The serum sodium concentration after surgical operation: Precision permits predictions

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Abstract
Summary: In view of previous conflicting reports, the distribution of a water overload was investigated in 6 healthy volunteers. The fall in serum sodium was predictable from prior measurement of total body water (tritium) confirming that the induced hyponatraemia was dilutional. Using a standard intravenous fluid regimen, a prospective study for 6 days on 50 patients undergoing major abdominal surgery established a confidence band for change in serum sodium concentration. This band not only enables precise prediction of whether observed sodium values in patients are ‘normal’ for that point in their postoperative course, but also demonstrates that changes are more prolonged than was at first thought. Their dilutional nature following uncomplicated surgery is supported by linear relationships between serum osmolality and serum sodium (s-Osm = 1·86 [s-Na+] + 18·74; P < 0·001) and between observed and calculated osmolality (Obs-Osm = 0·97 [Cale-Osm] + 15·11; P < 0·001). More detailed investigation of 14 patients by isotope dilution provided independent evidence of dilutional change. This conclusion is additionally supported by lesser changes in patients on a more restricted fluid regimen. As a practical confirmation of the utility of this approach, departures from the simple dilutional relationships were observed in 7 critically ill patients whose postoperative course was complicated. In this group osmolal ‘gaps’ of 20–50 mosmol/kg were identified. Current standard postoperative fluid regimens cause dilutional changes; these are predictable and provide a vardstick for use in management of both the healthy and the ill surgical patient.