Isotonic Saline Expands Extracellular Fluid and Is Inappropriate for Maintenance Therapy: In Reply

Abstract

In Reply.—Dr Holliday correctly emphasizes the important distinction between the use of intravenous (IV) fluid replacement to increase a contracted extracellular fluid (ECF) volume, with which antidiuretic hormone (ADH) secretion is frequently elevated, and fluid used to replace insensible fluid losses. We would argue that clinical evaluation of the degree of ECF contraction frequently overestimates the problem, resulting in significant volumes of IV fluid being administered,1 leading to volume overexpansion and the production of a hypertonic urine. This was seen in the study by Steele et al,2 in which the administration of large volumes of isotonic fluid led to the production of a hypertonic urine and a fall in plasma sodium (desalination). For the situation in which children are truly ECF volume contracted, Gerigk et al3 demonstrated that isotonic saline was more effective that oral rehydration or IV hypotonic saline in suppressing elevated ADH levels. We await with interest the publication of the data confirming this mentioned by Dr Holliday.We have also argued that formula used for estimating insensible losses results in the administration of excess electrolyte-free water,4 which can be particularly hazardous in patients who continue to have nonphysiologic-mediated ADH secretion. We believe that the point about the administration of isotonic saline causing hypernatremia and brain damage is not relevant in the context of IV fluid administration. The cases referred to by Dr Holliday occurred predominantly in infants with hypernatremia due to severe ECF contraction associated with large losses of free water and high hematocrits. On the contrary, the use of hypertonic saline to induce hypernatremia and a hyperosmolar state has now become commonplace in the intensive care management of patients with severe traumatic brain injury and raised intracranial pressure.If we have created the impression that all patients should receive normal saline regardless of fluid and electrolyte status, this is a misconception. We can only reiterate the concluding paragraph from our article, which states that hypotonic fluids should not be used routinely in the intraoperative or postoperative period or if a patient has a plasma sodium in the low-normal or distinctly hyponatremic range (<138 mmol/L). In addition, boluses of isotonic saline should only be given if there are clear hemodynamic indications for that infusion.5