.ncbi.nlm.nih.gov/30412709/
5. https://www.jacc.org/guidelines/bradycardia

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/