Airway Pressures at FRC - USMLE Forums
 USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
 Home USMLE Articles USMLE News USMLE Polls USMLE Books USMLE Apps
 USMLE Forums Airway Pressures at FRC

 USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam

#1
03-15-2012
 USMLE Forums Addict Steps History: Not yet Posts: 140 Threads: 52 Thanked 17 Times in 15 Posts Reputation: 27
Airway Pressures at FRC

hi
this is from FA 2011 p 505

I could not understand this statement:

(at FRC airways and alveolar pressure are 0 , and intraapleural pressure is negative (prevents pneumothorax))

#2
03-15-2012
 USMLE Forums Scout Steps History: Not yet Posts: 36 Threads: 2 Thanked 55 Times in 22 Posts Reputation: 65
Functional residual capacity + pneumothorax

Lung and chest wall

First, understanding what is FRC is and how it portray in the lung and chest wall.

Refer to the diagram in FA 2011 Respiratory pg. 505

FRC = functional residual capacity

- The center of the airway pressure-volume curve is the functional residual capacity (FRC) of the lung.
- FRC identifies the resting state where the airway pressure equals zero.

- Due to the elasticity of the lungs, the alveolar transmural pressure is always positive, resulting in a perpetual collapsing force on the lungs; this is why the curve marked "lung" always has a positive value.
- Inversely, the chest wall tends to transmit an expanding force to the lungs, resulting in a negative transmural pressure across the chest wall most of the time, except during maximal inspiration (see graph).

- These two forces, the positive alveolar transmural pressure, and the negative chest wall transmural pressure oppose one another equally at the functional residual capacity (FRC), resulting in an airway pressure of zero.
- Thus, at the FRC, the airway pressure is zero and there is no tendency for air to flow either into or out of the lungs at this point.

- When puncture of the pleura allows intrapleural communication with the atmosphere (pressure: 0 cm H2O), the negative intrapleural pressure equilibrates with the atmospheric pressure via entry of air into the intrapleural space; thus, a pneumothorax will develop as the lung collapses inward and the chest wall springs outward.

Hope this help! =)
 The above post was thanked by: Dr.Lacune (08-21-2012), DrThea (12-11-2015), hemlock (05-22-2014), Joanna (03-15-2012), makori (12-24-2013), mnajjj (07-23-2017), nelly (01-12-2017), patho2012 (03-16-2012), rapiddo (05-24-2012), Thrax_usmle (01-09-2014), Usmle16Forall (05-06-2017), voicesinmyhead (01-20-2014)
#3
01-20-2014
 USMLE Forums Scout Steps History: --- Posts: 75 Threads: 38 Thanked 16 Times in 14 Posts Reputation: 26

thanks. but it still doesnt make sense for FA to include that sentence without any mention about pleura rupture.

I mean in a normal indiv, why would this prevent pneumothorax.

Shouldnt the neg pleural pressure help to prevent collapse instead in a normal person.?

#4
01-20-2014
 USMLE Forums Veteran Steps History: Not yet Posts: 206 Threads: 13 Thanked 179 Times in 77 Posts Reputation: 189

Quote:
 Originally Posted by voicesinmyhead thanks. but it still doesnt make sense for FA to include that sentence without any mention about pleura rupture. I mean in a normal indiv, why would this prevent pneumothorax. Shouldnt the neg pleural pressure help to prevent collapse instead in a normal person.?
This must be an error, check the errata on their website. The negative pleural pressure is what causes a pneuomothorax, not prevent it
#5
12-11-2015
 USMLE Forums Scout Steps History: 1 + CK Posts: 28 Threads: 8 Thanked 6 Times in 5 Posts Reputation: 16

I am reading FA 2015 and there is the same statement.

Negative intrapleural pressure prevents lung collapse.

Pneumothorax is prevented via the integrity of parietal and visceral pleura.

On the other hand loss of this negative pressure results in Pneumothorax.
Maybe this is what they mean here when saying that negative intrapleural pressure prevents pneumothorax but really this negative pressure drives air in when pleura is damaged and doesn't prevent it.

So is this mistake in FA or I don't understand something?

#6
01-28-2016
 USMLE Forums Newbie Steps History: Not yet Posts: 1 Threads: 0 Thanked 0 Times in 0 Posts Reputation: 10

You seem to be correct in your understanding.

As long as negative intrapleural pressure exists, you do not have pneumothorax, but once you damage the chest and pneumothorax begins, negative intrapleural pressure is now disappearing from existence, so there does not exist anything to prevent the pneumothorax. Sounds like word play, doesn't it?

Perhaps there is confusion because FA seems to use the term "pneumothorax" interchangeably with "lung collapse" because they are so connected. Pneumothorax itself is defined as "air between chest wall and lung that LEADS TO lung collapse". Negative intrapleural pressure would only prevent lung collapse when there is NO air rushing into the space between chest wall and lung.
#7
01-29-2016
 USMLE Forums Scout Steps History: 1 + CK Posts: 28 Threads: 8 Thanked 6 Times in 5 Posts Reputation: 16

This confusion is because of word play, I agree.
I think saying that ''integrity of parietal and visceral pleura prevents pneumothorax'' is better than saying that "negative intrapleural pressure prevents pneumothorax". On the other hand, integrity of parietal and visceral pleura maintains negative intrapleural pressure which prevents lung from collapse.

 Tags Physiology-, Respiratory-

Message:
Options

## Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
User Name:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
 AMG IMG ---
USMLE Steps History
What steps finished! Example: 1+CK+CS+3 = Passed Step 1, Step 2 CK, Step 2 CS, and Step 3.

Choose "---" if you don't want to tell.

 Not yet Step 1 Only CK Only CS Only 1 + CK 1 + CS 1+CK+CS CK+CS 1+CK+CS+3 ---
Favorite USMLE Books
 What USMLE books you really think are useful. Leave blank if you don't want to tell.
Location
 Where you live. Leave blank if you don't want to tell.

## Log-in

 Similar Threads Thread Thread Starter Forum Replies Last Post flowerlady USMLE Step 1 Forum 5 03-15-2012 09:54 PM evavar USMLE Step 1 Forum 2 07-30-2011 03:19 PM eesfee USMLE Step 1 Forum 2 05-25-2011 12:24 PM drahmednawaz USMLE Step 1 Forum 3 02-18-2011 04:35 AM Pixie USMLE Step 1 Forum 7 08-02-2010 01:13 AM