Formula for Calculating Infusion Rate
In clinical settings, accurately calculating the IV (intravenous) infusion rate is a critical nursing skill. It ensures that patients receive the correct volume of medication or fluids over a specific period, preventing complications such as fluid overload or inadequate dosage.
The Two Main Methods of Calculation
Infusion rates are generally calculated in two ways depending on the equipment used:
- mL/hr (Milliliters per Hour): Used when an electronic infusion pump is available.
- gtt/min (Drops per Minute): Used for manual gravity infusions where the nurse counts the drops in the drip chamber.
1. Calculating Flow Rate in mL/hr
This is the simplest calculation, primarily used for setting electronic pumps.
Example: A doctor orders 1,000 mL of Normal Saline to infuse over 8 hours.
- Volume = 1,000 mL
- Time = 8 hours
- Calculation: 1,000 ÷ 8 = 125 mL/hr
2. Calculating Drop Rate in gtt/min
When using gravity tubing, you must calculate how many drops fall per minute. To do this, you need to know the Drop Factor of the tubing, which is printed on the package (measured in gtt/mL).
Note: Ensure you convert the time from hours to minutes by multiplying the hours by 60.
Real-World Example Calculation
Let's assume the same order: 1,000 mL over 8 hours. However, you are using standard macro-drip tubing with a drop factor of 15 gtt/mL.
- Convert Time: 8 hours × 60 minutes = 480 minutes.
- Apply Formula: (1,000 mL × 15 gtt/mL) ÷ 480 minutes.
- Multiply Numerator: 15,000 total drops.
- Divide: 15,000 ÷ 480 = 31.25.
- Result: Since you cannot count a partial drop, you round to the nearest whole number: 31 gtt/min.
Understanding Drop Factors
The "drop factor" or "drip factor" indicates how many drops it takes to make one milliliter of fluid.
- Macrodrip (10, 15, or 20 gtt/mL): Used for general adult IV fluids. The drops are large.
- Microdrip (60 gtt/mL): Used for pediatrics, neonates, or precise medication administration. The drops are very small.
Safety Considerations
Always double-check your math. A decimal point error can result in a 10-fold overdose or underdose. If the calculated rate seems unusually high or low for the specific patient situation, verify the order and your calculation with a colleague.