Case 1

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:

2. 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.

4. 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.

5. 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.