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In this lesson we’re going to do some more practice problems for med math calculations for injectable medications. Let’s just look at a few key points. One thing you want to always look at with these are the units. What is the problem actually asking for? Are we looking for mL? Are we looking for the actual dose, like micrograms? Make sure you know what you’re looking for.
Some medications that might be used for injectables would be IM, SubQ injections, interdemals, IV pushes, etc. These are things that have a volume, but may not have a time or a rate. As with any other problem, you always want to look at appropriateness. For example, did you get a result of giving 10 mls in an IM injection? That is not appropriate. We’re talking about, typically, two or less, maybe three or less. But 10, definitely not appropriate. You should reevaluate, see if you missed anything.
And then, again, same with orals. You’re typically looking at a per dose type of formula because you’re not looking at a rate or a time.
Let’s work some of these out. First one: The order is 7,000 units of Heparin SubQ every eight hours, and Heparin available is 5,000 units per mL. How many mL should you administer? Now, on these, I’m going to strictly use dimensional analysis because it’s my preferred method. If you want more on how to use the formula method, check out the oral medications lesson or the basics of med calculations lesson.
The first question is, what are we looking for? How many mL? I’m looking for the number of mL per dose. Always start with what you’re looking for. Transfer your top units over; mL. Insert what you know. Do I know anything? Am I given anything about mL? Yes, I am. I know that one mL is 5,000 units. Transfer my units up. What else do I know about units? Well, I know that I want 7,000 units in one dose. Cancel units, and am I left with what I want; mls per dose? Yes, I am. Multiply across the top, divide across the bottom. You end up with 7,000 divided by 5,000, which gets us 1.4 mL per dose.
The last step is always to verify. Does it make sense to give about 1.4 mL, or to give 1.4 mL specifically in a SubQ injection? Well, it’s a little bit high, but it’s not too high. It’s appropriate.
Let’s look at the next one. Haloperidol two mg intramuscularly one time. Available Haloperidol 5 mg per mL. How many mL should you administer? Again, start with what you’re looking for; mL per dose. Transfer your top units across; mL. What do I know about mL? I know that one mL is five mg. Transfer units again, what do I know? Well, I already used this one, so the other thing I know about mL is that I want two mg per one dose. Cancel mg, I’m left with mL per dose, which is exactly what I want. Multiply across the top, divide across the bottom. One times two, divided by five, divided by one, is going to give you 0.4 mL per dose.
Does 0.4 mL make sense for IM injection? Does it make sense for this calculation? Yes, it does. We have verified.
All right. Let’s look at the next one. A nurse has drawn up four mL of Furosemide to administer IV push to a patient. The vial contains 10 mg in one ml. How many mg is the nurse about to administer to the patient?
Okay. Start with what you you’re looking for; mg per dose. What do I know, what am I given about mg? Right here, I’ve got 10 mg is in one mL, so 10 mg, and again, because we transferred this unit across; one mL. Now, transfer the units up; mL. What else do I know about mL? Well, right here I know that she’s giving four mL in this dose that she’s about to give, or he. Let’s cancel mL, and are we left we what we want, mg per dose? Yes, we are. Multiply across the top, divide across the bottom. 10 times four, divided by one, divided by one, 40 mg in this dose. Is this an appropriate dose? Does this make sense? 10 per ml times four; 40. This makes sense. 40 mg IV push of Furosemide is perfectly acceptable.
All right. Let’s look at one more. For an initial bolus dose before an IV infusion of regular insulin, the provider has ordered 0.1 units per kilo of regular insulin IV push. First all, we know regular insulin’s the only one that can be given IV, so we know that that part is appropriate. The patient weighs 132 pounds. How many units should be administered?
Start with what we’re looking for; number of units per dose. Transfer your units across. What do we know about units? Well, we know that we want 0.1 units per kg, so 0.1 units in one kg. Transfer units up. Do we know anything else about kg? Actually, we don’t, so when you don’t know, you convert. One kg equals 2.2 pounds. Transfer our units up. What do we know about pounds? 132 pounds. Cancel kgs, cancel pounds, and we’ve got what we need. Units, and this is going to be 0.1 units per kilo per dose.
Multiply across the top; 0.1 times 132, divided by 2.2 is going to give you six units of regular insulin IV. Then ask yourself, “Does this make sense?” Well, if we’re starting an insulin infusion, we may have somebody in DKA, or HHNS. Six units of an initial bolus dose is actually appropriate, so that is going to be good to go. If you had come up with 60 or 600, that may have been inappropriate, but six, that’s good to go.
Last one. Protamine sulfate 25 mg IV push one time. Available Protamine sulfate five mg in two mL. How much are we going to give? Well, in this case, by how much, it wants to know volume. We’re going to be looking for mL per dose.
Transfer units across. What do we know about mL? We know that two mL is five mg. Transfer our units up. What else do we know about mg? Well, in this case, we know that we want to give 25 mg in one dose. Cancel mg. Are we left with what we want? Yes, we are. Multiply across the top, divide across the bottom. Two times 25, divided by five, divided by one is going to get you 10 mL per dose. Is 10 mL appropriate for an IV push? Sure it is, especially if this is the right calculation. Makes sense. This is about five times here. Five times two is 10, it’s about right. We verified, and we’re good to go.
All right guys, that’s it for some injectable medications. Please make sure that you also check out the IV medication, IV infusions, the complex calculations, oral meds, the basics and the dimensional analysis lessons so that you have a really great foundation for med math.
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