Electrocardiography (ECG / EKG) and cardiac conduction

Updated: Feb 20, 2020

Electrocardiography (ECG / EKG):

A tool used to visualize the electricity that flows through the heart.

Electricity flows through the heart?

- The cells become more positive when they depolarize. This causes the electricity towards the depolarized cells from the neighbouring unpolarized cells.

Brief overview of the cardiac conduction

  1. The depolarization in the heart occurs automatically at the Sinoatrial (SA) node in the R. atrium.

  2. This depolarization at the SA node quickly propagates to the L. atrium via Bachmann's bundle.

  3. The depolarization at the SA node also propagates to the Atrioventricular (AV) node via internodal tracts.

  4. The depolarization at the AV node propagates to the bundle of HIS, which will eventually flows through the Purkinje fibers located within the main mass of the ventricles.

How is each stage of cardiac conduction represented in ECG?

1. P wave: SA depolarization (When the atria are filled with the blood, the SA node depolarized)

2. P-Q segment: The time it takes for the signals to travel from the SA node to AV node

3. QRS complex: Ventricular depolarization (AV node depolarization)

- Q: Depolarization of interventricular septum

- R: Depolarization of the main mass of the ventricles (Purkinje fibers)

- S: Last phase of the ventricular depolarization

4. S-T segment: Plateau in the myocardical action potential

** This is when the ventricles contract to pump the blood out. **

5. T wave: Ventricular relaxation