Hey all question about Alveolar Arterial O2 gradient. Kind of confusing to write this out, hope someone can follow.
So I know that you have increased gradient when its a lung problem with say diffusion defect, v/q mismatch etc.
and that if you have problem 'outside the lung' - a la respiratory depression, airway obstruction etc. you do not have increased gradient.
my question is WHY is there a gradient with the with lung problem, and not with extrapulmonary problem? where's the difference in the equations?
i would think with both you'll have decreased arterial oxygen (hypoxemia), so problem has to be in alveolar air side. yeah ur not ventilating in the resp depression case, but you're still equilibrating cus theres no problem there. so is the lack of Aa gradient in the equation due to increased co2 so you're alveolar o2 is decreased in the equation??
[[ PAo2 - Pao2 = gradient ]]
[[ PAo2 = 150- Paco2/0.8 ]]
is the increase in co2 causing the decrease in PAo2, and thats why PAo2 (Alveolar o2) is just as low as Pao2 (arterial o2) and thus you don't have a gradient with w/ an extrapulmonary problem?
if thats true then with intrapulmonary problems do you have hypoxemia and a co2 closer to normal as the reason for the gradient in the equation? if that's true is it because you're ventilating still, and can still equilibrate co2, just not o2?? is o2 harder to equilibrate to alveolar air than co2?? (i get that theres no co2 in air u breathe in so can draw it down lower w/ hyperventilation)
that would kind of make sense, like in example of a PE where you're hypoxemic, but have resp. alkalosis (cus hyperventilating and blowing off co2), so you're o2 AND co2 are low, same thing with pure emphysema..
but there are also pulmonary conditions where o2 is low and co2 high - chronic bronchitis (where mucus plugs block co2 from leaving)
can someone explain, where am i going wrong here?? thanks
edit-
So i get that o2 wont be equilibrating with pulmonary diseases, and then you'll have a gradient b/w alveolar air oxygen and arterial oxygen. But whats the difference in the EQUATION so that there is NO gradient with extrapulmonary disease (i.e. respiratory depression) is it because co2 is higher and thus Alveolar air oxygen is lower (this makes sense but is this not true with pulmonary diseases too?)
i can think of it on its own and say yeah no airflow in, so it just equilibrates and theres no gradient, but what part of the equation accounts for that?
So I know that you have increased gradient when its a lung problem with say diffusion defect, v/q mismatch etc.
and that if you have problem 'outside the lung' - a la respiratory depression, airway obstruction etc. you do not have increased gradient.
my question is WHY is there a gradient with the with lung problem, and not with extrapulmonary problem? where's the difference in the equations?
i would think with both you'll have decreased arterial oxygen (hypoxemia), so problem has to be in alveolar air side. yeah ur not ventilating in the resp depression case, but you're still equilibrating cus theres no problem there. so is the lack of Aa gradient in the equation due to increased co2 so you're alveolar o2 is decreased in the equation??
[[ PAo2 - Pao2 = gradient ]]
[[ PAo2 = 150- Paco2/0.8 ]]
is the increase in co2 causing the decrease in PAo2, and thats why PAo2 (Alveolar o2) is just as low as Pao2 (arterial o2) and thus you don't have a gradient with w/ an extrapulmonary problem?
if thats true then with intrapulmonary problems do you have hypoxemia and a co2 closer to normal as the reason for the gradient in the equation? if that's true is it because you're ventilating still, and can still equilibrate co2, just not o2?? is o2 harder to equilibrate to alveolar air than co2?? (i get that theres no co2 in air u breathe in so can draw it down lower w/ hyperventilation)
that would kind of make sense, like in example of a PE where you're hypoxemic, but have resp. alkalosis (cus hyperventilating and blowing off co2), so you're o2 AND co2 are low, same thing with pure emphysema..
but there are also pulmonary conditions where o2 is low and co2 high - chronic bronchitis (where mucus plugs block co2 from leaving)
can someone explain, where am i going wrong here?? thanks
edit-
So i get that o2 wont be equilibrating with pulmonary diseases, and then you'll have a gradient b/w alveolar air oxygen and arterial oxygen. But whats the difference in the EQUATION so that there is NO gradient with extrapulmonary disease (i.e. respiratory depression) is it because co2 is higher and thus Alveolar air oxygen is lower (this makes sense but is this not true with pulmonary diseases too?)
i can think of it on its own and say yeah no airflow in, so it just equilibrates and theres no gradient, but what part of the equation accounts for that?
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