A physiological analysis of hyponatremia: Implications for patients on peritoneal dialysis

The basis for hyponatremia is a negative balance for sodium (Na+) plus pota ssium (K+) and/or a positive balance for water. In patients with normal ren al function, vasopressin is needed to prevent the excretion of electrolyte- free water. Vasopressin is not important when there is little residual rena l function. If hyponatremia is accompanied by a quantitatively appropriate gain in weight, this implies that a gain of electrolyte-free water was the basis for hyponatremia. In the absence of this weight gain, a loss of salts is to be suspected. If the extracellular fluid (ECF) Volume is obviously l ow, hyponatremia is due to a deficit of NaCl, unless there is a deficit of K+. With a KCI deficit and a contracted ECF volume, there should also be a large shift of Na+ into cells, so metabolic alkalosis would not be an expec ted finding. In contrast, those patients with no change in weight who have a normal or expanded ECF volume are subdivided into those with a gain of so lutes restricted to the ECF compartment (glucose, mannitol), or those with a deficit of solutes of intracellular fluid origin, which implies that a ca tabolic state (malnutrition) may be present.