1. Introduction to Prescribing - Part 2 3 rd year Medical Students

2. Content Part 1 –Principles of medicines management –Using the BNF –Tour of pharmacy to highlight practical issues Part 2 –Drug calculations Acute medicine skills week (Hotweek) –Prescribing day

3. Aim To prepare medical students to accurately undertake a range of calculations needed when prescribing medication

4. Objectives Students will be able to calculate: Volumes required for oral or IV doses Drug doses based on patient weight Ideal body weight and creatinine clearance Body surface area Dilutions for IV infusions Infusion rates

5. Divided doses or not? BNF can describe doses in 2 ways: 250mg every 8 hours (amoxicillin) 75-150mg in 2-3 divided doses (diclofenac) How could you prescribe diclofenac?

6. Calculating practical doses Prescribers usually state dose in unit of WEIGHT (grams, mg, micrograms, nanograms) Patients or nurses will have to covert that dose to the correct VOLUME (mls) for oral liquids or preparing IV drugs Ensure you are prescribing a sensible dose!

7. Converting units Try these examples 425ml = ……. Litres 62.5micrograms = …… mg 1020nanograms = …… micrograms 1020nanograms = …… mg 1.6g = …… mg

8. Formula for calculating volumes Dose (ml) = dose (mg) x volume available (ml) weight available (mg) Concentration = weight available in volume available Examples: 250mg in 5ml (antibiotic syrups) 100mg in 10ml (IV injection)

9. Question 1 - Volumes Child is prescribed clindamycin liquid 42mg tds. Liquid strength is 75mg /5ml. What volume is required? Adult is prescribed IV co-trimoxazole 1950mg qds. Injection strength is 96mg/ml. Each ampoule contains 5mls. What volume is required?

10. Percentages Some drugs labelled as % –Magnesium injection 20% –Sodium chloride 0.9% –Glucose 5% Xg in 100ml –20g in 100ml –0.9g in 100ml –5g in 100ml

11. Question 2 - Magnesium Your patient needs IV magnesium 4g for treating seizures in eclampsia The vial is labelled as magnesium sulphate 20% How many mls do you need to give the dose?

12. Ratios Very rare e.g. adrenaline –1 in 1000 –1 in 10000 1g in Xmls –1g in 1000ml –1g in 10000ml

13. Question 3 - Adrenaline You need to give 1mg IV adrenaline The ampoule is labelled as 1 in 10000 How many mls do you need to give?

14. Drug dosages Many standard doses for adults based on: –Average adult male = 68kg –Average adult female = 56kg Certain drugs require more accurate dosing: –Actual body weight –Body surface area

15. Body weight Drugs with narrow therapeutic index or risk of serious toxicity But need to consider whether patient is overweight / obese / oedematous Often there is a maximum dose stated Examples: –Antivirals (IV) –Aminoglycoside antibiotics –Low molecular weight heparins

16. Question 4 – IV foscarnet BNF states for CMV disease: ‘by intravenous infusion, initially (induction) 60 mg/kg every 8 hours or 90 mg/kg every 12 hours, for 2–3 weeks; maintenance 60 mg/kg daily, increased to 90– 120 mg/kg if tolerated; if disease progresses on maintenance dose, repeat induction regimen’ What is the induction and maintenance dose for a patient who weighs 73kg?

17. Question 5 - Nitrofurantoin BNFc states that dose for child 3 months to 12 years is 750mcg / kg 4 times a day Calculate dose required for 13kg child Nitrofurantoin suspension = 25mg/5ml Calculate volume to be measured for the dose

18. Ideal body weight - IBW Often needed for patients who are Overweight / obese Oedematous / ascites Male = 50kg + (2.3kg x every inch above 5ft) Female = 45.4kg + (2.3kg x every inch above 5ft)

19. Question 6 - Aminophylline Loading dose = 5mg/kg (max 500mg) for acute severe asthma or exacerbation of COPD. To avoid excessive dosage in obese patients dose should be calculated on ideal weight for height. Calculate loading dose of aminophylline loading dose for male patient who weighs 95kg and is 5’6” tall.

