Indications

• Don't need maintenance IVF: eating/drinking, euvolemic, and normal osmoregulation
• Normal fluid requirement - ~ 1.5 L/d = ~ 50-75 mL/hr

Fluid balance

• TBW = weight (kg) x 0.6 (M) or 0.5 (F)
• ICF = TBW x 2/3
• ECF = TBW x 1/3
• Interstitial fluid = ECF x 3/4
• Plasma = ECF x 1/4

IV Fluids

• Normal serum osmolality = 275-295 mOsm/kg
• ^Dextrose is metabolized quickly, so D5W/D5 1/2NS is functionaly hypotonic
• ^*LR also has 4 mmol/L K, 1.5 mmol/L Ca, 28 mmol/L lactate which is converted to bicarb in vivo
• The K in LR doesn't significantly affect plasma K
• Normal saline is isotonic, but still hypernatremic/hyperchloremic compared to plasma → hyperchloremic non-gap metabolic acidosis (increases HCl levels)
• Excess chloride anion forces bicarb anion intracellularly or prevents reabsoprtion by proximal tubule, leading to acidosis
• **hide** Further reading - "Stewart approach" and "Strong ion" theory
• Bicarb solutes are D5W base + 1-3 ampules of Na bicarb (50 mEq of each)
• Other situations
• Metabolic acidosis - give D5W + 3 amps Na bicarb = replace sodium (150 mEq) while also giving base (bicarb) and avoiding Cl which can worsen acidosis
• Hypocalcemia - avoid bicarb (will cause more free calcium to bind to albumin)