Effect of sodium administration on fluid balance and sodium balance in health and the perioperative setting. Extended summary with additional insights from the MIHMoSA and TOPMAST studies

Purpose: We aimed to provide an extended analysis of the physiological handling of of the sodium burden induced by maintenance fluids. Materials and methods: We revisited two studies that demonstrated, in healthy volunteers and in surgical patients, that maintenance fluids with 154 mmol/L of sodium lead to a more positive fluid balance than a regimen containing 54 mmol/L. We report different unpublished data on the renal handling of the imposed sodium burdens with specific attention to the resulting fluid and sodium balances. Results: The kidneys adapt to the sodium-rich fluids not only by altering sodium excretion, but also by retaining extra free water by concentrating urine. Realigning urinary sodium excretion with an increased administration takes around one day in health and much longer in the clinical setting. This difference may be explained by the presence of hypovolemia-induced aldosterone secretion in the latter group. Non-osmotic storage of sodium limits an unrestrained fluid retention even when very high amounts of sodium are administered but fluid accumulation will inevitably be further prolonged. Conclusions: Sodium administration induced by sodium-rich maintenance fluids leads, especially in the clinical setting, to prolonged fluid retention when compared with a regimen that resembles a healthy dietary sodium intake, even when kidney function is normal.