What is the 4-2-1 (four-two-one) rule for estimating maintenance fluid requirements in pediatric (peds) patients?

The 4-2-1 Rule for Pediatric Maintenance Fluids

The 4-2-1 rule calculates hourly maintenance fluid rates as: 4 ml/kg/hour for the first 10 kg of body weight, 2 ml/kg/hour for the next 10 kg (11-20 kg), and 1 ml/kg/hour for each kilogram above 20 kg. 1, 2

How to Calculate Using the 4-2-1 Rule

The Holliday and Segar formula remains the appropriate clinical standard for calculating maintenance water needs in children based on weight 3. The calculation breaks down as follows:

Weight-based hourly rates:

• First 10 kg: 4 ml/kg/hour 1
• Second 10 kg (11-20 kg): 2 ml/kg/hour 1
• Each kg above 20 kg: 1 ml/kg/hour 1, 2

Practical example: For a 30 kg child, calculate: (10 kg × 4 ml/kg/hr) + (10 kg × 2 ml/kg/hr) + (10 kg × 1 ml/kg/hr) = 40 + 20 + 10 = 70 ml/hour total 2

Critical Fluid Type Recommendations

Use isotonic fluids (not hypotonic) as the standard maintenance solution in acutely and critically ill children, especially during the first 24 hours, to reduce the risk of hyponatremia. 3, 2 This represents a major shift from traditional hypotonic maintenance fluids that were historically recommended but are now recognized as causing dangerous hyponatremia in hospitalized children 4, 5.

Balanced isotonic solutions should be favored over normal saline to reduce length of stay. 3

Essential Fluid Composition Elements

Glucose provision:

• Include sufficient glucose to prevent hypoglycemia, monitored at least daily 3, 1
• Avoid excessive glucose to prevent hyperglycemia in critically ill children 3

Electrolyte supplementation:

• Add appropriate potassium based on clinical status and regular monitoring 3, 1
• Keep chloride intake slightly lower than the sum of sodium and potassium (Na + K - Cl = 1-2 mmol/kg/day) to avoid iatrogenic metabolic acidosis 3, 1

When to Restrict the 4-2-1 Calculation

Individual patient needs may deviate markedly from standard calculations. 3 Specific restrictions include:

Reduce to 65-80% of calculated volume:

• Children at risk of increased antidiuretic hormone (ADH) secretion 1
• This prevents fluid overload and hyponatremia in acutely ill patients 3

Reduce to 50-60% of calculated volume:

• Heart failure patients 1
• Renal failure patients 1
• Hepatic failure patients 1

Total Fluid Accounting

The total daily maintenance fluid volume must include ALL fluid sources: IV fluids, blood products, all IV medications (infusions and bolus), arterial and venous line flush solutions, and enteral intake. 3, 1 This comprehensive accounting prevents "fluid creep" and unintended fluid overload 3.

Avoid cumulative positive fluid balance to prevent prolonged mechanical ventilation and increased length of stay. 3, 1

Monitoring Requirements

Reassess at least daily: fluid balance, clinical status, and electrolytes (especially sodium) in all acutely and critically ill children receiving IV maintenance fluids 1, 2. Adjust rates based on clinical circumstances such as fluid retention, dehydration, or excessive water losses 3, 1.

Common Pitfalls to Avoid

• Using hypotonic fluids: This traditional practice increases hyponatremia risk and should be abandoned in favor of isotonic solutions 3, 4, 5
• Failing to adjust for clinical conditions: Continuing full 4-2-1 rates in patients with heart failure, renal failure, or SIADH leads to dangerous fluid overload 1, 2
• Ignoring all fluid sources: Not accounting for medication diluents, flushes, and blood products causes unrecognized fluid excess 3, 1
• Applying the formula to neonates: The 4-2-1 rule applies to infants and children beyond the neonatal period; neonates require different fluid management strategies 3