20. Creatinine clearance (CrCl) Traditionally creatinine clearance used to estimate renal function Now eGFR provided via lab results (based on serum creatinine, sex & age) BUT CrCl more appropriate than eGFR when dosing high risk drugs (e.g. aminoglycosides) Considers patient weight More data about dose adjustment based on CrCl

21. Creatinine clearance - adults CrCl (ml/min) = N x (140-age) x IBW serum creatinine (micromol/L) N = 1.23 for males N = 1.04 for females Can only use if patient has stable renal function

22. Question 7 - CrCl What dose and frequency of gentamicin would you prescribe for a 72 year old male? Serum creatinine = 167micromoles/L Height = 5’8” Weight = 80kg Use guideline provided

23. Body surface area - BSA Many physiological parameters correlate better to body surface area Use in paediatrics and chemotherapy regimens Tables provide estimated values for children based on weight (see back of BNFc)

24. BSA for adults For adults it is more accurate to use a nomogram which takes patient weight, height and gender into account (DuBois formula) One available for ADULTS through microbiology webpage for CMFT (under general prescribing information – drug dose calculators)

25. Question 8 – Oral zidovudine BNF states dose as 180mg/m 2 (max 300mg) twice daily for children 1 month – 18 years Child (female) is 20kg and 110cm tall Determine BSA Calculate dose

26. Preparing IV infusions IV drugs presented as –Powder (reconstitute with small volume of fluid, usually water for injection) –Solution Drug will need to be diluted with compatible fluid e.g. sodium chloride 0.9%, glucose 5% How should it be diluted? –Chemical stability –Duration of infusion –Patient’s fluid status

27. Preparing IV infusions Fluid component needs to be prescribed Standard bag sizes –50ml, 100ml, 250ml, 500ml and 1000ml Information available –BNF appendix 4 –Package insert –SPC www.medicines.org.ukwww.medicines.org.uk

28. How much fluid to use? State as add to Xmls of infusion fluid Example: co-amoxiclav injection ‘reconstitute 1.2g initially with 20ml water for injections then dilute with 100ml infusion fluid’

29. How much fluid to use? Stated as obtaining a final concentration of X mg/ml Example: voriconazole ‘dilute dose in infusion fluid to concentration of 0.5-5mg/ml’ Volume (mls) = dose (mg) concentration (mg per 1ml)

30. Question 9 - AmBisome Patient is prescribed AmBisome 180mg daily. The final concentration of the prepared infusion must be 0.2 – 2 mg/ml How much fluid should this dose be diluted in? Which are the most appropriate sized bags to use?

31. Infusion rates Rate controlled by use of volumetric pumps Rapid infusion can result in toxicity –E.g. vancomycin & ‘red man’ syndrome (severe hypotension, wheezing, flushing) Rate = mls / hr Rate (ml/hr) = volume (mls) x 60 time (mins)

32. Question 10 - IV Vancomycin Patient prescribed IV vancomycin 750mg in 250ml of glucose 5% Trust policy is to give this over 90mins Calculate the infusion rate

33. Continuous infusions Dose often stated as –per min –per kg per min Examples –Lidocaine = 4mg / min for 30 mins then 2mg / min for 2 hours then 1mg / min –Dopamine = 2 micrograms/kg/min Issues to remember –Rate for pumps is set in mls / hr –Ensure amount prescribed will last for a suitable length of time

34. Calculating infusion rates Calculate dose needed per hr Dose (mg) per hr = dose (mg) per min x 60 Calculate mls needed per hr Rate (ml/hr) = dose (mg) per hr concentration (mg per 1ml)

35. Question 11 - Dopamine Dopamine = 3 micrograms/kg/min Patient weight = 65kg Nurse plans to prepare a bag of 200mg dopamine in 250ml bag of 0.9% sodium chloride What is the rate in ml/hr?

36. Question 12 - Lidocaine 4mg / min for 30 mins then 2mg / min for 2 hours then 1mg / min Infusion available = 2mg/ml in 500ml bags Calculate rate in ml/hr How long will one bag last for?

37. Conclusion Please complete feedback questionnaire Identify your areas of weakness (if any!) Practice doing calculations using prescriptions you see on the wards Ask nurse if you can shadow them preparing and administering IV doses

38. Extra Tuition…