A 58-year-old woman comes to the emergency department due to cramping mid-abdominal pain. Imaging shows a small bowel obstruction. Vital signs and laboratory studies, including complete blood count and comprehensive metabolic panel, are within normal limits. The patient is admitted to the hospital for nonsurgical management. Oral intake is withheld to promote bowel rest, and an isotonic saline infusion is administered for hydration. She has no vomiting or diarrhea and after 2 days the abdominal pain improves. Total administration of sodium chloride infusion was approximately 6 L. Which of the following changes have most likely occurred in this patient since the onset of her hospitalization?

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11ec160b_3cfc_466c_9249_db9942a2388d_MD0001_00 This patient likely has nonanion gap metabolic acidosis (NAGMA) due to infusion of excess normal saline. Excess sodium chloride increases serum chloride (Cl⁻) to cause hyperchloremia. Because Cl⁻ and bicarbonate (HCO₃⁻) are the predominant anions in the body, the increased serum Cl⁻ causes intracellular shifting of HCO₃⁻ to maintain electronegative balance. This "loss" of HCO₃⁻ (reduced serum HCO₃⁻) decreases blood pH. Infusion of excess normal saline also increases intravascular volume, which the kidneys respond to by increasing sodium (Na⁺) excretion, resulting in increased urine Na⁺. Other causes of NAGMA usually involve loss of HCO₃⁻ from the kidneys (eg, renal tubular acidosis) or gastrointestinal tract (eg, severe diarrhea). With these etiologies, serum Cl⁻ is increased to compensate for the loss of HCO₃⁻. Because of the inverse relationship between Cl⁻ and HCO₃⁻, NAGMA of any etiology is also referred to as hyperchloremic acidosis. (Choice A) Metabolic acidosis with hypochloremia (decreased serum HCO₃⁻, pH, and serum Cl⁻) can occur with diabetic ketoacidosis. The typical inverse relationship between serum HCO₃⁻ and Cl⁻ is disrupted due to the presence of unmeasured anions (ie, ketones) in the blood and profound Cl⁻ losses from osmotic diuresis (due to glucosuria). Urine Na⁺ is low due to hypovolemia and consequent increased aldosterone-mediated urinary Na⁺ reabsorption. (Choice C) Increased serum HCO₃⁻ does not explain decreased pH (acidemia), but the combination of the two can occur with metabolic compensation for primary respiratory acidosis (eg, chronic hypoventilation). Serum Cl⁻ is decreased in response to the increase in serum HCO₃⁻. Urine Na⁺ is unchanged in the absence of a change in volume status. (Choices D and E) Hypochloremic metabolic alkalosis (decreased serum Cl⁻, increased serum HCO₃⁻ and pH) can occur in the setting of hypovolemia (eg, poor oral intake/vomiting, diuretic overuse) or hypervolemia (eg, primary hyperaldosteronism). Urine Na⁺ is decreased with hypovolemic etiologies and increased with hypervolemic etiologies. Educational objective: The infusion of excessive normal saline (sodium chloride) is a common cause of nonanion gap metabolic acidosis. The excess intravascular chloride (Cl⁻) causes intracellular shifting of bicarbonate (HCO₃⁻), reducing serum HCO₃⁻ and decreasing blood pH. __________ References: Hyperchloremic acidosis. (http://www.ncbi.nlm.nih.gov/pubmed/29493965)

A 58-Year-Old Woman Comes to the Emergency Department Due to Cramping