Pediatric emergency medicine trisk 2851 2851
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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
