Airway Resistance: Definition, Formula & Issues

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  • 0:02 Airway Resistance
  • 0:57 1st Formula
  • 3:07 2nd Formula
  • 4:10 Issues with Formulas
  • 4:58 Lesson Summary
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Lesson Transcript
Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

In this lesson, you'll discover what airway resistance means and learn two different ways of calculating it. You'll also uncover issues with calculating a true airway resistance.

What Is Airway Resistance?

Riddle me this: It's something you can't touch but you can hold, and every living animal has it. What is it?

The answer is breath!

How do we breathe? Well, with our lungs, of course. Although we breathe all the time, there are actually many factors involved in a simple breath.

One is the resistance to airflow in the lung, known as airway resistance, which in science and medicine is abbreviated as Raw. In order for air to move into or out of your lungs, it has to want to, and it has to overcome friction, the force that stops two things from sliding past each other. Yes, even air has friction. Airway resistance is thus a measure of the resistance to lung airflow caused by friction.

The resistance of your lung to airflow is a determinant of how easy it is to breathe; therefore, scientists like to measure it. Knowing the airway resistance helps doctors tell if your lungs are functioning normally.

1st Formula for Airway Resistance

Why does air want to move in the first place? The answer is the difference in pressure.

Imagine a rubber band holding a bundle of toothpicks. What happens if you pull on two sides of the rubber band? The pressure of the rubber band that was holding the toothpicks together decreases. This makes the toothpicks loosen and move. If you release the rubber band, it moves back into place, putting pressure back on the toothpicks and tightening them.

A similar event occurs in your lungs. Your muscles pull the lung (rubber band) open, decreasing the pressure and leaving air free to flow inside (the loosened toothpicks). Upon exhaling, your muscles relax, no longer pulling the lung, thereby increasing the pressure (retightening the toothpicks). The increased pressure forces the air to leave the lungs.

The first formula for airway resistance involves determining the change in pressure from where air enters (your mouth) to where it ends up, the part of the lung called the alveoli. The pressure of the lung where air enters is the same as the pressure of the atmosphere (PB), while the pressure of the alveoli (Palv) is determined by other factors.

The most common unit of pressure used for the lung is centimeters of water (cmH2O).

So, now we know one part of the equation: change in pressure. The other part involves flow rate (V dot), or how fast air flows.

Imagine you're in a room full of people and see an empty room nearby. You go in there to escape the crowd, but people follow you. If you measure the number of people who've moved into the room over time, you'll get the flow rate of people. For example, 10 people in 10 minutes = 1 person per minute. Since air isn't a solid, it's most often measured by how much volume in liters (L) it occupies, and since it moves so fast, time's usually measured in seconds (sec).

One formula for airway resistance then is a ratio of the change in pressure to the flow rate of air. To calculate the change in pressure, all we need to do is subtract the alveolar pressure from the atmospheric pressure. Normal airway resistance is around 2 cmH2O per L per sec.

2nd Formula for Airway Resistance

What happens if you can't measure flow or pressure?

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