Calculating constant rate infusion is a procedure routinely done in hospitals and veterinary clinics. Constant rate infusion is delivering the continuous intravenous supply of medication, usually through a computerised delivery device, to maintain constant blood levels of medication. Examples of drugs that are delivered this way are propofol, ketamine, insulin and dopamine. Performing this calculation is very important. Calculating too little may not induce the desired effects of the drug administered. Calculating too much may create health complications.
Find the number of micrograms (ug) of the medication to be administered, and the person or animal's weight, in kilograms (kg). Micrograms of drug are given in dosage orders. If weight is not given in the doctor's orders, ask the patient or pet owner. If unsure, use a scale.
Calculate the number of minutes that a given intravenous fluid must last for a specific individual or animal. Perform this calculation by dividing the volume in the dosage bag by the fluid administration rate per hour (hr). Then multiply the number of hours by 60 to determine the total number of minutes (min).
Once you have calculated the number of minutes your bag of intravenous fluid must last, take the volume of the bag and divide it by the prescribed running rate of fluid. If the supplied running rate is in hours, multiply by 60 to obtain the running rate in minutes.
Place the information into the constant rate infusion formula -- micrograms of medication, multiplied by kilograms of patient weight, multiplied by number of minutes needed to drain the intravenous bag.
Change the number of micrograms needed to milligrams (mg). Solve this by dividing micrograms by 1,000. Then divide the number in milligrams by the given drug concentration per bag in millilitres (ml). Once the number of necessary millilitres of drug is found, remove that found volume of intravenous solution and replace it with the same volume of drug.
Administer a 10 ug/kg/minute constant rate infusion of dopamine at a 40 mg/ml concentration to an 80kg male. Add the dopamine to a 1,000ml intravenous solution that is running at a rate of 50ml/hour. How much dopamine must be placed in the 1,000ml solution?
Calculate the amount of time the solution will last:
1,000ml divided by 50ml/hr = 20 hours
Convert hours to minutes:
20 hours at 60 min/hour = 1,200 minutes
Plug the results into the formula "micrograms of medication, multiplied by kilograms of patient weight, multiplied by number of minutes needed to drain the intravenous bag" to determine the number of micrograms of drug needed:
10 ug times 80kg times 1,200 min = 960,000 ug of drug needed
Convert micrograms to milligrams:
960,000 divided by 1,000 = 960 mg of drug needed
Find out how many milligrams are needed per 1,000ml bag of intravenous solution by dividing by the given concentration of the drug being administered. In this case the concentration is 40 mg/ml.
960 mg divided by 40 mg/ml = 24ml per bag of intravenous solution.
Remove 24ml of intravenous solution from the 1,000ml bag, and put in 24ml of dopamine solution.
Keep tables of drug information handy that include suggested concentrations and dosage ranges.
Always double-check your math.
Tips and warnings
- Keep tables of drug information handy that include suggested concentrations and dosage ranges.
- Always double-check your math.