How do I calculate a patient's fluid requirements?

How to Calculate Fluid Requirements

For maintenance fluid requirements in adults, use 35 mL/kg/day for patients aged 18-60 years and 30 mL/kg/day for those over 60 years, then adjust based on clinical conditions such as fever (add 2-2.5 mL/kg per day for each 1°C rise above 37°C) and ongoing losses. 1

Basic Maintenance Calculations

Adults

• Ages 18-60 years: 35 mL/kg body weight per day 1
• Ages >60 years: 30 mL/kg body weight per day 1

These baseline calculations assume normal organ function without abnormal losses and should be adjusted based on the patient's fluid status, medical condition, activity level, and organ function. 1

Pediatric Patients

The Holliday-Segar formula remains the standard method for calculating maintenance fluid requirements in children, calculated as: 2, 3

• 100 mL/kg/day for the first 10 kg of body weight
• Plus 50 mL/kg/day for the next 10 kg (10-20 kg)
• Plus 25 mL/kg/day for each kg above 20 kg

For hourly rates, this translates to 4 mL/kg/hour for the first 10 kg, 2 mL/kg/hour for the next 10 kg, and 1 mL/kg/hour for each kg above 20 kg. 3

Example: A 25 kg child requires approximately 1,600 mL per day (≈65 mL/hour) under standard euvolemic conditions. 3

Adjustments for Specific Clinical Conditions

Increased Fluid Requirements

Add to baseline maintenance for: 1

• Fever: Add 2-2.5 mL/kg per day for each 1°C rise in body temperature above 37°C per 24-hour period of pyrexia 1
• Hyperventilation, hypermetabolism, or gastrointestinal losses: Increase volume above standard calculation as clinically indicated 2, 3

Decreased Fluid Requirements

Reduce maintenance volume to 65-80% of calculated amount for patients with: 2, 3

• High-risk ADH secretion states (pneumonia, CNS infection, postoperative state, dehydration)
• Mechanical ventilation
• Temperature-controlled environments

Reduce to 50-60% of calculated volume for patients with: 2, 3

• Renal failure
• Heart failure
• Hepatic failure

Patients with Ongoing Fluid Losses

These individuals must be assessed on a daily basis to account for stomal losses, drain output, or other measurable losses. 1 The fluid prescription should reflect these losses in addition to baseline maintenance requirements.

Fluid Type Selection

Adults

Balanced crystalloid solutions (Ringer's lactate or Hartmann's solution) are preferred as they have an electrolyte concentration closer to physiologic levels. 1 These have historically been the most commonly used for fluid resuscitation and maintenance. 1

Pediatric Patients

Isotonic fluids (0.9% saline or balanced crystalloids) should be used for maintenance hydration in acutely ill children, especially during the first 24 hours. 2, 3 This recommendation is based on Level 1+ evidence showing that hypotonic fluids significantly increase the risk of hospital-acquired hyponatremia and potentially fatal hyponatremic encephalopathy. 2

Balanced/buffered crystalloids are preferred over 0.9% saline for initial resuscitation in critically ill children, as they modestly shorten length of stay and avoid hyperchloremic metabolic acidosis. 3

Special Populations

Sepsis and Septic Shock

Initial fluid resuscitation begins with 30 mL/kg of crystalloid within the first 3 hours. 1 This fixed volume enables clinicians to initiate resuscitation while obtaining more specific information about the patient. 1 Many patients will require more fluid than this, and further fluid should be given in accordance with functional hemodynamic measurements. 1

Burn Patients

The Modified Parkland formula calculates fluid requirements as 3-4 mL/kg/% TBSA over 24 hours, with half administered in the first 8 hours and the remainder over the next 16 hours. 4 This applies to adults with burns ≥10% TBSA and children with burns ≥10% TBSA. 4

Children with burns >10% TBSA require additional maintenance fluids calculated by the 4-2-1 rule (Holliday-Segar) added to the Modified Parkland formula. 4 Retrospective data shows children require approximately 6 mL/kg/% TBSA over the first 48 hours. 4

Total Fluid Accounting

The calculated maintenance volume must incorporate ALL fluid sources, including: 2, 3

• Intravenous maintenance fluids
• Blood products
• Intravenous medications (both infusions and boluses)
• Arterial and venous line flush solutions
• Enteral intake

This is critical to prevent "fluid creep", which represents a striking 32.6% of mean daily total fluid volume and can lead to inadvertent volume overload. 5 Maintenance and replacement fluids account for 24.7% of mean daily total fluid volume, far exceeding resuscitation fluids at 6.5%. 5

Monitoring and Titration

Clinical Endpoints

Fluid resuscitation rates should be adjusted based on clinical and hemodynamic parameters, with hourly urine output being the easiest and fastest method. 1

Target urine output: 1, 4

• Adults: 0.5-1 mL/kg/hour
• Children: 0.5-1 mL/kg/hour
• Electrical burns with myoglobinuria: 1-2 mL/kg/hour to prevent acute kidney injury 4

Additional Monitoring Parameters

Other parameters that may be used in combination with urine output include: 1

• Arterial lactate concentration
• Echocardiography
• Cardiac output monitoring
• Central venous pressure measurements (though CVP alone can no longer be justified for guiding fluid resuscitation) 1

Dynamic measures of fluid responsiveness (passive leg raises, fluid challenges against stroke volume measurements, or variations in systolic pressure/pulse pressure/stroke volume) have demonstrated better diagnostic accuracy at predicting fluid responsiveness than static measurements. 1

Frequency of Reassessment

Reassess acutely and critically ill patients at least daily regarding fluid balance and clinical status. 2, 3 Regular monitoring of serum electrolytes, especially sodium levels, is essential to prevent hyponatremia and fluid overload. 2

Critical Pitfalls to Avoid

Avoid hypotonic fluids (sodium <130 mEq/L) for routine maintenance in hospitalized children, as they are associated with fatal hyponatremic encephalopathy. 3

Account for "hidden" fluid sources (medications, flushes, blood products) to prevent inadvertent fluid overload. 3 Fluid creep represents a median of 645 mL (IQR 308-1039 mL) per day. 5

Do not continue full maintenance rates without reassessment in patients who develop fluid overload, oliguria, or whose clinical condition changes. 3 Fluid overload (defined as >10% increase in cumulative fluid balance from baseline) is an independent predictor of morbidity, mortality, prolonged mechanical ventilation, and increased length of stay. 2

Avoid both under-resuscitation and over-resuscitation, as both are associated with increased morbidity. 1 In burn patients, over-resuscitation ("fluid creep") is a major complication, with studies showing 76% of patients receive more than the upper Parkland limit, averaging 6.3 mL/kg/%TBSA. 4

Electrolyte Supplementation

Potassium supplementation of 20-40 mEq/L (2/3 KCl and 1/3 KPO4) is recommended once renal function is assured and serum potassium is known. 2 Sodium requirements are 1-3 mmol per 100 kcal, and potassium requirements are 1-3 mmol per 100 kcal. 2

Glucose provision (2.5-5% dextrose) should be included in sufficient amounts to prevent hypoglycemia but not excessive to avoid hyperglycemia, with monitoring at least daily. 3