Post-surgical hyponatraemia: The role of intravenous fluids and arginine vasopressin

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Abstract
In 14 patients having undergone abdominal surgery, plasma sodium, urine volume and osmolality and the urinary excretion of arginine vasopressin (AVP), the antidiuretic hormone in man, were measured daily for 7 days. Each patient was given 31 of intravenous fluids daily. In 8 patients the fluid was saline and in the other 6 it was dextrose-saline. The plasma sodium did not change in the saline group whereas it fell in the dextrose-saline group during the day of operation and then remained low. On the day of operation the urinary AVP increased to very high levels in both groups of patients. On that day and during the postoperative period the AVP level and urine osmolality were much higher in the saline group than in the dextrose-saline group, which suggests that a loss or redistribution of extracellular fluid was not the cause of the high AVP. These results taken together suggest that it was the higher AVP level in the saline group that was required to keep the plasma sodium normal. On the other hand, the results suggest that in the dextrosesaline group, although the AVP was lower than in the saline group, it produced a urine osmolality greater than that necessary for balance so that the patients retained water and became hyponatraemic. The fall in plasma sodium was small compared with that seen in patients with symptoms of water intoxication, but it may nevertheless have clinical effects and is best avoided. The results suggest that in these patients hyponatraemia can be avoided if saline rather than dextrose-saline is given on the day of operation.