Understanding Heart Blocks on an EKG

Introduction

Heart blocks are abnormalities in the electrical conduction system of the heart, which can be identified using an electrocardiogram (EKG/ECG). They occur when the signal from the sinoatrial (SA) node is delayed or completely blocked before reaching the ventricles. There are three main types of heart blocks: first-degree, second-degree (which includes Mobitz type I and II), and third-degree heart block.

Types of Heart Blocks and Their EKG Patterns

1. First-Degree AV Block

• Definition: A delay in conduction through the atrioventricular (AV) node.
• EKG Features:
  • PR interval is prolonged (>200 ms, or 5 small squares)
  • Every P wave is followed by a QRS complex (no dropped beats)
  • Typically benign unless associated with other cardiac conditions

2. Second-Degree AV Block

This type is further divided into two subtypes:

Mobitz Type I (Wenckebach)

• EKG Features:
  • Progressive lengthening of the PR interval until a QRS complex is dropped.
  • Usually occurs due to transient conduction block at the AV node.
  • Often asymptomatic or may cause mild symptoms like dizziness.

Mobitz Type II

• EKG Features:
  • PR interval remains constant, but some P waves are not followed by QRS complexes.
  • Indicates a block below the AV node (in the His-Purkinje system).
  • More serious than Type I as it can progress to complete heart block.

3. Third-Degree (Complete) AV Block

• Definition: Complete disconnect between atrial and ventricular activity.
• EKG Features:
  • P waves and QRS complexes occur independently (no relationship between atrial and ventricular activity).
  • Ventricular rhythm is usually slow (<40 bpm if relying on ventricular escape rhythm).
  • Requires urgent intervention, often with a pacemaker.

Symptoms of Heart Block

• Dizziness or fainting (syncope)
• Fatigue
• Shortness of breath
• Chest pain (if ischemia is involved)
• Sudden cardiac arrest in severe cases

Diagnosis and Treatment

• EKG is the primary tool for diagnosing heart block.
• Holter monitoring may be used for intermittent heart block.
• First-degree and Mobitz I may not require treatment unless symptomatic.
• Mobitz II and third-degree block often require a pacemaker.
• Underlying causes (e.g., medications, electrolyte imbalances, heart disease) must be evaluated and managed.

Conclusion

Heart blocks can range from benign to life-threatening conditions. Understanding their EKG patterns is crucial for early diagnosis and appropriate management. If you experience symptoms such as dizziness, fainting, or palpitations, consult a cardiologist immediately.

Source recommendations

1. American Heart Association Guidelines on Bradycardia and Heart Block

1. https://www.heart.org/en/health-topics/arrhythmia/about-arrhythmia/conduction-disorders
2. https://cpr.heart.org/-/media/cpr-files/cpr-guidelines-files/algorithms/algorithmacls_bradycardia_200612.pdf
3. https://www.heart.org/en/health-topics/arrhythmia/about-arrhythmia/bradycardia--slow-heart-rate
4. https://cpr.heart.org/en/resuscitation-science/cpr-and-ecc-guidelines/algorithms
5. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000628

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/