Lesson 3: Calculations used when compounding medications

1 Lesson 3: Calculations used when compounding medicationsPharmacists and pharmacy technicians all compound medications in one way or another, at least at some point intheir careers, and you will try your hand at it soon, if you have not already. The most common compounding you arelikely to do is preparation of intravenous (IV) solutions . This compounding is fairly straightforward and will involveprimarily proportional Calculations . There will be times when you will need to determine the osmolarity of an IVsolution. Finally, you may need to change the concentration of an already-mixed IV solution, so you will need toknow how to dilute or concentrate that will likely also compound medications for topical application or administration via some body orifice.

2 Thereare Calculations you will need to perform to ensure that ingredients are properly weighed and that solutions formucous membranes are isotonic, so they do not harm tissues. You may have to dilute or concentrate a topical orother compounded product. You will need to understand how buffers work, when they are needed, and be able toperform buffer solution purpose of this Lesson is to review all of the Calculations you will need in order to do those things just described,which include: proportional Calculations determination of osmolarity isotonicity Calculations dilution and concentration of previously-prepared medications aliquots buffer system lingo used in the IV roomThere are a few important terms that you need to become familiar with prior to moving on to Calculations in thislesson.

3 The first one is stock solution. A stock solution is the most concentrated form of a drug that you can put yourhands on. Sometimes a stock solution will be pure drug in powder or crystalline form (I know, I know: it s not asolution then, but people may call it that, anyway). At other times it will be a liquid or a solid paste or second term you will need to understand is the word bag. In the world of pharmacy, a bag is a flexible, softplastic container filled with a sterile fluid (it looks vaguely like a heavy-duty, half-empty water balloon). Bags oftenhave liquid forms of medication added to them (which are not-so-imaginatively referred to as additives ), and thecontents of the bag are then infused straight into a patient s vein (through tubing that is usually connected to apump).

4 Different sizes of bags will hold different volumes of fluid. Bag volumes used commonly contain 50ml,100ml, 250ml, 500ml, or 1000ml amounts of fluid, commonly D5W or NS. Larger bags of fluid (250ml, 500ml, and1000ml) will have medication added to them which will infuse in over a long period of time (hours), and are oftenreferred to as drips. Little bags (50ml, 100ml bags) are often called IV piggybacks, abbreviated IVPB, since theyare piggybacked onto the tubing of a solution already infusing into the patient. Use of these small bags formedication administration has declined over the past decade in favor of syringes, which are placed in an pump formedication term you will need to know is standard concentration.

5 A standard concentration is a set volume andconcentration of a commonly-made medication. The medication concentration and volume is pre-determined bynursing and pharmacy, and then all bags of that medication are prepared based on the specified recipe. To illustratehow standard concentrations work, let s say that a physician decided that a patient needed an intravenously-administered drug called dopamine. The physician would just write the word dopamine on the order, plus thedesired infusion rate, for example 5 mcg/kg/minute, and the pharmacy will automatically place 400mg of dopaminein a 250ml bag of D5W.

6 Every institutional and home health care pharmacy will have available a list of standardconcentrations for intravenous medication infusions. An example of a list of standard concentrations such as mightbe found in a hospital can be seen in the box below. Some standard concentrations are so standard betweenpharmacies that manufacturers have decided to prepare and market them commercially. For instance, when I wasback in pharmacy school (yes, dinosaurs were extinct by then), we would prepare a lidocaine drip by placing 2g oflidocaine in a bag containing 500ml D5W. In the mid- 80s, manufacturers started selling a pre-mixed lidocaine bagat the same concentration, so if you work in a hospital pharmacy you will probably never need to mix this particular2lidocaine drip.

7 It is important to understand what standard concentrations are, because a doctor or nurse may ask youto double concentrate a drip. This means you will have to prepare the drip at double the standard example of a list of standard concentrations for a D5 Wdobutamine250mg/250ml D5 Wdopamine400mg/250ml D5 Wheparin25,000 units/500ml D5 Winsulin25 units/250ml NSlidocaine2g/500ml D5 Wmagnesium40g/500ml D5 Wnitroglycerin50mg/250ml D5W (glass bottle)nitroprusside50mg/250ml D5W (wrap)oxytocin30 units/1000mlprocainamide1g/250ml D5 WProportional CalculationsYou will use proportional Calculations a lot in pharmacy, especially in the IV room.

8 This is great, because youalready know how to do these types of Calculations . Proportional Calculations involve using an available drugconcentration and a desired final drug amount (in either weight or volume) to determine the amount of eachindividual ingredient to add. Just like that. If you cook, you ve probably done proportional Calculations , even if youcook using only a can-opener and a Alert!! An important message regarding proportional Calculations . Some students try to do proportionalcalculations using a series of Calculations that look like this:a = xsolving for x by multiplying (a)(c) and dividing by (b)b cIf this is the way you are used to doing proportional Calculations , then realize that although you can get the correctanswer this way, it is also possible to get the incorrect answer.

9 This is particularly true if you don t write out theunits for (a), (b), and (c), and if there other variables such as (d) and (e) involved. Because you are dealing withmedications that could harm and in some cases even kill patients if the dose is miscalculated, you must use a methodthat will allow you to get the right answer every time. If you didn t learn about dimensional analysis in your highschool mathematics classes, it would be in your best interest to learn it now. Dimensional analysis is a method ofchecking an equation or solution to a problem by the way you set up the dimensions (units of measurement).

10 If thetwo sides of an equation do not have the same dimensions, the equation is wrong. If they do have the samedimensions, then the equation is set up correctly. (I learned about this in high school, but not by the fancy name. Itwas called the make sure all your units cancel out, leaving the ones you want method. I think. Or maybe Mr. Kaystold us the name and I wasn t listening. Always a possibility.) This is how it works:1. Write out your final units (the units you would like to arrive at) on the right hand side of the page first. Leave asmall space to the left, and then write an = On the left hand side of the page, begin to line up all of your available data.