Clinical meaning
Children have proportionally more total body water (TBW) than adults: neonates ~75% TBW, infants ~65%, compared to ~60% in adults. A larger percentage exists in the extracellular compartment, making children more vulnerable to rapid fluid shifts. The immature kidneys have limited concentrating ability (maximum urine osmolality ~600 mOsm/kg in neonates vs. 1200 in adults), making them less able to compensate for fluid losses. Dehydration classification by tonicity is critical: isotonic (isonatremic, Na 130-150 mEq/L) is most common (~80%), hypotonic (hyponatremic, Na <130) causes cellular swelling and seizure risk, and hypertonic (hypernatremic, Na >150) requires slow correction to prevent cerebral edema. The nurse performs comprehensive fluid status assessment, calculates fluid deficits, manages IV rehydration protocols, monitors electrolytes, and recognizes signs of dehydration-related shock.
Exam relevance
Risk factors: - Age <12 months (highest risk due to immature renal function and high BSA:weight ratio) - Acute gastroenteritis (rotavirus, norovirus, Salmonella, Shigella) - Fever >39°C (insensible losses increase 12% per degree above 37°C) - Burns (massive insensible losses proportional to BSA) - Diabetic ketoacidosis (osmotic diuresis) - Pyloric stenosis (projectile vomiting with hypochloremic metabolic alkalosis) - Cystic fibrosis (excessive salt losses in sweat) - Adrenal insufficiency (salt-wasting)