Originally Posted by iceman_purav
UWorld says- Atelectasis ABG: hypoxia and respiratory alkalosis (CO2 washout due to hyperventilation). Q.Id 4931
MTB 2 says- Acute Exacerbation of Chronic Bronchitis ABG: hypoxia and RESPIRATORY ACIDOSIS
My doubt- if pt is hypoxic in AECB and hyperventilating shouldn't he be in respiratory alkalosis???
I think you're confusing hyperventilation with tachypnea. Their respiratory rate might be increased (tachypnea), but "hyperventilation" implies that they have increased total amounts of air entering/leaving the lungs.
If a patient is truly hyperventilating, they will inhale a lot of oxygen and exhale a lot of CO2, thereby leading to respiratory alkalosis, as you suggest.
In COPD, patients are tachypneic, but they're not hyperventilating. Each breath contains very small volumes of air. They're still producing a lot of CO2, but they're retaining it in their lungs because they can't manage to blow out enough air. That's why we call them "CO2 retainers." If you look at the blood gases of a patient with COPD who is otherwise stable, you'll see that their CO2 is high, but their pH is normal because of renal compensation.
In acute exacerbations of COPD, their lung function gets even worse than usual. At this point, they're breathing even harder, so you might think that they'll blow off the CO2... but you have to remember that the reason why they're breathing harder is because they have a stronger respiratory drive from increased CO2 concentrations in the blood. They've increased their respiratory rate (tachypnea), but they're NOT hyperventilating... in fact, they've increased their resp rate BECAUSE they're hypOventilating. And that's what causes the increased pCO2.
So, when you look at the ABG in a patient with an acute exacerbation, you'll see a drop in pH. Their pCO2 won't tell you much because they're a CO2 retainer, so you don't know what their normal range is... but you DO know that their pH should be normal (due to renal compensation), so if the pH suddenly drops to 7.3, it means that their CO2 is higher than normal.