Fluid calculation
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Fluid Calculation - Complete Overview
Fluid management involves three components: maintenance, deficit replacement, and ongoing losses. Here is a structured reference covering all key formulas.
1. Maintenance Fluid Calculation (Holliday-Segar / 4-2-1 Rule)
This is the most widely used formula for calculating hourly IV maintenance fluid requirements.
Hourly Rate (4-2-1 Rule)
| Body Weight | Rate |
|---|---|
| First 10 kg | 4 mL/kg/h |
| Next 10 kg (10-20 kg) | 2 mL/kg/h |
| Each kg above 20 kg | 1 mL/kg/h |
Formula:
[4 × 10] + [2 × 10] + [1 × (weight - 20)]Worked Examples
45 kg patient (from Sabiston Textbook of Surgery):
- 10 kg × 4 = 40 mL/h
- 10 kg × 2 = 20 mL/h
- 25 kg × 1 = 25 mL/h
- Total = 85 mL/h
70 kg patient (60 kg example from Fischer's):
- 10 kg × 4 = 40 mL/h
- 10 kg × 2 = 20 mL/h
- 40 kg × 1 = 40 mL/h
- Total = 100 mL/h
Daily Rate (100-50-20 Rule)
| Body Weight | Daily Requirement |
|---|---|
| First 10 kg | 100 mL/kg/day |
| Next 10-20 kg | 50 mL/kg/day |
| Each kg > 20 kg | 20 mL/kg/day (reduce to 15 mL/kg/day in elderly or cardiac patients) |
Source: Mulholland and Greenfield's Surgery, Table 11.5
Typical Maintenance Fluid Composition
- Dextrose 5% in 0.45% NaCl (D5 1/2 NS) is the standard
- Add 20-40 mEq/L KCl as needed
- Daily electrolyte targets: Na 1-2 mEq/kg/day, K 0.5-1 mEq/kg/day
- Insensible losses (skin + respiratory): estimated at 8-12 mL/kg/day, increased by fever, tachycardia, hyperventilation
2. Dehydration / Deficit Replacement
Grading Dehydration
| Grade | Body Weight Loss | Signs |
|---|---|---|
| Mild | ~5% (50 mL/kg) | Thirst, dry mucous membranes, slightly decreased skin turgor |
| Moderate | ~10% (100 mL/kg) | Sunken eyes, tachycardia, oliguria, orthostatic hypotension |
| Severe | ≥15% (150 mL/kg) | Hypotension, poor capillary refill, altered mental status |
Calculating Fluid Deficit
Fluid deficit (mL) = % dehydration × body weight (kg) × 10
(or: body weight loss in kg × 1000 mL/kg)
Replacement Strategy
- Mild: Oral rehydration therapy (ORS) preferred - 50 mL/kg over 4 hours
- Moderate: ORS 100 mL/kg over 4 hours OR IV isotonic crystalloid
- Severe: IV bolus first (see Resuscitation below), then replace deficit over 24-48 h
Phased replacement (general principle):
- Give half the deficit in the first 8 hours
- Give the remaining half over the next 16 hours
- Always add ongoing maintenance requirements on top of deficit replacement
3. Resuscitation Fluids (Acute Shock)
Adults (Hypovolemic / Septic Shock)
- Initial bolus: 30 mL/kg isotonic crystalloid (Lactated Ringer's preferred; normal saline acceptable)
- Give rapidly; reassess after each bolus
- If septic shock persists after ~30 mL/kg, add vasopressors (norepinephrine first-line)
- Avoid overhydration - may cause pulmonary edema and intra-abdominal hypertension
Pediatric Trauma/Shock
- First bolus: 20 mL/kg warm isotonic crystalloid (NS or LR) over 10 minutes
- Second bolus: Repeat 20 mL/kg if no improvement
- After 2 boluses without improvement: pRBCs 10 mL/kg
- Massive transfusion threshold: >40 mL/kg (adolescent) or >50 mL/kg (child/infant) - add plasma and platelets
Source: Rosen's Emergency Medicine, Box 160
4. Burn Fluid Resuscitation (Parkland Formula)
The Parkland formula is standard for major burns (>20% TBSA).
Formula
Total fluid (first 24 h) = 4 mL × body weight (kg) × % TBSA burned
Fluid used: Lactated Ringer's solution
Delivery Schedule
| Time Period | Volume | Fluid |
|---|---|---|
| First 8 h from time of burn | 2 mL/kg × %TBSA | Lactated Ringer's |
| Next 16 h (hours 8-24) | 2 mL/kg × %TBSA | Lactated Ringer's |
| Second 24 h (hours 24-48) | 0.8 mL/kg × %TBSA (D5W) + 0.015 mL/kg × %TBSA (5% albumin) | D5W + albumin |
Important: The clock starts from the time of the burn, not time of hospital arrival.
Worked Example: 70 kg patient, 40% TBSA burn
- Total first 24h: 4 × 70 × 40 = 11,200 mL
- First 8h: 5,600 mL (~700 mL/h)
- Next 16h: 5,600 mL (~350 mL/h)
Source: Miller's Anesthesia 10e, Box 43.1 (citing Baxter CR, Surg Clin North Am 1978)
5. Common IV Fluid Compositions (Quick Reference)
| Fluid | Na (mEq/L) | K (mEq/L) | Cl (mEq/L) | Osmolarity (mOsm/L) |
|---|---|---|---|---|
| 0.9% Normal Saline | 154 | 0 | 154 | 308 |
| Lactated Ringer's | 130 | 4 | 109 | 273 |
| Plasma-Lyte A | 140 | 5 | 98 | 294 |
Lactated Ringer's is the most physiologic crystalloid for resuscitation and burns. Source: Fischer's Mastery of Surgery 8e, Table 3.2
6. Pediatric Nuances
- The 4-2-1 formula (Holliday & Segar, 1957) was designed to replace insensible + urinary losses
- Caution with hypotonic fluids post-op - surgical stress causes SIADH-like state, leading to dilutional hyponatremia if hypotonic fluids are infused in large volumes
- Preferred post-op strategy: use 1/2 to 2/3 of calculated 4-2-1 volume, and use balanced isotonic crystalloids rather than hypotonic solutions
- Avoid glucose-containing fluids intraoperatively in most children (risk of hyperglycemia and neurological harm during hypoxic/ischemic events) except in premature infants or those with hypoglycemia risk
- Intraoperative fasting replacement: 25 mL/kg for children ≤3 years, 15 mL/kg for ≥4 years
Source: Miller's Anesthesia 10e, Pediatric section
Key takeaway: Always calculate maintenance + deficit + ongoing losses separately, then add them together for the total fluid plan. Reassess frequently using clinical endpoints (HR, BP, urine output >0.5 mL/kg/h in adults, >1 mL/kg/h in children).
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