What is the difference between depolarization vs repolarization of the heart that creates cardiac action potential?
In order to understand how the PQRST waveform is created on the ECG, you have to understand these two terms. Depolarization and repolarization are words that can be used to describe how a heart cell is responding to the electrical conduction system of the heart.
The heart cells I’m referring to are specialized heart cells that are part of the electrical conduction system of the heart. They make up the atria and ventricles and can receive electrical signals to cause the atria and ventricles to contract.
When these cells are resting they are negatively charged, or another way of saying this is that they’re polarized. To get these cells to move (hence contract), they have to become depolarized (less negative) by an electrical stimulation.
How does this happen? The cells become depolarized (hence contract) when the ion concentration is shifted. When the cell is resting there are more sodium ions on the outside of the cell than the inside, and they can’t get inside of the cell because of the impermeable cell membrane.
But when the cell is stimulated (hence by electrical activity from let’s say the SA node) it causes the cell membrane to become receptive to sodium and sodium can now get inside. When sodium rushes inside the cell, it makes the cell positively charged, which changes its polarized state to a depolarized state, and this leads to the contraction of the heart cell.
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Then right after the heart cell contracts it needs to relax. This is referred to as repolarization. Repolarization is where the cell goes back to its negatively charged state on the inside (hence it repolarizes). Therefore, this leads the heart muscle cell to relax.
**The take away from this is to remember that depolarization of the heart cells leads to contraction, and repolarization leads to relaxation (to keep them straight, remember that both repolarization and relaxation start with “re”.)
This process of depolarization and repolarization creates the ECG reading, which is caused by the electrical conduction system of the heart.
Click the link to learn more about how to interpret an ECG.