Dosage Calc, part 34: Body Surface Area

  • 00:00 Intro
  • 1:23 BSA Equation
  • 1:58 Example 1
  • 2:37 Example 1A
  • 3:44 Example 1B
  • 4:43 Example 1C
  • 4:48 Example 1C Dimensional Analysis
  • 5:43 Example 1C Ratio & Proportion
  • 6:27 Example 1C Formula method

Full Transcript: Dosage Calc, part 34: Body Surface Area

Hi. This is Cathy with Level Up RN. In this video, I'll be sharing the formula used to calculate a patient's body surface area or BSA. I will then work through a dosage calculation problem that is based on the patient's BSA. And I'll be working through this problem using three different methods including dimensional analysis, ratio and proportion, and the formula method. You can find all the information that I'll be covering in this video in our Level Up RN dosage calculation workbook. If you are in nursing school, then you know how important it is to master dosage calculations, and our workbook will help you do just that. In a nutshell, our workbook contains all different types of dosage calculation problems that you are likely to encounter in nursing school. And we demonstrate how to solve each problem using multiple methods so you can pick the way that makes the most sense to you.

Medications that use BSA for their dosing typically include chemotherapy agents because they have a very low therapeutic index, and it's going to be really imperative that we get the dosing just right for our patient. So the equation to figure out BSA is the square root of the patient's weight times their height divided by 3,600. So you're going to need your calculator to make this calculation. So you need to figure out on your calculator how to get the square root. So everybody's calculator is a little different, so definitely figure out how yours works. And then when we do this math, we will end up with the patient's BSA, which will be in meters squared. Okay. So for our problem here, we are caring for a patient who is ordered to receive vinblastine IV single dosage 3.7 milligrams per meter squared. The patient's height is 165 centimeters, and the patient's weight is 70 kilograms. And vinblastine 10 milligrams in 10 mls of solution is available, and we need to round all of our answers to the nearest hundredth. Okay. So get out your calculator, and let's work through the parts of the problem. Okay.

So for part A, we're being asked, what is the patient's BSA or body surface area? And we're going to use this equation over here. So part A, BSA equals the patient's weight in kilograms, which is 70, times the patient's height in centimeters, which is 165, divided by 3,600. So if you do this math in your calculator, then you should end up with 1.79 meters squared. So keep that number in your calculator. Do not clear that out because we're going to need the full number in your calculator to work through part B. But we are going to round this particular answer to the nearest hundredth per the instructions here. Round all answers to the nearest hundredth. So the patient's PSA is 1.79 meters squared. Now, let's move to part B. And again, keep that answer in your calculator. Part B asks us how many milligrams are to be administered in this dose. Use the non-rounded answer from part A in your calculator to calculate the dose. So the ordered dose is 3.7 milligrams per meter squared, and if we multiply this dose that's ordered by the patient's BSA, which is also in meters squared, then those will cross off and we'll be left with the milligrams. But again, this number that you're going to multiply 3.7 by is going to be the full number in your calculator. So when you do that math, you end up with 6.63 milligrams, and that is rounded also to the nearest hundredth. So that is the answer to part B.

All right. Now let's work through part C of this problem, which asks us how many milliliters will need to be administered. So we can solve this part of the problem using dimensional analysis, ratio and proportion, or the formula method. So let's first solve it with dimensional analysis. So we need to give 6.63 milligrams to our patient, and we can figure out the amount of milliliters to administer by multiplying this amount by the available concentration of vinblastine. So if we multiply by 10 milliliters over 10 milligrams, which is the concentration-- I was careful to put milliliters on top and milligrams on bottom such that my milligrams cross off. And if I calculate that out, I end up with 6.63 milliliters. And that's how many milliliters I would need to administer. I could also solve this part of the problem with ratio and proportion as well. If you like that method, you would first start off by writing down your known ratio, which is our concentration. So 10 milligrams in 10 ml. And then on the other side of the equation, we would write down our desired dose and an X for ml. And then we would cross multiply here. So 10X equals 10 times 6.63. If you solve for X, you will end up with 6.63 milliliters. And then finally, with the formula method, the formula method is desired over have times the vehicle. So we desire 6.63 milligrams, and what we have is 10 milligrams in 10 mls of solution. So if we multiply this out, we end up with 6.63 milliliters again. Three different ways to get to the same answer. And that's it for our body surface area problem.

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