Arrhythmia of the Heart: Terms, Definition & ECG Detection

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  • 0:06 Electrocardiogram (ECG)
  • 0:56 Arrhythmias
  • 1:36 Heart Block
  • 2:29 Bradycardia
  • 3:34 Tachycardia
  • 4:13 Fibrillation
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

An electrocardiogram, or ECG, can be used to detect irregular heartbeats or arrhythmias. In this lesson, you will learn about arrhythmias detected by ECG, including bradycardia, tachycardia, fibrillation, and heart block.

Electrocardiogram (ECG)

An electrocardiogram, or ECG, is a common test that records the electrical activity of the heart. A normal ECG has three distinct waves, and we call them the P wave, QRS wave, and the T wave, and you notice that they follow in alphabetical order. The P wave represents the spread of the electrical impulse through the atria, and this is referred to as atrial depolarization. This is what causes the atria to contract.

The QRS wave, which is sometimes called the QRS complex, represents the spread of the electrical impulse through the ventricles, or ventricular depolarization. As the wave of electrical activity spreads through the ventricles, they contract. And then the T wave represents the period of recovery, or relaxation, of the ventricles, or ventricular repolarization.


A diagram of the AV node assisting in moving impulses through the heart
AV Node Function Diagram

From these waves, we record the normal electrical activity of the heart, but we can also detect irregular heartbeats, called arrhythmias, by looking closely at the shapes of the waves and the intervals of time that separate the waves. In this lesson, we will look at some common abnormalities that can be detected on ECG.

We previously learned that the SA node initiates the electrical impulse that travels through the heart. But, this impulse cannot freely pass from the muscle fibers of the atria to the muscle fibers of the ventricles and must be relayed by the AV node. The AV node is like a traffic cop that allows the impulse to move from the atria to the ventricles.

Heart Block

Therefore, any damage or scar tissue at the AV node could result in an increased time between the contraction of the atria and the contraction of the ventricles. If the damage is severe, it could block the electrical signal completely, and the ventricles would no longer be under the influence of the SA node, causing them to beat completely on their own. This is a very inefficient way for the heart to move blood, and it's a condition known as heart block.

On an ECG, we would see an increased distance or time between the P wave and the Q part of the QRS wave. The reason for this increased distance or time is because the P wave represents contraction of the atria and the QRS wave represents contraction of the ventricles. This is called an increased P-R interval, and it's sometimes referred to as an increased P-Q interval.

What an increased P-R interval looks like on an ECG
Increased P R Interval


An ECG can detect a number of changes in the way that the heart is beating. Because the ECG machine is set at a standard speed, the beats per minute can be regulated. A normal heart rate is between 60 to 100 beats per minute. This number can vary among individuals, and generally, a slower heart rate at rest implies that the heart is pumping efficiently, and it is often a trait seen in very fit individuals. For example, a marathon runner might have a normal resting heart rate closer to 50 beats per minute.

Yet, interestingly, a heart rate slower than 60 beats per minute has a term applied to it, which is bradycardia. You can recall this term by remembering the word 'brady' is Greek for 'slow' and 'cardia' is Greek for 'heart.' So, literally, it's a slow heart. As stated, a slow heart rate can be normal and healthy, but it can also be a sign of a disease that damaged or slowed the heart's electrical system or a result of taking a medication. On an ECG, you would see more spacing between the heart beats.

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