Learning: Acid-Base Disorders

I always had trouble with Acid-Base Disorders, so I thought I'd share this. People always say that they're not too difficult if you know what you're doing. Sure you can look at the pH, but how do you tell if the offense is a metabolic acidosis vs. a respiratory acidosis?

First of all, you're dealing with pH, PCO2, and bicarbonate.

1. Look at the pH. Is it low or high? High pH= alkalosis. Low pH = acidosis.

2. Now all you have to do is figure out which organ (lungs or kidney) is messing up and causing the pH dysfunction. For example, let's say we have a low pH. What can cause that? Either low bicarbonate or high PCO2. Note: you will never have both. So if you have a low bicarbonate, you will have a low PCO2, and visa versa. You can take my word for it, or you can see the explanation below(*). Now all you have to do is look at which one corresponds to the pH level. If you have a low pH and you have a low bicarbonate, you KNOW it's the kidney that isn't making enough bicarbonate to balance all that acid. Thus, metabolic acidosis. If you have low pH and high PCO2, you know that your lungs are messing up, making it a respiratory acidosis. Essentially the name of the disorder (i.e. respiratory acidosis) refers to the organ that isn't functioning properly (lungs in this case) and the pH value (acidosis in this case).

Just two steps. Not too shabby.

*here's an example to understand. If you have a low pH (acidosis), you're really just asking what causes it. The other organ, either the lung or the kidney, is going to try to compensate for that lack. So again, low bicarbonate is causing the acidosis, your lungs are going to try to compensate by expelling out CO2 (decreasing PCO2). Reason your way through this with any scenario. Another example if you're still confused: if you have a respiratory alkalosis, that means your lungs are messing up (low PCO2 gives you a high pH) and thus your kidneys will try to compensate by decreasing the bicarbonate.