Mechanism of Postoperative Limitation in Sodium Excretion: The Role of Extracellular Fluid Volume and of Adrenal Cortical Activity1

Abstract

The renal excretion of Na before and after the intravenous infusion of 2,000 ml of 0.9% NaCl solution over a 90 minute period was measured in 3 patients on the 2d-4th day prior to a major orthopedic surgical procedure and on the 2d postoperative day. In each case the rate of Na excretion before the challenging infusion was lower postoperatively. In 2 of the 3 cases the natriuretic response to the infusion postoperatively was only 1/3 of the preoperative response. That endogenous adrenal cortical activity is not involved in this phenomenon is suggested by the fact that a patient with Addison''s disease maintained on a constant dose of hydrocortisone exhibited a similar response following cholecystectomy. In 3 other patients, studied both before and after similar major orthopedic surgery, iso-tonic saline was infused in the postoperative study until the basal rate of Na excretion increased to the preoperative level. At this time the 2,000 ml challenging infusion was administered. The natriuretic response in each case was equal to or greater than that of the preoperative study. These observations are consistent with the concept that a postoperative decrease in the "effective" extracellular fluid volume may be responsible for both Na retention and the decreased natriuretic response to challenge following surgery.