How do I calculate maintenance intravenous fluid requirements for an adult and a pediatric patient in the clinic?

Maintenance Intravenous Fluid Calculation and Management

Adults

For euvolemic adults, administer isotonic crystalloid at approximately 30 mL/kg/day (roughly 1.25 mL/kg/hour for a 70-kg adult = ~90 mL/hour), using 0.9% saline or preferably a balanced isotonic solution. 1

Fluid Volume Calculation

• Daily maintenance for adults is estimated at 30 mL per kilogram of body weight 1
• For a 70-kg adult, this equals approximately 2,100 mL/day or 90 mL/hour 1
• This formula applies to euvolemic patients at presentation 1

Fluid Type Selection

• Use isotonic solutions (0.9% saline or balanced crystalloids) rather than hypotonic fluids like 5% dextrose or 0.45% saline 1
• Hypotonic solutions distribute into intracellular spaces and may exacerbate cerebral edema in conditions like acute stroke 1
• Isotonic solutions distribute more evenly into extracellular spaces (interstitial and intravascular) 1

Special Considerations for Adults

• Hypovolemic patients: Provide rapid volume replacement first, then transition to maintenance fluids 1
• Patients with renal or heart failure: Exercise extra caution to avoid intravascular volume overload 1
• Monitor plasma osmolality: Elevated osmolality (>296 mOsm/kg) is associated with increased mortality 1

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Pediatric Patients

For hospitalized children, calculate maintenance fluids using the Holliday-Segar formula (4 mL/kg/hour for first 10 kg, 2 mL/kg/hour for next 10 kg, 1 mL/kg/hour for each kg above 20 kg) and administer isotonic balanced crystalloid solutions with 2.5-5% dextrose. 1, 2, 3

Fluid Volume Calculation: Holliday-Segar Formula

Hourly Rate Method:

• 4 mL/kg/hour for the first 10 kg 3
• 2 mL/kg/hour for the next 10 kg (10-20 kg) 3
• 1 mL/kg/hour for each kg above 20 kg 3

Daily Volume Method:

• 100 mL/kg/day for the first 10 kg 3, 4
• 50 mL/kg/day for the next 10 kg 3, 4
• 20-25 mL/kg/day for each kg above 20 kg 3, 4

Example: A 25-kg child requires approximately 1,600 mL/day (65 mL/hour): (10 kg × 100) + (10 kg × 50) + (5 kg × 20) = 1,600 mL/day 3

Fluid Composition: Critical Safety Point

Always use isotonic fluids (sodium 130-154 mEq/L) with added dextrose—never hypotonic solutions. 1, 2, 3

• Isotonic solutions significantly reduce hospital-acquired hyponatremia compared to hypotonic fluids (number needed to harm with hypotonic fluids = 7.5) 2
• Hypotonic maintenance fluids increase the risk of potentially fatal hyponatremic encephalopathy 2, 3
• This represents a major patient-safety improvement; multiple fatal hyponatremia cases prompted national safety alerts 2

Preferred Fluid Type:

• Balanced isotonic crystalloids (e.g., Isolyte P, PlasmaLyte, Lactated Ringer's) are superior to 0.9% saline 1, 2, 3
• Balanced solutions modestly shorten hospital length of stay and prevent hyperchloremic metabolic acidosis 1, 2, 3
• Normal saline causes dose-dependent hyperchloremic acidosis due to equal sodium and chloride concentrations 2

Glucose and Electrolyte Requirements:

• Add 2.5-5% dextrose to prevent hypoglycemia 2, 3
• Monitor blood glucose at least daily 1, 2, 3
• Add potassium based on clinical status after confirming adequate urine output (>1 mL/kg/hour) 1, 2, 3
• Target sodium: 2-3 mmol/kg/day for stable children 2
• Target potassium: 1.5-3 mmol/kg/day 2

Critical Volume Adjustments Based on Clinical Condition

Reduce to 65-80% of calculated volume in these high-risk situations: 1, 3

• Pneumonia, CNS infection, or postoperative state (increased ADH secretion)
• Mechanical ventilation with humidified gases
• Temperature-controlled environments
• Any condition with impaired free water excretion

Reduce to 50-60% of calculated volume for: 1, 3

• Renal failure
• Heart failure
• Hepatic failure

Increase volume by 10-20% for: 2

• Phototherapy (increased insensible losses)
• Fever, hyperventilation, or ongoing GI losses

Total Fluid Accounting: Preventing "Fluid Creep"

Calculate total daily fluid balance including ALL sources: 1, 3

• Primary IV maintenance fluids
• Blood products
• All IV medications (continuous infusions AND bolus doses)
• Arterial and venous line flush solutions
• Enteral intake (oral or tube feeding)
• Exclude: Replacement fluids for acute losses and massive transfusion 3

This comprehensive accounting prevents inadvertent fluid overload, which prolongs mechanical ventilation and increases length of stay 1, 3

Monitoring Requirements

Daily reassessment is mandatory: 1, 3

• Fluid balance and clinical status (perfusion, capillary refill, weight changes) 1, 2
• Serum sodium, potassium, and glucose levels 1, 2, 3
• Signs of fluid overload or dehydration 3
• Urine output (should exceed 1 mL/kg/hour before adding potassium) 2

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Special Population: Neonates

For term neonates in stable growth phase (>1 week old), administer 140-160 mL/kg/day (~6-7 mL/kg/hour) of balanced isotonic solution with 10% dextrose, providing sodium 2-3 mmol/kg/day and potassium 1.5-3 mmol/kg/day after confirming adequate urine output. 2

Very Low Birth Weight Infants (<1500 g)

Day 1: Start at 70-100 mL/kg/day with 0-2 mmol/kg/day sodium; delay potassium until urine output >1 mL/kg/hour 2

Days 2-3: Increase to 90-140 mL/kg/day; begin sodium supplementation (0-5 mmol/kg/day) after diuresis begins 2

Stable growth phase: 140-160 mL/kg/day with sodium 3-5 mmol/kg/day and potassium 2-3 mmol/kg/day 2

Critical Neonatal Pitfalls

• Never start potassium before confirming urine output >1 mL/kg/hour—VLBW infants are at high risk for life-threatening hyperkalemia 2
• Never use hypotonic maintenance fluids—they markedly increase hyponatremia risk 2
• Adjust volume upward by 10-20% during phototherapy 2
• Adjust volume downward by 10-20% on mechanical ventilation with humidified gases 2

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Common Pitfalls to Avoid Across All Ages

1. Never use hypotonic maintenance fluids (0.45% saline, 0.18% saline, 5% dextrose alone)—they cause fatal hyponatremic encephalopathy 2, 3
2. Never ignore "hidden" fluid sources from medications, flushes, and blood products—this causes inadvertent overload 1, 3
3. Never continue full maintenance rates without daily reassessment—clinical conditions change rapidly 1, 3
4. Never add potassium before confirming adequate urine output—especially critical in neonates and renal failure 2
5. Never use standard maintenance volumes in conditions with increased ADH secretion—reduce to 65-80% of calculated volume 1, 3