The 4-2-1 Rule Explained
The Holliday-Segar method was published in 1957 by Holliday and Segar. The authors calculated metabolic rate in children and determined that daily water requirements approximate 100 mL per 100 kcal metabolised. The method assumes resting metabolic rate of 100 kcal/kg for the first 10 kg, 50 kcal/kg for the next 10 kg, and 20 kcal/kg beyond 20 kg — giving the memorable 100-50-20 rule for daily volumes and the 4-2-1 rule for hourly rates.
Step-by-Step Calculation
For a 35 kg child:
- First 10 kg: 4 mL/kg/hr × 10 = 40 mL/hr
- Next 10 kg: 2 mL/kg/hr × 10 = 20 mL/hr
- Remaining 15 kg: 1 mL/kg/hr × 15 = 15 mL/hr
- Total: 75 mL/hr (1800 mL/day)
NICE 2015 Fluid Type Recommendations
NICE 2015 (NG29) recommends the following for maintenance IV fluids in hospitalised children and adults:
- Fluid of choice: Isotonic saline (0.9% NaCl) with 5% dextrose — provides sodium and calories to prevent starvation ketosis
- Add potassium: 20 mmol/L KCl to the bag unless patient is hyperkalaemic, anuric, or has acute kidney injury
- Avoid hypotonic fluids (0.18% NaCl, 0.45% NaCl) as routine maintenance — risk of hospital-acquired hyponatraemia
- Hartmann's solution: Acceptable alternative, especially post-operative. Contains lactate which is metabolised to bicarbonate
When to Adjust Maintenance Rates
| Condition | Adjustment | Reason |
|---|---|---|
| Fever (per 1°C above 37) | Increase by 10–12% | Increased insensible losses |
| Burns (>10% TBSA) | Add Parkland formula | Massive fluid shifts |
| Oliguria / AKI | Reduce to insensible losses (~400 mL/m²/day) + urine output | Impaired excretion |
| Mechanical ventilation | Reduce by 15–20% | Humidified air reduces insensible losses |
| SIADH | Restrict to 50–75% maintenance | Free water retention |
| High ambient temperature / sweating | Increase by 10–30% | Increased sweat losses |
| Diarrhoea / vomiting | Add measured losses mL for mL | Ongoing losses must be replaced separately |
Maintenance vs Resuscitation Fluids
Maintenance fluids are distinct from resuscitation boluses. Resuscitation fluids (10–20 mL/kg NS or Hartmann's in children; 250–500 mL in adults) are given rapidly for haemodynamic instability and are separate from the maintenance calculation. Maintenance fluids run continuously at a low rate to replace daily obligatory losses (urine, insensible losses via skin and lungs, stool).