Pathophysiology
Clinical meaning
Pediatric fluid and electrolyte management differs fundamentally from adult management due to children's higher metabolic rate, greater body surface area-to-weight ratio, higher total body water percentage (75-80% in newborns vs 60% in adults), and immature renal concentrating ability. These factors make children more susceptible to fluid imbalances and more rapid progression to dehydration. Maintenance fluid requirements are calculated using the Holliday-Segar method: 100 mL/kg/day for the first 10 kg, 50 mL/kg/day for the next 10 kg, and 20 mL/kg/day for each kg above 20 kg. The equivalent hourly rate is the 4-2-1 rule: 4 mL/kg/hr for first 10 kg, 2 mL/kg/hr for next 10 kg, 1 mL/kg/hr for each kg above 20 kg. Dehydration is classified as mild (3-5% body weight loss in infants, 3% in adolescents), moderate (6-9% in infants), and severe (>=10% in infants). Isotonic dehydration (proportional loss of water and sodium) is most common (80% of cases). Hypotonic dehydration (greater sodium than water loss) leads to intracellular edema and is more hemodynamically dangerous. Hypertonic dehydration (greater water than sodium loss) causes intracellular dehydration and neurological symptoms and...