What is AFib (Atrial Fibrillation)? - Definition & Causes

Instructor: Artem Cheprasov
This lesson will teach you about AFib. You'll learn what happens during AFib from an electrical standpoint and some of the causes of atrial fibrillation.

What is AFib?

Did you know that your heart can have an irregular rhythm? Atrial Fibrillation (AF), also abbreviated as AFib, is a kind of arrhythmia or irregular heartbeat. In fact, it is the most common type of arrhythmia.

According to the Centers for Disease Control and Prevention (CDC), about 2.7-6.1 million people in the U.S. alone have AFib. Roughly 2% of the population younger than 65 and around 9% of those 65 and over have AFib. Since it's so common, let's figure out what happens during AFib and some of its many causes.

The Heart's Electrical System

To understand AFib, you first need to understand how the heart works. Your heart is able to beat thanks to an internal electrical system. Basically, like wires in a home power lights, appliances, and other processes, the hearts electrical system powers the heart's ability to beat within a certain rate and rhythm in order to pump blood throughout the body.

So how does it work! Well, the heart has four chambers. There are two upper chambers called atria, and two larger lower chambers called ventricles. These are the chambers that relax to fill with blood and then contract to pump it forward. The heart also has an internal pacemaker, called the SA node, located in the right atrium.

The SA node discharges about 60-100 times a minute when a healthy adult is at rest. This discharge sends an electrical signal through the atria, which causes them to contract to push blood into the ventricles. The electrical signal then reaches a gatekeeper of sorts, called the AV node, which is located between the atria and the ventricles. Here, the electrical signal slows down just a bit, which allows the ventricles to fill with blood. After the gatekeeper opens the gateway, so to speak, the electrical signal spreads through the ventricles, which causes them to contract and push blood forward.

Atrial Fibrillation

In AFib, the atria discharge (beat) at a very fast rate. This is because many electrical signals begin throughout the atria and pulmonary veins, instead of just the SA node. Meaning, multiple packets of electrical activity randomly circulate within the atria at the same time. Because of this gigantic mess, the atria cannot contract effectively. If you were to compare the ECG (electrocardiogram) of a person with AFib, you'd notice a lot of small, rapid, and irregular waves as a result of this chaotic electrical activity.

Note how neatly the electrical impulse spreads from the SA node in a normal heart and how the ECG has a regular and neat rhythm to it. Compare this to AFib on the right, where the packets of electrical signals travel haphazardly and the ECG has lots of small, rapid, and irregular waves

The one saving grace of this condition is that the AV node, our gatekeeper, is unable to conduct every crazy atrial impulse into the ventricles. If this were to actually occur, an extremely dangerous condition, called ventricular fibrillation, would result.

Instead, in AFib, some of the impulses (electrical signals) traveling from the atria are completely blocked by the gatekeeper, while others only partially make it through to the ventricles. The latter signals don't activate the ventricles to contract, but they may block or delay other impulses trying to do so. This causes a completely irregular ventricular rhythm during AFib.


The rate at which the ventricles will beat during AFib is thus dependent on how the AV node is controlled. If the person is at rest, a high vagal tone will cause a slow ventricular rate. The vagal tone is determined by the vagus nerve, which helps the heart slow down. Think of this scenario as the gatekeeper being told by his boss, Mr. Vagus, to slow down the rate at which cars enter a highway on a ramp.

On the flipside, if the person is active, then the sympathetic system (which speeds things up) will allow more of the signals to reach the ventricles, and this can result in a very fast ventricular rate. However, even though the ventricles may have a fast rate, they won't beat as fast as the atria, which beat at a much faster rate in this disorder.

Either way, a big hallmark of this arrhythmia is actually a completely irregular ventricular rhythm, regardless of how fast or slow the ventricular rate actually is (aka the main symptom is an irregular heartbeat both at rest and during activity). Because of this incoordination between the atria and ventricles, the amount of blood pumped into the ventricles and then into the body is random. Sometimes, the body will receive small amounts of blood in quick succession, while other times it will receive larger amounts of blood. This can lead to the other symptoms like lightheadedness and shortness of breath.

Causes of AFib

The causes of AFib are many. They include:

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