Predicts the change in serum Na per litre of infusate:

ΔNa per L = (Infusate Na − Serum Na) / (Total Body Water + 1)

TBW = 0.6 × weight (men) or 0.5 × weight (women). Once you know ΔNa per litre, divide your target 24-hour Na rise by it to get the volume to infuse — then spread evenly over 24h.

• 3% hypertonic saline: 513 mEq/L (first-line for symptomatic severe hyponatraemia)
• 0.9% normal saline: 154 mEq/L
• Ringer's lactate: 130 mEq/L (avoid in severe hyponatraemia — too dilute)
• 0.45% half-normal saline: 77 mEq/L (raises Na slowly — used in hypernatraemia not hypo)

Osmotic demyelination syndrome (ODS), historically central pontine myelinolysis, occurs when chronic hyponatraemia is corrected too quickly. Caps:

• Chronic (or unknown duration) hyponatraemia: ≤8 mEq/L per 24h (max 10), ≤18 mEq/L per 48h
• Acute (<48h): Brain hasn't adapted — can correct up to 10-12 mEq/L per 24h safely
• High-risk for ODS: Na <105, alcoholism, malnutrition, hypoK, liver disease — be more conservative (≤6 mEq/L per 24h)
• If overshoot: Stop hypertonic, give 5% dextrose IV 3 mL/kg, consider DDAVP 2-4 mcg IV to re-lower Na

For severe symptomatic hyponatraemia (seizures, coma, severe vomiting), don't titrate slowly — give boluses:

• 3% saline 100-150 mL IV over 10 min, may repeat up to ×3 until symptoms resolve
• Then continue as Adrogue-Madias guided infusion to complete 24h target
• Recheck Na within 1-2h of each bolus

• 3% saline is not always commercially available — many Indian ICUs prepare in-house by adding 23 mL of 3% NaCl concentrate (or 30 mL of 23.4% NaCl) to a 500 mL bag of NS. Confirm pharmacy SOP.
• SIADH from TB drugs (especially rifampicin, INH), neuro infections (TBM, JE), and lung infections is common — always rule out before labelling idiopathic.
• Thiazide-induced hyponatraemia is increasingly recognised in elderly Indian outpatients on telmisartan-HCTZ.