Principles of quantitative water and electrolyte replacement of losses from osmotic diuresis

Osmotic diuresis results from urine loss of large amounts of solutes distributed either in total body water or in the extracellular compartment. Replacement solutions should reflect the volume and monovalent cation (sodium and potassium) content of the fluid lost. Whereas the volume of the solutions used to replace losses that occurred prior to the diagnosis of osmotic diuresis is guided by the clinical picture, the composition of these solutions is predicated on serum sodium concentration and urinary sodium and potassium concentrations at presentation. Water loss is relatively greater than the loss of sodium plus potassium leading to hypernatremia which is seen routinely when the solute responsible for osmotic diuresis (e.g., urea) is distributed in body water. Solutes distributed in the extracellular compartment (e.g., glucose or mannitol) cause, in addition to osmotic diuresis, fluid transfer from the intracellular into the extracellular compartment with concomitant dilution of serum sodium. Serum sodium concentration corrected to euglycemia should be substituted for actual serum sodium concentration when calculating the composition of the replacement solutions in hyperglycemic patients. While the patient is monitored during treatment, the calculation of the volume and composition of the replacement solutions for losses of water, sodium and potassium from ongoing osmotic diuresis should be based directly on measurements of urine volume and urine sodium and potassium concentrations and not by means of any predictive formulas. Monitoring of clinical status, serum sodium, potassium, glucose, other relevant laboratory values, urine volume, and urine sodium and potassium concentrations during treatment of severe osmotic diuresis is of critical importance.