Changes in the plasma sodium concentration after minor, moderate and major surgery.

The plasma sodium concentration, [Na]P, falls following major surgery and this fall is exacerbated by the administration of sodium-free or sodium-low intravenous fluids in the postoperative period. As a prelude to prospective, but controlled, studies we observed the range of [Na]P on admission for surgery in 477 patients. In 309 patients the extent of the change in [Na]P by the first postoperative day (delta [Na]) was related to the severity of the operation undergone (minor, moderate or major) and the volume of potentially hypo-osmolar intravenous fluid given. The mean admission [Na]P was significantly lower with increasing severity of proposed operation. A highly significant fall occurred in [Na]P following surgery of every grade; such falls in [Na]P were seen even when no potentially hypo-osmolar fluids were administered. A significant inverse relationship between admission [Na]P and delta [Na] was demonstrated. We conclude that [Na]P falls following surgery of any severity and that such falls occur even when exogenous dilution cannot be implicated as a cause.