Understanding Leads I, II, and III on an ECG

Introduction

An Electrocardiogram (ECG or EKG) is a diagnostic tool used to assess the electrical activity of the heart. It uses different leads to capture the electrical signals generated by the heart from various angles. Leads I, II, and III are part of the standard limb leads and provide important information about cardiac function.

What are Leads I, II, and III?

Leads I, II, and III are bipolar limb leads that form a triangle around the heart, known as Einthoven’s Triangle. They measure the electrical differences between two limb electrodes: - Lead I: Records the voltage difference between the right arm (RA) and left arm (LA). - Lead II: Records the voltage difference between the right arm (RA) and left leg (LL). - Lead III: Records the voltage difference between the left arm (LA) and left leg (LL).

Clinical Significance

These leads focus on the electrical activity of the heart’s frontal plane and are especially useful for detecting: - Right or left axis deviation, which may indicate heart disease. - Inferior myocardial infarction, which affects the lower part of the heart. - Atrial abnormalities, particularly in Lead II, which is often used for rhythm analysis.

Interpretation Tip

• Lead II is commonly used for rhythm analysis because it closely follows the typical conduction pathway of the heart (from the sinoatrial node to the ventricles).
• A normal ECG should exhibit a positive deflection in Lead I and Lead II, indicating that the electrical impulse is moving in the correct direction.
• Abnormalities in these leads may suggest ischemia, enlargement, or conduction block.

Conclusion

Understanding Leads I, II, and III on an ECG provides critical insights into cardiac function and potential disorders. If abnormalities are present in these leads, further clinical evaluation may be necessary.

References

For more in-depth information, refer to the following clinical guidelines:

Source recommendations

1. American Heart Association (AHA) Guidelines on Electrocardiography

1. https://www.heart.org/en/health-topics/heart-attack/diagnosing-a-heart-attack/electrocardiogram-ecg-or-ekg
2. https://www.ahajournals.org/doi/10.1161/circulationaha.108.191095
3. https://cpr.heart.org/en/courses/acls-prep-ecg-pharmacology
4. https://www.ahajournals.org/doi/10.1161/circulationaha.106.180200
5. https://pubmed.ncbi.nlm.nih.gov/19281932/

2. European Society of Cardiology (ESC) Guidelines on the Interpretation of the ECG

1. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines
2. https://academic.oup.com/eurheartj/article-abstract/39/16/1466/2965923
3. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines/Neonatal-Electrocardiogram-Guidelines-for-the-interpretation-of-the
4. https://pubmed.ncbi.nlm.nih.gov/23564906/
5. https://www.jacc.org/doi/10.1016/j.jacc.2017.01.015