Respiratory Pathophysiology 4

Last post on respiratory pathophysiology! Just like the last posts, this one will probably be short since so much has been covered before. In fact, the first part of this lecture is about the mechanics of breathing, which has already been explained here. That post also covers flow-volume plots, which are important too. The next part is about compliance, which has also conveniently been explained here. If you need to re-read these posts, just pay closer attention to how these things are affected by obstructive and restrictive lung disease, because that's important. Actually, I highly recommend reviewing flow-volume plots and their changes in obstructive/restrictive disease.

So what's new? Dynamic compression and expiratory flow limitation are new. Expiratory flow limitation is the idea that for any given volume, there's a maximum rate of airflow that you can produce, no matter how much effort you put in. This happens due to a little thing called dynamic compression.

As I briefly mentioned in an earlier post, the pleural pressure can become positive during forced expiration. If the pleural pressure is more positive than the pressure in the airways, this can cause the airways to become compressed, restricting airflow. Remember that the airflow resistance is directly proportional to 1/(radius^4), so small changes in radius will strongly affect flow.

An important thing to note here is that the pressure in the airways tends to vary along the length of the airway. The pressure in the alveoli tends to be the greatest, and then pressures decrease along the length of the airways. The point at which the airway pressure is equal to the pleural pressure is the equal pressure point (EPP). Anything beyond that is prone to compression during forced expiration.

Dynamic compression can be affected by several different factors. An increase in peripheral resistance, due to narrower airways etc., can cause a greater decrease in pressure along the airways, making them more prone to collapse. A reduction in lung recoil due to low lung volumes or emphysema may also make airways more vulnerable to collapse. On the flipside, stiffer airways may become less vulnerable to collapse.

And believe it or not, we're done for this part of the course! There were a few more slides on COPD and exercise as well as asthma, but we didn't actually get time to cover that in class due to technical difficulties.

This post is also the last post covering stuff on the PHGY350 midterm. Good luck in your studies!