I understand basics of ABG/BMP and acid base rules w/ a primary d/o w/ compensations or two primary disorders. However, I remember seeing some triple acid base problems in the past and couldn't identify the 3 disorders in the problems. anyone got sample acid base problems w/ 3 disorders and how you arrived at the findings? thanks.
I have an exam on this topic tomorrow, so I've been looking at a few of these. Here might be an example. A Type II DM patient who is taking Metformin is admitted to the ICU after an acute exacerbation of his concurrent COPD which happened after he he became ill and began vomiting. This patient may have a relatively normal pH, but he is at risk for 3 different disorders. He is taking Metformin, which can cause a High AG Metabolic Acidosis. He may have a high pCO2 from the COPD, which causes a respiratory acidosis (We'll say acute here, because he is having an acute exacerbation of previously stable COPD). With all of this however, he has been vomiting, which means that he is losing H+ from the stomach, putting him at risk for metabolic alkalosis.
Let's say that his ABG comes back with the following: pH=7.39, pCO2=70, HCO3-=36, AG=30. These numbers may not work out EXACTLY, but the concept is there. His pH is normal, but his pCO2 is MUCH higher than any normal compensation. This means that he has a respiratory acidosis. Because the pH is normal, we already suspect a second disorder, an alkalosis. Remember that compensation NEVER makes pH normal. He has an elevated HCO3-, making it likely a metabolic alkalosis.
However, none of this explains the REALLY LARGE anion gap. This can only be caused by a high AG Metabolic Acidosis. A good rule of thumb then would be to subtract the difference between expected AG and the real AG from the expected HCO3-. So 30-14=16. Since HCO3- should be ~24 in a normal individual, we subtract 16 and get an expected HCO3- of 8( Remember UA-UC=Na-Cl-HCO3-). But the patient has an HCO3- of 36. This is a severe metabolic alkalosis that is compensating for both the Respiratory and High AG Metabolic Acidosis. A Triple Acid Base Disorder.
Good Luck.
