A 53 year old previously well man presents with diarrhoea for 1 day with transient abdominal pain. On examination his vital signs are normal. He is clinically dehydrated, and his abdominal examination is unremarkable. His bloods on admission to ED show; EUC; Na  141, K  3.9 ,Cl 115, Bicarb 13, Urea  6.9, Cr  62 He is admitted overnight to the short stay unit and given 2L of intravenous NS. His diarrhoea and abdominal pain settled and on repeat examination he appeared well hydrated. His repeat bloods  the next morning show; Na 142, K 4.5, Cl 115, Bicarb 18, Urea 4.5, Cr 85 VBG; pH 7.28, CO2 25, BE -11, Bicarb 16, Lactate 0.49 UA - moderate ketones, pH 6 What is going on biochemically? Approach to the Abnormal Blood Gas Here's one approach to acid-base disorders pitched at a pretty basic level.  Acid-base disorders can be approached with three questions: 1) What is wrong? 2) What caused it? 3) What's is the body doing to correct it? 1) "What is wrong?" - Look at the pH, PaCO2, and HCO3. If all three values are normal, the answer to "What is wrong?" is "Nothing," and the other two questions can be ignored. If either pH, PaCO2, or HCO3- are abnormal, check the pH. If it's below 7.4, the answer to "What is wrong?" is "Acidosis." If above 7.4, "Alkalosis." If the pH is within the normal range (7.36-7.44), but the PaCO2 or HCO3- (or both) are abnormal, there is a acid-base