Hypernatremic hypovolemia In children who present with hypernatremic hypovolemia, the total fluid deficit is composed of both a free water deficit and an isotonic deficit A pure water deficit is consistent with dehydration Hyperosmolality initially promotes water movement out of the cells, including brain cells Over several days, idiogenic osmoles are generated within the brain cells, prompting water movement into the intracellular space, restoring normal brain volume Once cerebral adaptation has occurred, rapid correction of the serum sodium can result in cerebral edema and severe neurologic consequence The goal of therapy in children with a serum sodium concentration above 150 mEq/L is to correct the hypernatremia at a rate of less than 10 to 12 mEq/L in 24 hours The total fluid deficit can be inferred by the estimated weight loss Calculation of the free water deficit is based upon the serum sodium and estimated current body water: Free water deficit = TBW(c) × [(serum Na/140) − 1] The difference between the total fluid deficit and the free water deficit is the estimated isotonic deficit Table 100.4 estimates the sodium and water deficits and outlines a plan for fluid management of a child with hypernatremic hypovolemia and serum sodium of 155 mEq/L After the patient has received the initial isotonic fluid bolus to emergently restore intravascular volume, subsequent therapy should correct the remaining isotonic deficit, free water deficit, ongoing losses, and maintenance requirements Depending on the acuity and severity of the process, the free water deficit should be replaced gradually to allow judicious correction of the serum sodium at the desired rate In general, D5 ¼ NS at one-and-a-half times maintenance would be expected to correct deficits over 36 to 48 hours Given the uncertainty of any correction plan and the possibility of ongoing losses, the best approach is to measure the sodium frequently as it corrects and adjust fluid content and rate as indicated DISORDERS OF SODIUM HOMEOSTASIS Goals of Treatment Hyponatremia (serum sodium less than 135 mEq/L) and hypernatremia (serum sodium greater than 150 mEq/L) are both associated with severe