The influence of high and low sodium intakes on post-operative antidiuresis

There remains some doubt over the influence of intravenous fluid and electrolyte therapy on the development of postoperative hyponatraemia. We believe the effect of varying sodium intakes on fluid balance during both the early antidiuresis phase and the later postoperative period has received insufficient emphasis. Therefore, the effects of intravenous regimens, containing either 450 mmol/24 h of sodium (saline group) or no sodium (dextrose group), on salt and water balance, plasma sodium and plasma water concentrations have been studied in two groups of 6 patients undergoing upper abdominal surgery. Hyponatraemia was not seen in the saline group but marked salt (+ 1023 mmol) and water retention (+ 3509 ml) were seen by day + 4. The dextrose group became relatively hyponatraemic (131.8 mmol/l) by day +1 but, following a phase of fluid retention during the 24 h following surgery, they developed negative fluid balance with some recovery of plasma sodium levels. This study confirms the importance of the sodium content of intravenous fluids both on the early development of hyponatraemia and on water and salt balance during the later postoperative period.