Introduction to Prescribing - Part 2 3rd year Medical Students 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 Aim • To prepare medical students to accurately undertake a range of calculations needed when prescribing medication 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 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? 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! Converting units Try these examples • 425ml = ……. Litres • 62.5micrograms = …… mg • 1020nanograms = …… micrograms • 1020nanograms = …… mg • 1.6g = …… mg 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) 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? 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 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? Ratios • Very rare e.g. adrenaline – 1 in 1000 – 1 in 10000 • 1g in Xmls – 1g in 1000ml – 1g in 10000ml 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? 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 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 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? 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 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) 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. 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 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 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 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) 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) Question 8 – Oral zidovudine • BNF states dose as 180mg/m2 (max 300mg) twice daily for children 1 month – 18 years • Child (female) is 20kg and 110cm tall • Determine BSA • Calculate dose 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 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.uk 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’ 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) 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? 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) 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 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 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) 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? 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? 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 Extra Tuition…