Why does bronchoconstriction in asthma affect expiration more than inspiration?

[sprinkaan]

I'm confused.
As an asthmatic myself, I remember the feeling of not getting enough air into my lungs during attachs.

Now I'm studying the mechanisms and lung functions in asthma, and I repeatedly come across "expiratory obstruction" and "expirational wheeze" and "outflow obstruction".

My question is:
if your airways are hyperresponsive and/or hypersensitive and you have bronchoconstriction (either in atopic or non-atopic asthma) then why is the obstruction more on inspiration than expiration.
My mind just can't get around it.
If you have a narrowed airway, surely air should struggle as much getting in as getting out...??

Thank you for helping!

 

[SW VandeCarr]

I'm confused.

if your airways are hyperresponsive and/or hypersensitive and you have bronchoconstriction (either in atopic or non-atopic asthma) then why is the obstruction more on inspiration than expiration.
My mind just can't get around it.
If you have a narrowed airway, surely air should struggle as much getting in as getting out...??

Thank you for helping!

It's a good question. The pathophysiology of asthma is still poorly understood. Measures of airway resistance, peak flows, etc do not correlate well with disease severity. There's no doubt that initial hyperventilation plays a major role in air retention and this causes blood CO2 to fall leading to compensatory blunting of respiratory drive and decreased expiratory flows. In addition, small airway obstruction may play a role in air retention in the lung periphery. Finally, decreased elastic recoil means that it's more difficult for the lungs to expel air. With inspiration, the thoracic muscles and diaphragm actively expand the lungs, while recoil is more passive.

http://chestjournal.chestpubs.org/content/123/3_suppl/405S.2.full

 

[sprinkaan]

Thank you so much. That makes sense now, and I also read more about the elastic recoil causing increased resistance with exparation (compared to inspiration), even in normal lungs so it makes more sense now.

Thanks again for the reply!

 

[SW VandeCarr]

Thank you so much. That makes sense now, and I also read more about the elastic recoil causing increased resistance with exparation (compared to inspiration), even in normal lungs so it makes more sense now.

Thanks again for the reply!

You're welcome. Note that air trapping in the alveolar sacs may increases the "stiffness" of the lung periphery which could further reduce recoil.

 

[sprinkaan]

On the topic of asthma... I know there are two types - allergic and non allergic...
When it comes to triggers for non-atopic asthma, drugs and chemials like preservatives are possible triggers..

Again from personal experience, I get bronchospasms after only a few mouth-fulls of fruit juice preserved with e.g. Sodium Benzoate. This is usually an immediate reaction. How is this not allergic??

I don't know if my confusion is because of my lack of immunology knowledge, but if you have any ideas..?

 

[Borek]

I have a feeling that it can be easily explained in terms of simple physics. It is easier to squeeze the sack to create a high pressure that forces air out, than to create low pressure that will suck the air in. You can get much higher pressure difference between lungs and the air outside when blowing than when taking a breath.

But that's just an intuition, so I can be wrong. I guess it will be enough to find and check information on the pressure inside the lungs during breathing, but I don't know any source and I am in hurry now, so no time for googling.

 

[SW VandeCarr]

I have a feeling that it can be easily explained in terms of simple physics. It is easier to squeeze the sack to create a high pressure that forces air out, than to create low pressure that will suck the air in. You can get much higher pressure difference between lungs and the air outside when blowing than when taking a breath.

Yes, but that's exactly the opposite of what we have with asthma. Here we have more difficulty expelling air. Airway constriction is the reason, but the OP wanted to know why expiration is more difficult than inspiration.

Note there's more effort required for the normal person to exhale beyond their vital capacity. The only muscle that supports this in a significant way is the diaphragm. That's why the anterior abdominal wall is sucked in with forced exhalation. With inspiration, the powerful thoracic muscles are utilized in addition to the diaphragm to expand the chest cavity and lungs. Unforced exhalation is accomplished with elastic recoil for the most part. This is compromised in asthma. The reasons for this are probably related to air trapping (see my previous posts), although there is much about asthma that is not known.

 

[sprinkaan]

Thanks again for all the replies.. It helps so much to put book-knowledge into understandable concepts that I can work with.

 

[mtc1973]

The reason is that airways are pulled open by radial tension as lungs expand - but as lungs deflate the airways narrow as the radial tension falls. Hence airway resistance is higher upon deflation than inflation. Also somethng called dynamic airway compression happens during expiration especially forced expiration, that is not present during inhalation. Thats a long explanation - but essentially the effort of exhalation leads to airway narrowing because the intrapleural pressure is higher than the airway pressure which squeezes airway and acts as a valve - this only occurs on expiration.