introduction to prescribing - part 2
DESCRIPTION
Introduction to Prescribing - Part 2. 3 rd 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. - PowerPoint PPT PresentationTRANSCRIPT
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Introduction to Prescribing- Part 2
3rd year Medical Students
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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
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Aim• To prepare medical students to
accurately undertake a range of calculations needed when prescribing medication
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ObjectivesStudents 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
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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?
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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!
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Converting units
Try these examples
• 425ml = ……. Litres
• 62.5micrograms = …… mg
• 1020nanograms = …… micrograms
• 1020nanograms = …… mg
• 1.6g = …… mg
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Formula for calculating volumes
Dose (ml) = dose (mg) x volume available (ml) weight available (mg)
Concentration = weight available in volume availableExamples: • 250mg in 5ml (antibiotic syrups)• 100mg in 10ml (IV injection)
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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?
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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
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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?
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Ratios
• Very rare e.g. adrenaline– 1 in 1000– 1 in 10000
• 1g in Xmls– 1g in 1000ml– 1g in 10000ml
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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?
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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
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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
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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?
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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
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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)
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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.
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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
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Creatinine clearance - adults
CrCl (ml/min) = N x (140-age) x IBW serum creatinine (micromol/L)
N = 1.23 for malesN = 1.04 for females
Can only use if patient has stable renal function
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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
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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)
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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)
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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
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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
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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
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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’
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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)
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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?
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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)
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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
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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
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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)
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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?
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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?
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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
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Extra Tuition…