Solution to "An Ileal Bypass Patient With Watery Diarrhea"

Robert M. Centor, MD


November 09, 2006

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

These were the patient's lab values:

Electrolyte panel: Sodium 130 mEq/L; potassium 3.2 mEq/L; chloride 102 mEq/L; bicarbonate 10 mEq/L; blood urea nitrogen (BUN) 13; creatinine 1.2 mg/dL; glucose 96 mg/dL

Arterial blood gas: pH 7.02; pCO2 40; pO2 180; calculated bicarbonate 11

The first thing we notice is that the serum bicarbonate is decreased. I always check the anion gap in these situations.

  1. Anion gap = 18 (Formula: Na - (Cl + HCOx) = 130 - [102 + 10])

  2. Delta gap calculation = (observed gap - expected gap) + observed bicarbonate = (18 - 12) + 10 = 16. Thus, the patient started out with decreased bicarbonate, which suggests an underlying normal gap acidosis.

  3. Winter's equation = 1.5 (calculated bicarbonate) + 8 (+ or -) 2 [expected pCO2] = 1.5 (11) + 8 = 14.5. This suggests hypoventilation, which is inappropriate for the degree of metabolic acidosis. Thus, this case represents the rare triple acidosis: anion gap, normal gap metabolic acidosis, and respiratory acidosis.

As I analyze this case, I suspect the following causes:

  1. Anion gap acidosis in an ileal bypass patient is likely secondary to D-lactic acidosis. D-lactic acidosis is an unusual form of lactic acidosis that is most often seen in patients who have undergone ileal bypass or small bowel resection. In these cases, short bowel syndrome can occur, resulting in increased bacterial metabolism of carbohydrate to D-lactic acid because of local overgrowth. In D-lactic acidosis, the anion gap rises initially but may fall over time because renal tubular reabsorption of D-lactate is inefficient compared with the reabsorption of L-lactate.

  2. The normal gap acidosis probably is secondary to the diarrhea (remember that stool is basic). Large-volume diarrhea is one cause of normal gap acidosis. We would like to measure her urine anion gap to confirm diarrhea as a cause, but we do not have those values.

  3. Respiratory acidosis is probably secondary to excess narcotics, which suppressed her respiratory drive.

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


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