FAQ on Electrocardiograms / ECG / EKG

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What is a 12-lead ECG? Why is it useful?

The 12-lead ECG is the measurement of the electrical conductivity of the heart through electrodes placed on the surface of the body.

It is important because it is possible to determine if the conductivity is abnormal, if the heart is enlarged, or if the heart is diseased or damaged.

What determines whether a wave is positive (upright) or negative? The P wave and the T wave are both positive in a Lead II ECG - why is this?

A positive wave represents depolarization, that is, positive charge in cytosol of the cardiac cells. The P and T waves represent atrial depolarization and ventricular repolarization (right after a long, maintained depolarization), hence, they both are seen as positive.

What is the relationship of the ECG to electrical and mechanical events in each heart-beat?

diagram of electrocardiogram / ECG / EKG

Waves seen in an electrocardiogram:

• P wave: atria are contracting due to depolarization.

• PQ/PR interval: conducting time from beginning of atrial excitation to the beginning of ventricular excitation.

• QRS complex: rapid ventricular depolarization.

• ST segment: time when the ventricular contractile fibers are depolarized during the plateau phase of action potential.

• T wave: ventricular repolarization, which is slower than depolarization.

What part of the ECG corresponds to atrial systole? What part corresponds to ventricular contraction?

The P wave represents atrial systole. Ventricular systole or contraction is represented by the QRS complex.

What part of the ECG corresponds to repolarization of the ventricles and atria?

Repolarization of the ventricles is shown by the T wave. Repolarization of the atria is not seen because it occurs together with ventricular repolarization.

What is the significance of the PR interval?

It is the conducting time from beginning of atrial excitation to the beginning of ventricular excitation.

Why is the height of the P wave less than the QRS complex?

This is because QRS complex represents action potential travelling over a large mass, namely, the ventricles, as opposed to action potential travelling on smaller mass (atria) represented by the P wave.

Why is a 12-lead ECG used in clinical practice?

The 12-lead ECG is used to detect:

1. An abnormal conducting pathway.

2. An enlarged heart.

3. Damaged regions in the heart.

Additional Reading:

Basic Cardiology

1. Electrical Activity of the Heart
2. Heart Muscle Mechanics
3. Heart Sounds and Murmurs
4. Additional FAQ on Heart Sounds and Murmurs
5. Cardiac Conduction Diagram
6. Blood Pressures in Cardiac Chambers
7. What is Pulsus Paradoxus?
8. FAQ on Heart Murmurs and Mechanisms of Turbulent Flow
9. Notes on Fetal Circulation
10. FAQ on Ischemic Myocardial Infarction
11. FAQ on Electrocardiograms / ECG / EKG
12. FAQ on Cardiac Conduction
13. The Heart as a Pump, the Cardiac cycle and Cardiac Output
14. What are the most common causes of aortic stenosis?
15. What is Pulseless Electrical Activity?
16. Causes and Complications of Arteriovenous Fistulas
17. CHADS2 Score for Atrial Fibrillation Stroke Risk
18. How to Reduce Blood Pressure without Medications?
19. Types of Shock
20. Locations of Heart Murmurs on Chest Wall
21. Types of Heart Blocks

Electrocardiogram (EKG/ECG) Topics

1. EKG Chest Leads
2. EKG Limb Leads
3. Quick 12-Lead ECG/EKG Format

Cardiology Videos

1. Video of Cardiology Examination in a Clinical Setting

Medical Images

Useful Medical Images & Diagrams (link opens in a new window)

Related Topics

1. Thorax Anatomy
2. Vascular Disorders
3. Heart Disorders
4. Histology of the Cardiovascular System
5. Jugular Venous Distention Workup
6. ER Chest Pain Workup
7. Cardiac Examination for Internal Medicine
8. FAQ on Blood Pressure
9. FAQ on principles of fluid and flow dynamics of Blood

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