Depolarization vs. Repolarization of the Heart (Explained)

The heart is an astonishing organ, beating approximately 100,000 times per day to pump blood throughout the body.

The specialized cells of the heart’s electrical conduction system work together to generate impulses that cause the heart to contract and relax, resulting in the rhythmic beating of the heart.

The depolarization and repolarization process is important, as it enables the cells to contract and relax in a coordinated manner.

In this article, we will explore the difference between depolarization and repolarization and how they create the electrical activity of the heart that is essential for maintaining a healthy cardiovascular system. 

Depolarization vs. Repolarization

The heart is composed of specialized cells that work together to maintain a rhythmic heartbeat.

The conduction system of the heart is responsible for generating electrical signals that cause the heart to contract and relax.

The depolarization and repolarization of these cells play a crucial role in this process.

Depolarization

Depolarization is the process by which the cells of the heart become less negative and contract. When the cells are at rest, they are negatively charged or polarized.

However, when an electrical impulse is generated, the cells become depolarized. This occurs when the concentration of ions changes, specifically when sodium ions rush into the cells.

The sodium ions move into the cells through ion channels, which are specialized proteins embedded in the cell membrane.

Once the sodium ions enter the cells, they cause the cells to become positively charged, which results in the depolarization of the cells. This causes the cells to contract and push blood through the heart.

After depolarization and contraction, the cells need to relax, which is referred to as repolarization.

Repolarization

Repolarization is the process by which the cells return to their negatively charged state. This occurs when the cell membrane becomes more permeable to potassium ions, which exit the cell.

The loss of positive ions from the cell causes the cells to become negatively charged again, leading to relaxation.

The process of depolarization and repolarization creates the electrical activity of the heart, which is represented as a PQRST waveform on an electrocardiogram (EKG).

What is an Electrocardiogram?

An electrocardiogram (ECG or EKG) is a noninvasive diagnostic test that measures and records the electrical activity of the heart.

It is a commonly used diagnostic tool that can detect various heart conditions and is often the first step in diagnosing cardiac arrhythmias.

During an EKG test, electrodes are placed on the skin of the chest, arms, and legs, and connected to a machine that records the electrical signals produced by the heart.

The test is painless and takes only a few minutes to complete. The recorded signals are then used to create a visual representation of the heart’s electrical activity, called an EKG waveform.

EKG Waveform

The EKG waveform shows the pattern of electrical impulses produced by the heart, including the depolarization and repolarization phases, and can detect abnormalities in the heart’s electrical activity.

For example, an EKG can identify irregular heartbeats, such as atrial fibrillation or ventricular tachycardia, and can also help diagnose heart attacks and other cardiac conditions.

Cardiac Electrophysiology

Cardiac electrophysiology focuses on the study of the electrical properties and activity of the heart, including the following:

• Sinoatrial (SA) node
• Atrioventricular (AV) node
• Purkinje fibers
• Bundle of His

The sinoatrial node, also known as the pacemaker, is responsible for setting the heart’s rhythm. The wave of depolarization that originates from the SA node is responsible for causing the atria to contract.

This is visualized as the P wave on an EKG tracing.

The impulse is received by the AV node, which causes a short delay. This delay is visualized as the PR interval on the EKG tracing.

Then the stimulus moves through the bundle of His, through the left and right bundle branches, and into the Purkinje fibers.

This produces ventricular depolarization, and contraction occurs, which can be seen as the QRS complex.

Then, the heart enters into a short period of repolarization, which is a period where no electrical activity can be detected.

This is known as the ST segment on an EKG tracing.

Finally, the heart enters a period of recovery where the SA node is recharged, and another cycle can begin.

What is an Action Potential?

An action potential is a rapid, temporary change in the electrical potential of a cell caused by the movement of ions across the cell membrane.

In the context of the heart, an action potential refers to the electrical changes that occur in the heart cells during each heartbeat.

When the heart muscle cells are stimulated by an electrical impulse, such as from the sinoatrial (SA) node, the action potential is initiated, resulting in the depolarization and repolarization of the cells.

The sequence of depolarization and repolarization produces the electrical signal that triggers the contraction of the heart and the ejection of blood.

Normal Sinus Rhythm

Normal sinus rhythm is the rhythm of the heart when all of the electrical impulses are passing through the heart in a regular pattern.

This involves an upright P wave, consistent PR intervals that last from 0.12 to 0.20 seconds, and identical QRS complexes that are no longer than 0.12 seconds.

A normal sinus rhythm also involves flat ST segments, regular R-R intervals, and a heart rate that is between 60-100 beats per minute.

However, if the electrical impulse becomes irregular, this means that an arrhythmia is present.

Cardiac Arrhythmias

A cardiac arrhythmia is an abnormal heart rhythm that occurs when the electrical impulses that control the heart’s contraction and relaxation are disrupted.

Types

There are several different types of cardiac arrhythmias, including:

• Sinus tachycardia
• Sinus bradycardia
• Sinus arrhythmia
• First-degree heart block
• Second-degree heart block
• Third-degree heart block
• Atrial flutter
• Atrial fibrillation
• Premature ventricular contractions
• Ventricular tachycardia
• Ventricular fibrillation
• Pulseless electrical activity

Each type of cardiac arrhythmia has its unique characteristics and can be caused by various factors, such as genetics, lifestyle, medications, and underlying health conditions.

Final Thoughts

The depolarization and repolarization of the heart are critical components of the cardiac action potential. Without these electrical changes, the heart would not be able to contract and pump blood effectively.

Recap: Depolarization leads to the contraction of the heart, while repolarization leads to relaxation.

This process occurs thousands of times daily and is essential to keeping us alive. However, sometimes things can go wrong, leading to various types of arrhythmias.

By understanding the intricacies of depolarization and repolarization, healthcare professionals can diagnose and treat these conditions effectively.

With advances in technology and research, we can continue to deepen our knowledge of the heart and its electrical system, leading to better outcomes for patients with cardiovascular disease.