Pathophysiology
Clinical meaning
The clinician managing pediatric dehydration must integrate developmental physiology, fluid compartment dynamics, and electrolyte correction strategies. Neonates have ~75% TBW with 40% in the extracellular fluid (ECF), making them vulnerable to rapid dehydration. The immature nephron has limited concentrating ability (600 mOsm/kg vs. 1200 in adults), limited sodium conservation, and a lower GFR. Isotonic dehydration (Na 130-150) reflects proportional water-sodium loss and is the most common (80%). Hyponatremic dehydration (Na <130) from hypotonic fluid replacement causes water to shift intracellularly, producing cerebral edema risk. Hypernatremic dehydration (Na >150) from free water loss causes water to shift extracellularly, and overly rapid correction can produce cerebral edema from reverse osmotic shift. The clinician must classify dehydration by severity and tonicity, calculate fluid deficit and maintenance requirements, prescribe electrolyte correction protocols, and investigate underlying etiology.