A 20-year-old man presents with obtundation. Past medical history is unobtainable. Blood pressure is 120/70 without orthostatic change, and he is well perfused peripherally. The neurological examination is nonfocal. His laboratory values are as follows:
Which of the following is the most likely acid-base disorder?a. Pure normal anion-gap metabolic acidosis
The first step in analyzing an acid-base disturbance is simply to look at the pH. This patient has an acidosis. Then look at the HCO3 and the PCO2 to determine the primary disturbance; that is, is it a metabolic acidosis or a respiratory acidosis? The serum HCO3 has decreased from 24 to 5 mEq/L, so this must be a metabolic acidosis. The PCO2 is below the normal value of 40 mm, so this cannot be a respiratory acidosis (the PCO2 would be above 40 in a respiratory acidosis). The first two steps are straightforward and unambiguous. The third (and most difficult) step is to assess the compensatory response. This patient has a metabolic acidosis, so you need to assess the respiratory compensation. That is to say, has the PCO2 decreased appropriately to compensate for the metabolic acidosis? The normal compensatory response in metabolic acidosis is for the PCO2 to decrease by 1 to 1.5 mm Hg for each 1-mEq decrease in HCO3 . This patient’s 19 mEq/L drop in bicarbonate is matched by a 25-mm drop in the P CO2 . Hence, this is a compensated metabolic acidosis. Another method of assessing compensation in a metabolic acidosis is to use the Winters formula, which says that the appropriate P CO2 equals 1.5 (HCO3 ) + 8. This would give an appropriate PCO2 of 15.5, very close to the measured PCO2 . Again, the compensatory response is appropriate for the degree of acidosis; the patient does not have a respiratory acid-base disorder.
The fourth step is to calculate the anion gap Anion Gap= Sodium Na - (Chloride Cl + Bicarbonate HCO3). The normal anion gap is 8 to 12 mEq/L; in this case the value is 29 mEq/L. Therefore, this is an anion-gap metabolic acidosis with appropriate respiratory compensation. A brief differential of anion-gap metabolic acidosis is as follows: