Hello and welcome to the Zero to Finals podcast. My name is Tom and in this episode I'm going to be

talking to you about viral induced wheeze. And if you want to follow along with written notes on this topic,

you can follow along at zero definals.com slash viral induced whys or in the respiratory section

of the Zero Definals Pediatrics book. So let's get straight into it.

Viral induced wheeze describes an acute wheezy illness caused by a viral infection.

We need to talk about the pathophysiology of a viral induced wheeze so that you understand what's

happening in these children.

Small children, typically under the age of three years, have small airways.

And when these small airways encounter a virus, commonly the RSV or rhinovirus,

they develop a small amount of inflammation and edema, which is swelling in the walls of the airway, and this restricts the space for air to flow.

This inflammation also triggers the smooth muscles of the airway to constrict, which further

narrows the space for the air to flow.

This swelling and constriction of the airway caused by a virus has little noticeable effect on the larger

airways of an older child or an adult. However, in the smaller diameter of the child's

airway, the slight narrowing leads to a proportionally larger restriction in airflow.

This is described by Poizzoil's law, which states that flow rate is proportionate to the radius of the tube to the power of four.

Therefore, halving the diameter of the tube decreases the rate of flow by 16 fold.

So this means in a small airway where there's a decrease in the diameter of the tubes by 50%.

There's actually a decrease in the flow rate by 16 times.

Airflowing through these narrow airways causes a wheeze

and the restricted ventilation leads to respiratory distress.

For some reason certain children are much more prone to this

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