Expressions of Infusion Rate

Physicians write orders for the rate at which an IV is infused into a patient. Infusion rates can be expressed as number of milliliters per minute, milliliters per hour, amount of drug per hour, and often as length of time for a volume to be infused. For example:

• Infuse at 125 ml/hr
• Infuse, 1000 ml over 8 hours
• Infuse 10 mg per minute

Infusion rate in ml/hr or ml/min: If the volume of infusion and time of infusion are known, then the rate of infusion can be calculated using the formula:

Solved Problem: A patient was administered 1000 ml of D5W over 8 hours. What is the infusion rate in ml/hr? Approach: $$ Infusion \ Rate ={1000 \ ml \over 8 \ hr} = 125ml/hr $$ Answer: 125 ml/hr

The infusion time or volume of infusion can also be calculated using the above formula provided the other two variables are known.

Solved Problem: A 1 liter bag of D5W is to be run at 200 ml/hr. The bag is started at 9:00 am. What time will the bag finish? Approach: Convert the liter to ml, use the formula for calculating infusion time. $$ Infusion \ Time ={1000 \ ml \over 200 \ ml/hr} = 5 hr $$ Answer: 2:00 pm

Infusion rate in drops/min(drip rate, gtt/min): To calculate infusion rate in drops/min also called the drip rate, one needs to know the drop factor of the primary IV tubing. Drop factor is printed on the package of the IV tubing. The following formula can be used to calculate the drip rate:

Take a note that drip rate has a denominator in minutes, but the drop factor has a denominator in ml. Drip rates are calculated to the tenths place and rounded off to the nearest whole number. For example:

Solved Problem: The flow rate of an IV infusion ordered by a physician is 125 ml/hr. The IV set to be used for the infusion is calibrated at 15 drops/ml. Calculate the drip rate. Approach: Convert flow rate from ml/hr to ml/min, then use the formula for drip rate. $$ {125 \ ml \over 60 \ min} = {x \ ml \over min} = {2.1 \ ml \over min } $$ Use the formula for drip rate to calculate drops/min. $$ Drip \ rate = {Drop \ factor \times Infusion \ Rate }$$ $$ Drip \ rate = {15 \ gtt \over ml }{\times}{2.1 \ ml \over min } = 31.5 = 32 $$ Answer: 32 drops/min

Solved Problem: If a standard microdrop infusion set is used to administer 100 ml of an infusion over a 2-hour period, calculate the rate of delivery, in drops/min Approach: Convert flow rate from ml/hr to ml/min. $$ {100 \ ml \over 120 \ min} = {x \ ml \over min} = {0.83 \ ml \over min } $$ Use the formula for drip rate to calculate drops/min. $$ Drip \ rate = {Drop \ factor \times Infusion \ Rate }$$ A microdrop infusion set has a drop factor of 60. $$ Drip \ rate = {60 \ gtt \over ml }{\times}{0.83 \ ml \over min } = 50$$ Answer: 50 drops/min