Drug Calculation Basic concepts - pptx

Provide Basic concepts related to drug calcualtion

1. Drug C alculation Multidisciplinary Nursing Orientation Prepared by: EL Shafee Ahmed

2. On December 1, 1999, the Institute of Medicine (IOM) released To Err is Human: Building a Safer Health System, which revealed that an estimated 44,000 to 98,000 people die annually from medical errors. According to the Report, “more people die in a given year as a result of medical errors than motor vehicle accidents (43,458), breast cancer (42,297), or AIDS (16,516)”. Deaths from medication errors that take place both in and out of hospitals—more than 7000 annually—exceed those that occur as a result of workplace injuries.

3. First Rule THE “ONE UNIT” RULE Always ensure that you are working with the same units in your calculations. Convert units if necessary to achieve this. For Example: mg or micrograms but NOT BOTH TOGETHER!

4. Second Rule CHECKING THE ANSWER – DOES IT SEEMS REASONABLE?? Is 20 tablet dose of furosemide REASONABLE??? NO!!!

5. Conversion Nanograms Micrograms Milligrams Grams Kilograms mL L 1000 1000 from a larger unit to a smaller unit, multiply by 1,000 from a smaller unit to a larger unit, divide by 1,000

6. Methods of Calculation Simple Dose Calculations. Percentages. Drug Dosages based on Patient Parameters. Infusion Rate Calculations.

7. Simple Dose Calculations Amount to administer = Dose prescribed Dose in Hand Volume it’s in Example 1: A patient is prescribed 75 micrograms of levothyroxine but the strength of the tablets available is 25 micrograms. How many tablets are required? 75 micrograms 25 micrograms 1 = 3 tablets 25 micrograms 1 tablet 75 micrograms x tablets ??? x

8. Example 2: A patient is prescribed 2 g of flucloxacillin to be given orally but is available in 500 mg capsules. How many capsules do you give now? Use the “ ONE UNIT ” RULE Convert 2 grs to mgs 2 grs 1000 = 2000 milligrams 2000 milligrams 500 milligrams 1 = 4 capsules Amount to administer

9. Percentages Per cent means ‘part of a hundred’ or ‘a proportion of a hundred’. % w/v = number of grams in 100 mL So Lidocaine 2% means 2 grams in 100 mLs = 20 mg/ mL. Example 1: How many grams do we have in 10 mls Calcium Gluconate 10%??

10. Drug Dosages based on Patient Parameters Weight Total dose required = dose/kg × patient’s weight Example 1: A baby is prescribed Vancomycin 15 mg/kg/dose and his/her weight is 4 kgs. How many mgs of Vancomycin the baby shall receive? 15mg/kg × 4 kgs = 60 mgs ----- Total dose required

11. Surface Area Total dose required = dose/m2 × body surface area Drug Dosages based on Patient Parameters Example 2: The dose is 500 mg/m2 and the patient’s body surface area equals 1.89 m2. What is the total dose required? 500mg/m2 × 1.89 m2= 945mg ----- Total dose required

12. Example 3: You need to give Acyclovir as an infusion at a dose of 5 mg/kg every 8 hours. The patient weighs 76 kg and Acyclovir is available as 250 mg vials. How many vials do you need for each dose? 5 mg/kg × 76 kgs = 380 mgs every 8 hours You will need 2 vials of Acyclovir to administer each dose.

13. Using Mosteller BSA formula Drug Dosages based on Patient Parameters Example 4: Find out the BSA for a patient weighing 96 kg and with a height of 180 cm. Calculate the answer to two decimal places.

14. Infusion Rate Calculations Drip rate calculations (drops/min) • In all drip rate calculations, you have to remember that you are simply converting a volume to drops (or vice versa) and hours to minutes. drops/min= drops/mL of the giving set × volume of the infusion (mL) number of hours the infusion is to run × 60 The standard giving set (IV set) has a drip rate of 20 drops per mL for clear fluids and 15 drops per mL for blood. The microdrop giving set or burette has a drip rate of 60 drops per mL .

15. Infusion Rate Calculations Example 1: One litre of sodium chloride 0.9% is to be given over 8 hours. What drip rate is required using the regular IV set? 1 Liter = 1000 mL 20 drops/mL × 1000 mL 8 hrs × 60 = 20,000 drops 480 mins = 41.67 = 42 drops/ minute (approximation)

16. Infusion Rate Calculations Conversion of dosages to mL/hour mL/hour = volume to be infused (mL) × dose (mcg/kg/min) × wgt (kg)× 60 amount of drug (mg) × 1,000 Note: Doses are expressed as mcg/kg/min. 60 converts minutes to hours 1,000 converts mcg to mg

17. Infusion Rate Calculations Example 1: You have an infusion of dopamine 800 mg in 500 mL. The dose required is 2 mcg/kg/min for a patient weighing 68 kg. What is the rate in mL/hour? mL/hour = volume to be infused × dose × wgt × 60 amount of drug × 1,000 = 500 mL × 2 mcg/kg/min × 68 kgs × 60 800 mgs × 1000 = 5.1 mL/hour

18. Infusion Rate Calculations Conversion of mL/hour back to a dose mcg/kg/min = rate (mL/hour) × amount of drug (mg) × 1,000 weight (kg) × volume (mL) × 60 Note: Doses are expressed as mcg/kg/min. 60 converts hours to minutes 1,000 converts mg to mcg

19. Infusion Rate Calculations Example 2: An infusion pump containing 250 mg dobutamine in 50 mL is running at a rate of 3.5 mL/hour. You want to convert back to mcg/kg/min to check that the pump is set correctly. Noting that patient’s weight is 75 kgs mcg/kg/min = rate (mL/hour) × amount of drug (mg) × 1,000 weight (kg) × volume (mL) × 60 = 3.5 mL/hour × 250 mgs × 1000 75 kgs × 50 mLs × 60 = 4 mcg/kg/min

20. Calculating the length of time for IV infusions Manually controlled infusions: number of hours the infusion is to run= volume of the infusion × drip rate of giving set rate (drops/min) × 60 Infusion Rate Calculations Example 1: The doctor prescribes 1 litre of 5% glucose to be given over 8 hours. The drip rate for the infusion is calculated to be 42 drops/min. You wish to check the drip rate. How many hours is the infusion going to run? (drip rate: 20 drops/mL) Answer: 7.93 (app. 8 hours)

21. Exercises Example 1: The doctor has decided to intubate your 100 kg patient who has gone into acute respiratory distress. As the doctor prepares to intubate, he ask you to give 2.5 mg/kg of propofol (10mg/ml) IVP. How many mLs of Propofol will you give? Example 2: 600 mL of antibiotic is to be infused over the 180 minutes by an infusion pump. Calculate the flow rate (mL per hour). Example 3: If you want to administer 3000 ml of IV solution in 24 hours and each liter contains 40 meq of potassium chloride, how much potassium chloride will the patient receive after 12 hours?   

22. Example 4: Find:  The total daily dose if a child weighing 10 kg is to be given 60 mg/kg/day            The individual dose if the total daily dose is to be given in 6 divided doses?  Example 5: A patient is receiving a heparin drip 25000 units in 250 ml of 0.45% Sodium Chloride. He is supposed to receive 1000 units/hour. At what rate should you infuse the drug?    a. 1 ml/hour.  b. 10 ml/hour.  c. 250  ml/hour.  d. 25 ml/hour  Exercises

23. References Lapham, R; (2016). Drug Calculations for Nurses, A step-by-step approach. Taylor and Francis Group. Fourth edition. http://www.taylorandfrancis.com