Drug calculations

Lecture By
Mr.Rami Al-Khateeb
King fahd Medical city, Nursing Educator

1. Drug Calculations Rami AL- Khateeb

2. Outlines Introduction Metric Conversions MG/KG Dose Conversion Flow Infusion Rates

3. Introduction When calculating how much of a drug is required, working with the formula helps the accuracy of the calculation.

4. Always remember this formula: What you want X Quantity it comes in What you have

5. A patient requires 4 mg of Morphine IV. Morphine is available as 10mg/ml. How many mls will you draw up? Please answer questions 1-3

6. Metric Conversions Metric weights and measurements involve a step-by-step conversion from one unit to another. With weight we often convert to smaller (and more numerous units) thus: Kg ------gm ------mg ------mcg grams milligrams micrograms

7. Metric Conversions Each of these steps involves the heavier unit being multiplied by 1000 to bring up the number of the smaller units for the same weight.

8. Metric Conversions Kg → g 1kg X 1000 = 1000 gm g →mg 1g X 1000 = 1000 mg = 1000 mcg 1mg X 1000 mg → mcg

9. Metric Conversions If we are converting from a lighter unit to a heavier unit we move the decimal point three places to the left for each conversion.

10. Metric Conversions If we are converting from a lighter unit to a heavier unit we move the decimal point three places to the left for each conversion

11. Metric Conversions mcg → mg 1000 mcg /1000 = 1mg mg →gm 1000mg / 1000 = 1 gm = 1kg 1000gm / 1000 gm → kg

12. Please answer questions from 4-7

13. Metric Conversions If we are moving across two conversions we repeat the process twice. For example from grams to micrograms 3gm converted to micrograms Step one: = 3 x 1000 = 3000mg Step two: 3000mg x 1000 = 3,000,000mcg

14. Please answer questions from 8-10

15. When we are converting volumes the process is the same. Liters → 1000 ml

16. Please answer questions from 11-13

17. MG/KG Dose Conversion Often we need to check the dose of a drug or solution that is based on the patient’s weight. We multiply the prescribed mg dose by the kg weight of the patient

18. MG/KG Dose Conversion 25mg per kg - Prescribed dose Patient’s weight-66kg What is the dose required?

19. MG/KG Dose Conversion 25mg x 66kg = 1650mg = 1.65 gm Please answer questions 14 a & b

20. Flow Infusion Rates

21. Flow Infusion Rates To obtain the hourly rate, divide the volume of fluid to be infused by the number of hours fluid to be infused over. Eg: a 1000ml infused over 12 hours 1000/12 = 83.3 ml / hr

22. Flow Infusion Rates Please answer questions 15 & 16

23. Flow Infusion Rates Fluids are infused using a giving set, requiring a “drop per minute” rate. The giving sets are: Standards IV Set = delivers 20 drops per ml. Micro giving set = delivers 60 drops per ml. Drop per minutes: total fluid in ml X giving set calibration total hours X 60 mins

24. Flow Infusion Rates A patient is prescribed Sodium Chloride 1000ml to be infused over an 8-hour period. A standard giving set is being used. Calculate the drops per minute. 1000 X 20 = Drops per mins 8 X 60

25. Flow Infusion Rates Please answer question number 17

26. Answers 1. 9 mls 2. 0.6 mls 3. 3.5 mls 4. 600 mcg 5. 10 mg 6. 0.36 gm 7. 0.125 mg 8.1200000 mcg 9. 0.000125 gm 10. 1200 mcg 11. 0.45 L 12. 0.064 L 13. 4300 ml 14.a) 1125 mg b) 375 mg 15. 167 ml 16. 83 ml 17. 166.6

27. References Dodds , K.G. (2012). Safety in Numbers– Essential Calculations for Nurses. Palmerston North. Dunmore Press Ltd. Christchurch Hospital Department of Nursing Drug Calculation Prompt Card (15/06/00)