Introduction

It is helpful to have an idea of how much fluid a patient should receive during an operation. Below are some rules of thumb to calculate the expected needs of a patients. Fluids should be thought of as a medication and response to fluids should be monitored (urine output, blood pressure & heart rate response, biochemical improvement)

Fluid Requirements

Maintenance fluids can be estimated by the 4-2-1 rule:

Weight Range	Fluid Rate Calculation	Hourly Fluid Rate
First 10 kg	4 mL/kg/hr	40 mL/hr
Next 10 kg (11-20 kg)	2 mL/kg/hr	40 mL + (2 mL × weight over 10 kg)
Above 20 kg	1 mL/kg/hr	60 mL + (1 mL × weight over 20 kg)

Example Calculations:

• Patient weighing 15 kg:
  • First 10 kg: 4 mL × 10 = 40 mL
  • Next 5 kg: 2 mL × 5 = 10 mL
  • Total hourly fluid rate = 40 + 10 = 50 mL/hr
• Patient weighing 25 kg:
  • First 10 kg: 4 mL × 10 = 40 mL
  • Next 10 kg: 2 mL × 10 = 20 mL
  • Above 20 kg: 1 mL × 5 = 5 mL
  • Total hourly fluid rate = 40 + 20 + 5 = 65 mL/hr

Allowable Blood Loss

Blood volume for a patient can be calculated as follows: $Blood~Volume = Weight (kg) \times Blood~Volume~per~kg$

Age Group	Average Blood Volume (mL/kg)
Adult man	75
Adult woman	65
Infant	80

Using the final acceptable hemoglobin (Hgb_f, generally ~70), the starting hemoglobin (Hgb_i), and the blood volume, the allowable blood volume can be calculated as follows:

$$Allowable~Blood~Loss = Blood~Volume \times \frac{Hgb_i - Hgb_f}{Hgb_i}$$