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

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

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 availableExamples: • 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 malesN = 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…