PAco2 and PaCO2, any difference in alveolar gas equation? - USMLE Forums
 USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
 Home USMLE Articles USMLE News USMLE Polls USMLE Books USMLE Apps
 USMLE Forums PAco2 and PaCO2, any difference in alveolar gas equation?
 Register FAQs Members List Search Today's Posts Mark Forums Read

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

#1
08-25-2011
 USMLE Forums Newbie Steps History: Not yet Posts: 8 Threads: 4 Thanked 2 Times in 2 Posts Reputation: 12
PAco2 and PaCO2, any difference in alveolar gas equation?

Ok this has been bugging me for a while now...

In the alveolar gas equation what value is used in the calculation, alveolar CO2 or arterial CO2?

I've checked numerous sources online, and some say one thing and others indicate different.

I've checked with Costanzo's Physiology book, and she uses alveolar CO2 similar to First Aid 2010...HOWEVER...according to FIRST AID ERRATA:

"508 Alveolar gas equation
In three instances, in the abbreviation for arterial carbon dioxide
pressure, change the capital A to a lower case a, to indicate we‟re
measuring arterial carbon dioxide pressure instead of Alveolar. "

I'm probably looking too much into this, but it really is driving me crazy!

#2
08-26-2011
 USMLE Forums Addict Steps History: Step 1 Only Posts: 145 Threads: 8 Thanked 76 Times in 57 Posts Reputation: 86

Ok, let me try to explain you

When we inspire air, it has PCo2 = 0. -->

Air reaches alveoli, now PACo2 (alveolar) = 40. -->

This is mix up with the pul blood and now pul blood have PaCo2 = 40 same as alveoli. -->

This blood goes to tissue where CO2 is formed and CO2 diffuses back to this blood raising this blood conc. from 40 to 46(because of CO2 from tissues). so PaCO2 = 46 -->

Now, this goes back to pul capillaries and to alveoli and it now changes from 46 to 40. This is how it keeps on going.

Hope it is bit clear now

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

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.