Infusion Confusion – How To Calculate Drug Infusion Rates
The erosion of math and analytical skills that occurs with medics is truly astounding. Not surprising some might argue, what with it being such a memory oriented field. One area that many medics struggle with is drug dosage calculations. In the ER, one often doesn’t have the luxury of time and instant thinking is absolutely critical. Numbers need to be played out in seconds and optimal drug regimens have to be formulated. I was helping a colleague understand calculations for dopamine infusion the other day and thought like sharing with you folks some of the things we talked about.
Dopamine is used especially in ER settings to increase perfusion/blood pressure by means of its vasopressor, inotropic and chronotropic effects. When re-establishing blood pressure in a patient, attention not only needs to be paid to drugs that might be used but also fluid replacement for any amount of fluid loss from the body. Two questions need to be asked before starting a dopamine infusion:
- How much dopamine?
- How much fluid and how fast?
The usual dosage of dopamine is somewhere between 5-10 μg/kg/min. For the following example I’ll use 10 μg/kg/min.
1μg = 0.001mg.
For a patient weighing x kg, the dosage is therefore 0.01x mg/min. Now that you’ve established how much dopamine you need to infuse per minute, here comes the second part.
Suppose you intend to infuse y ml of fluid (as part of the dopamine infusion, i.e. aside from any other fluid infusions already in place). Say also that you’ve added z mg of dopamine to form the infusate. Dopamine is supplied in liquid form, so any amount of dopamine occupies a certain volume in ml, which in most situations is negligible.
y ml of infusate = volume of Normal Saline, etc. + volume of dopamine
If z mg of dopamine is contained in y ml of infusate,
0.01x mg dopamine is contained in [0.01x/z] * y ml of infusate.
Thus you’re interested in giving [0.01x/z] * y ml of infusate every minute and a simple formula is derived where:
rate of dopamine infusion in ml/min = [0.01x/z] * y
and therefore, z = [0.01x/(rate of infusion in ml/min)] * y
x = body weight in kg
z = amount of dopamine added in mg
y = total volume of infusate in ml
For any drug infusion:
rate of infusion in ml/min = [(total drug dose in mg/min)/(amount of drug added in infusate in mg)] * volume of infusate in ml
This infusate is typically given via an infusion set that specifies a unique drops per ml ratio. At our pediatrics ER for example, infusion sets come in two forms – microdrip infusion sets (1 ml = 60 drops) and macrodrip infusion sets (1 ml = 20 drops). Simply multiply the rate of infusion in ml/min with 60 or 20 to get the infusion rate in drops/min for micro and macro IV sets respectively.
As seen from the formula above, when deciding to add a given amount of drug to form the infusate, three things need to be fixed first:-
- Dose of drug in the mg/min format (should be appropriate to the clinical condition of the patient).
- Total volume of infusate in ml (again, this depends on the clinical condition and hemodynamic stability of the patient).
- Speed or rate of fluid replacement in ml/min (this is important as sudden fluid-volume changes in the body can be problematic in certain cases and you want to go for a rate that is optimal, neither too slow nor too fast.)
And with that I end this post. Hope readers find this useful. Comments and corrections are welcome!
Readability grades for this post:
Flesch Index: 65.7/100 (plain English)
Fog Index: 12.7
Lix: 39.4 = school year 6
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