Transcript Introduction_to_Prescribing

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…