 | Prep for USMLE |
| Forum | Resources | New Posts | Register | Login | » |
|
| Author | 4 Posts | |||||||||||||||||||||||||||||||
|
chesstitans
Forum Newbie Topics: 3 Posts: 7 |
could somebody pls explain to me how the A-a gradient is increased in a diffusion defect but is normal at high altitude. thanks in advance
|
Prep4USMLE.com
|
Advertisement
|
|
Dr_Cosme
| Forum Junior  Topics: 7 Posts: 60
Diffusion is inversely proportional to the thickness (think of pulmonary fibrosis) of the membrane and directly proportional to surface area (think of emphysema), and to Pressure gradient and solubility. In a diffusion defect, you have no trouble in PAO2, the problem is diffusing O2 through the alveolar membrane, which is a structural problem. In high altitude, you are situated in a high atmospheric pressure environment (over 760 mmHg), so a low PAO2 will be present. Even though you will compensate by hyperventilating due to the stimulation of the chemoreceptors, your PAO2 will increase, but also your PaO2 will not change because here the problem is not structural or anatomical (as the previous explenation), so the diffusion here is normal. I don't know if this helps, let me know. 
|
gag_pol_env
| Forum Junior Topics: 16 Posts: 45
DR_COSME FIRST of all at high altitude pressure decreses not increases.I think you r totally cofused.A-a gradient is difference between alveolar and arterial PO 2 ....At high altitude if pressure in alveoli decreses ,low Oxygen will cross the alveoli to blood.......then obviously pressure in arterial blood will decrease.BUT the A-a gradient will be the same.BUT u r right in acute stages there is some hyperventilation but after some time there is increase in hematocrit which acts as increased carriers of oxygen to tissues.WHEN WE TALK ABOUT HIGH ALTITUDE WE R TALKING ABOUT PERFUSION LIMITED SITUATION ,WHERE ALVEOLAR OXYGEN AND BLOOD OXYGEN ARE AT EQUILIBRIUM. WHEREAS IN DIFFUSION LIMITED SITUATION THERE IS NO EQUILIBRIUM.HENCE INCREASED A-a GRADIENT. AS DR COSME EXPLAINED ABOUT DIFFUSION IMPAIRMENT ITS TRUE. BUT WITH HYPER VENTILATION U CAN NOT CHANGE PAO 2 . PAO2 IS ALVEOLAR OXYGEN PRESSURE ,WHICH IS A MEASURE OF UR ATMOSPHERIC PRESSURE,WHICH DECREASES AT HIGH ALTITUDE. I THINK IT MAKES SENSE. DR_COSME IS WRONG AT MANY POINTS IF I AM WRONG PLEASE CORRECT ME.
|
Dr_Cosme
| Forum Junior Topics: 7 Posts: 60
Hello again, I see what the problem was, when I intended to say low PAO2 instead of high PAO2, guess I got distracted when I said that the patient will hyperventilate. If you read well, I said at the beginning that at high atmospheric pressure wee have a low PAO2, and that is the only point that I was wrong, instead of writing low I wrote high. READ WELL.
|
|
| | |||||||||||||||||||
|
This thread is closed, so you cannot post a reply.
 Similar forum topicsA-a gradient A-a gradient... help! hypoxemia and Aa gradient | Related resources |
Advertise | Support | Premium | Contact |
Your Privacy Choices