Providing intravenous (IV) fluids is an essential element of nursing care, as a multitude of medications and other substances are given by this route, particularly in inpatient hospital settings. IV administration has the advantage of allowing drugs to enter the body at a fixed and very precise rate, and factors such as the fraction of a medication that is ultimately absorbed from the stomach, the amount of undesired liver metabolism of orally given drugs and the time it takes for a substance to reach the bloodstream do not enter the picture.
For IV administration to be effective, however, you have to know how much of a substance you are providing per unit volume of IV solution, how fast it is entering the body, and the total amount you are administering. This relationship can be summarized by the following equation:
Infusion rate in gtts/min = (volume in ml)(gtts per ml) ÷ (time in min)
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In medicine, the term "gtt" is used for "drops" (the Latin word "gutta" translates to "drop"). The "gtt per ml" term is called the drop factor, and it is a measure of how drug-dense the IV fluid being used is. A microdrop, or µgtt, has a drop factor of 60.
Therefore, for calculations involving µgtts, this equation becomes:
R = 60V/t
Where R is the infusion rate, V is the total volume of fluid infused and t is the time in minutes.
Sample Problem 1
How many µgtt per minute are required to infuse 120 ml per hour?
In this scenario, a volume of 120 ml of solution is entering the patient every hour, or 60 minutes. This is a rate of 120 ÷ 60 = 2 ml/min.
However, this is the rate of solution flow, not drug flow. For the latter, multiply by the constant 60:
(2 ml/min)(60 µgtt/ml) = 120 µgtt/min
Sample Problem 2
At an infusion rate of 75 µgtt/min, how long will it take to infuse 300 ml of a microdrop solution?
Here, you have R and V but need t:
75 µgtts/min = (60 µgtt/ml)(300 ml) ÷ t
t = 18,000 µgtt ÷ 75 min = 240 min = 4 hours
Fluid and medication administration miscalculations can have serious consequences.
Always ask competent professionals to verify your calculations.
Always verify the drop factor is calibrated for the correct administration set.