Solution to "A 58-Year-Old Disoriented, Schizophrenic Male"

Robert M. Centor, MD


July 07, 2006

This is the solution to a case we presented recently. You may review the case here.

As you will recall, this patient was brought to the hospital because of strange behavior. He was disoriented and provided no history, so all we had available to evaluate his condition were the lab results presented in the original case.

After examining the numbers, we first note that the patient is alkalemic, is hyperventilating, and has a decreased serum bicarbonate level.

Second, we measure the anion gap: AG = Na - (Cl + HCO3). In this case, it is 139 - (90 + 14) = 35. Thus, we have an anion gap acidosis.

Third, since we have an increased anion gap, we can use Winter's equation to see if the hyperventilation is appropriate or inappropriate for the level of bicarbonate. Winter's equation estimates the appropriate pCO2 that compensates for a decreased serum bicarbonate level. Expected pCO2 = 1.5 (calculated HCO3) + 8. The expected pCO2 should be within 2 of the observed pCO2. In our patient, expected pCO2 = 1.5(11) + 8 = 24.5. Thus, the observed pCO2 is significantly lower than we can explain as compensation for the decreased serum bicarbonate. This means that the patient has a respiratory alkalosis.

Fourth, with all increased anion gap acidoses, we need to check the delta gap. The delta gap helps us determine what the bicarbonate would be if the anion gap were not present. This calculation can reveal either an underlying normal gap metabolic acidosis or an underlying metabolic alkalosis. Delta gap = observed gap - normal gap. We add the delta gap to the observed serum bicarbonate. The resulting number is the revealed bicarbonate - a measure of whether the patient has an underlying metabolic acidosis or alkalosis. In our patient we calculated the delta gap = 35 - 12 = 23. Adding 23 to the observed bicarbonate of 14 gives us 37. Thus, the patient has a metabolic alkalosis.

So, after all of our calculations, we know we have a patient with a complex acid-base disorder. He has an increased anion gap acidosis, a primary respiratory alkalosis, and a primary metabolic alkalosis.

The combination of increased anion gap acidosis (see the differential diagnosis we presented in an earlier case) and a primary respiratory alkalosis makes salicylate overdose the most likely diagnosis.

We measured a salicylate level that was markedly elevated. The metabolic alkalosis puzzled us at first. We postulated vomiting (but the patient did not have hypokalemia). Then, a family member brought in an empty bottle of Alka-Seltzer found near the patient's bedside; Alka-Seltzer contains significant amounts of bicarbonate. Thus, his metabolic alkalosis came from exogenous bicarbonate ingestion.

The patient recovered with conservative management.

Final diagnosis: Alka-Seltzer overdose causing a triple acid-base disorder -- anion gap metabolic acidosis, primary respiratory alkalosis, and primary metabolic alkalosis.

Read and participate in the discussion here, and watch for another new case soon.