A 44 year old moderately dehydrated man was admitted with a two day history of acute severe diarrhea. Electrolyte results: Na+ 134, K+ 2.9, Cl- 108, HCO3- 16, BUN 31, Cr 1.5.
ABG: pH 7.31 pCO2 33 mmHg
HCO3 16 pO2 93 mmHg
What is the acid base disorder?
Answer (using the step by step approach)
1. History: Based on the clinical scenario, likely acid base disorders in this patient are:
- Normal anion gap acidosis from diarrhea or
- Elevated anion gap acidosis secondary to lactic acidosis as a result of hypovolemia and poor perfusion.
Look at the pH.
The pH is low, (less than 7.35) therefore by definition, patient is acidemic.
3. What is the process? Look at the PCO2, HCO3-
PCO2 and HCO3- are abnormal in the same direction, therefore less likely a mixed acid base disorder. Need to distinguish the initial change from the compensatory response. A low PCO2 represents alkalosis and is not consistent with the pH. A low HCO3- represents acidosis and is consistent with the pH, therefore it must be the initial change. The low PCO2 must be the compensatory response. Since the primary change involves HCO3-, this is a metabolic process, i.e. Metabolic Acidosis.
Calculate the anion gap
The anion gap is Na - (Cl + HCO3-) = 134 -(108 + 16) = 10
Since gap is less than 16, it is therefore normal.
Is compensation adequate? Calculate the estimated PCO2.
Using Winter's formula; PCO2 = 1.5 × [HCO3-]) + 8 ± 2 = 1.5 ×16 + 8 ± 2 = 30-34.
Since the actual PCO2 falls within the estimated range, we can deduce that the compensation is adequate and there is no seperate respiratory disorder present.
Assessment: Normal anion gap acidosis with adequate compensation most likely secondary to severe diarrhea.