13. Drug Dosing and Calculations Flashcards
Complete the table with the name of the unit and the factor.
a) deci, 10-1
b) centi, 10-2
c) milli, 10-3
d) micro, 10-6
e) nano, 10-9
What 2 things must you check when calculating doses?
Body surface area (BSA) can be used to calculate doses. What are the 2 main methods used?
If doses are based on estimated GFR (eGFR), when may they not be as specific?
Usually in dose per weight. Maximum dose.
Dubois formula. Boyd method. N**eed senior guidance.
For hepatic dysfunction/liver injury.
Mr N is prescribed 25mg spironolactone in liquid form (5mg/ml). What volume should he take?
Concentration = dose/volume
So volume required = dose/concentration
= 25/5 = 5ml
A 35kg 8 yr old boy is prescribed amoxicillin at 30mg/kg IV. The drug is available as a 500mg/mL preparation. How much volume is required?
Dose required = 30mg/kg x 35kg = 1050mg
Volume required = dose required/conc = 1050/500mg/mL
= 2.1mL
What is the typical rate (range) of a syringe driver?
What is the formula for calculating rate?
What is the formula for calculating volume required?
Following a bolus, Mr N (80kg) is prescribed 10mg/kg amiodarone over 24hrs. IV amiodarone is made to a dilution to 500mg/500mL. Give the rate (mL/min) required.
0.1 - 20ml/hr
Rate required = volume required/time (TVR)
Volume required = dose required/concentration (CDV)
Dose required = 10mg/kg x 80kg = 800mg
Volume required = Dose required/ conc
= 800mg/ 500mg/500mL
= 800mL/ 1mg/mL
=800mL
Rate required = Volume required/ time
= 800mL/ 24hrs
= 800mL/ 24x60mins
= 0.56mL/min
Dextrose and saline are prescribed as w/v (weight/volume). Describe this relationship in a 5% glucose solution; how much glucose is there in a L?
Solutions can also be in ratio form (1:1). What does 1:1000 adrenaline mean in g/ml, and how much adrenaline per ml?
During a femoral block proceedure, 20ml of 0.5% w/v bupivacaine has been used. What is the quantity (mg) of bupivacaine that has been used?
= grams of solute/100ml of total solvent. So:
5% glucose solution = 5g/100mL
= 50g/L
= 1g/1000mL
= 1mg/ml
0.5% = 0.5g/100ml
Therefore 20ml = 20/100 x 0.5g = 0.1g = 100mg
What does % (v/v) mean?
A 30 year-old man presents to A+E having consumed 200ml of methylated spirits (5% methanol). How much methanol (ml) has he consumed?
For opiate conversion, what is everything compared to?
Volume of solute/total volume e.g. wine that is 12% ABV has 12ml ethanol in 100ml.
5% x 200ml = 0.05 x 200ml
= 10ml methanol
Morphine. Depends on route too. Conversion tables also for oral antipsychotics and glucocorticoids.
Miss N, a 10 year-old girl, has been diagnosed with otitis media, and is prescribed amoxicillin. She weighs 45kg. Please prescribe the first dose (in mg).
Otitis media, 500mg every 8 hours; CHILD 40mg/kg daily in 3 divided doses (max. 1.5g daily)
Daily dose = 45kg x 40mg/kg/day
= 1800mg daily
BUT max dose 1500mg so = 500mg as first dose
Mr F usually takes ferrous fumarate 210mg three times daily. He has recently had swallowing difficulties. It is planned that he only has liquids. What is his equivalent syrup dose (in ml)?
Tablets: derrous fumarate 210mg (68 mg iron)
Dose: ADULT and CHILD over 12 years, prophylactic, 1 tablet 1-2 times daily; theraputic, 1 tablet 2-3 times daily
Syrup: ferrous fumarate approx. 140mg (45 mg iron)/5ml
68mg elemental iron three times a day
= (68/45mg) x 5ml three times a day
= 7.6ml (rounded up) three times a day
Mr OP has been taking 30mg MST (morphine sulphate) tabs BD (twice a day) and 10ml Oramorph (10mg/5ml morphine) oral solution PRN (5ml, twice a day) for malignancy-associated bone pain. He now has difficulty swallowing and the pain team suggested switching to a patch.
Which patch should be used?
MST 30mg BD = 60mg per day
Oramorph oral solution = 10mg/5ml morphine
So, 5ml BD = 20mg per day
So, total daily dose = 80mg morphine
Consider Transtec ‘35’ patch
A 4 year old child (weight 12kg) is to be given vincristine injection. The recommended dosage is 2mg/m2. What dose should be prescribed?
Mr N, a 75kg man, requires inotropic support with severe sepsis. He is prescribed noradrenaline at 0.1micrograms/kg/min. Noradrenaline is available at 40micrograms/mL. Please give the required rate (mL/hr).
Convert weight to SA so 12kg = 0.56m2
0.56m2 x 2mg = 1.12mg
Dose required = 75kg x 0.1 micrograms/kg/min
= 7.5 micrograms/min
= 7.5 x 60 micrograms/hr
= 450 micrograms/hr
Rate required = 450 micrograms/hr / 40 micrograms/mL
= 11.25 mL/hr
A 48 year old woman weighing 62kg requires a subcutaneous treatment dose of tinzaparin 175 u/kg daily. Her eGFR is 36mL/min. Subcutaneous tinzaparin in available in 10000u/mL pre-filled syringes.
What amount of tinzaparin should the patient be given?
What volume of tinzaparin should the patient be given?
62kg x 175u = 10850u
10850/10000 = 1.085mL = 1.1mL (depending on syringe gradations)
A 78 year old man with atrial fibrillation developed an embolic stroke that caused him to have an unsafe swallow. An NG tube was placed for nutrition and medications. His daily oral digoxin dose of 0.125mg in tablet form needs to be converted to an equivalent liquid dose. Digoxin elixir is available in 50 micrograms/mL.
What volume of liquid should the patient be given?
Convert 0.125mg to micrograms = 125 micrograms.
125/50 = 2.5mL
A 28 year old man weighing 90kg presented to the ED with 4 tonic-clonic seizures in quick succession. He was treated as status epilepticus. He was on no regular medications. He required IV phenytoin loading at 20mg/kg at a maximum rate of 50mg/min.
What is the quickest time this infusion can be given over?
20mg x 90mg = 1800mg required for the man.
1800/50 = 36 minutes.
A 68 year old woman weighing 72kg required initiation of dobutamine at 2.5 microgram/kg/min following cardiac surgery. 20mg dobutamine was made up to 100mL crystalloid ready for infusion.
What infusion rate does she require per hour?
2.5 x 72 = 180 micrograms/minute
= 10.8mg/hour
20mg/100mL, so in one hour = 10.8 / (20/100) = 54ml/hr
