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Year 3 - Script > Dosing and calculation > Flashcards

Dosing and calculation Flashcards

1
Q

what are the common calculation errors seen in practise

A
  • miscalculating the concentration of a medicine in a solution
  • moving the decimal point in the wrong direction when converting dose units for example milligrams to grams
  • over or under estimating the time interval between doses
  • miscalculating the number of tablets to be administered
  • miscalculating the rate of administration
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2
Q

describe how paediatrics calculate dosage

A
  • dose needs to be calculated based on age and weight
  • dosing recommendations can be misinterpreted if the information is not read in full
  • liquid preparations vary in strength
  • there can be confusion between ml and mg for the prescriber or for the person administrating the dose
  • if medicines are not licensed in a liquid form, the use of non standard unlicensed preparations can cause confusion
  • a small dosing mistake with minimal effect in an adult can have a significant effect in a child
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3
Q

it is good practise for a …

A

second practioniner to check your dose calculation independently in order to minimise the risk of error

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4
Q

What is a sense check

A
  • what is the approximate answer you are expecting from the calculation
  • ask yourself if the dose seems reasonable to prescribe for the drug in question
  • check that the calculated dose does not exceed the adult dose or maximum daily dose especially when dosing for neonates and children
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5
Q

the dosing regimen for some drugs is…

A

dependent on the weight of the patient

- therefore every patient should have a recent weight documented in their medical record

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6
Q

What is the actual body weight

A
  • this is the weight you get when you stand a patient on a set of scales
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7
Q

Actual body weight….

A

works well provided the patient is neither too large or too small

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8
Q

How is lean body weight calculated

A

calculated by subtracting body fat weight from actual body weight

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9
Q

What are most drug trials conducted on

A
  • average weight patient so limited data available for patients at the extremes of weight
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10
Q

drugs that mainly distribute into water…..

A

water are likely to need dosing on lean or ideal body weight

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11
Q

drugs that mainly distribute into fat…

A

are more likely to be dosed on actual body weight

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12
Q

What formulates are used in acute paediatric emergencies before weight can be used

A
  • 0-12 months = (0.5 x age in months) + 4kg
  • 1-5 years = (2 x age in years) + 8Kg
  • 6-12 years = (3 x age in years) +7kg
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13
Q

what does body surface area (BSA) correlate more with

A
  • it is thought to correlate more closely with variations in age, body size and body composition
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14
Q

What drugs are dosed according to body surface area

A
  • toxic drugs
  • systemic anticancer therapies
  • some antibacterials/antivrials
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15
Q

Name an example of a drug dosed according to body surface area

A
  • intravenous aciclovir for the treatment of herpes simplex for children
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16
Q

What method do you use to measure the BSA in children

A
  • The Boyd method - estimates BSA from the weight only and is used in children
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17
Q

How are solid doses expressed as

A
  • they are expressed as weight per unit
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18
Q

how are liquid doses expressed

A
  • expressed as weight per ml or weight per 5ml or some other variation
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19
Q

How are injectable preparations expressed

A
  • percentage
  • amount per ml
  • total quantity of drug per total volume
  • ratio
20
Q

How many

  • Grams in a Kg
  • how many mg in a g
  • how many micrograms in an mg
  • how many nanograms in a microgram
A
  • 1Kg = 1000g
  • 1g = 1000mg (milligrams)
  • 1mg = 1000 micrograms
  • 1 microgram = 1000 nanograms
21
Q

what is % w/w

A
  • this represents percentage weight per weight
  • this is used when a defined weight of a medicine or chemical is added to a weight of a diluent
  • in a 1% w/w preparation there is 1g of drug in 100g of final product

for example

  • hydrocortisone acetate BP cream 0.5% w/w
  • this means that there is 0.5g of hydrocortisone in 100g of the cream
22
Q

What is % w/v

A
  • this represents percentage weight per volume
  • this is used when a defined weight of a medicine (or chemical) is added to a volume of diluent
  • in a 1% w/v solution there is 1g of drug in 100ml of the final product

for example

  • sodium chloride 0.9% w/v for infusion
  • 0.9g of sodium in 100ml of the infusion
23
Q

what is % v/v

A
  • this represents percentage volume per volume
  • tends to be used for liquid chemicals
  • in a 1% v/v solution there is 1ml of liquid drug/chemical in 100ml of the final product
    for example
  • chloraPrep containing isopropyl alcohol 70% v/v
  • contains 70ml of isopropyl alcohol in 100ml of solution
24
Q

how much does

  • 1 stone in pounds
  • 1 pound in grams
  • 1 stone in kilograms
A
  • 1 stone contains 14 pounds
  • 1 pound = 450g
  • 1 stone = 6.35kg
25
Q

How much is

  • 1 foot in inches
  • 1 inch in millimeters
  • 1 foot in millimetres
A
  • 1 foot contains 12 inches
  • 1 inch is equal to 25.4 mm
  • 1 foot is equal to 304.8 mm
26
Q

in obese patients what should you use to calculate the dose needed for gentamicin

A

ideal body weight

27
Q

what do paediatric doses vary on

A
  • age
  • weight
  • body surface area
28
Q

what is a drug dose equivalents

A
  • this is when you need to change a patients treatment regimen to another medicine in the same class as the drug is not available or not available in the formulation that you need
29
Q

What do you have to think about when using an equivalent drug dose

A
  • bioequivalence
  • pharmacological profile
  • renal function
  • liver impairment
30
Q

How do you withdraw benzodiazepines

A
  • change this to an equivalent dose of diazepam and then carefully reduce the dose of this to 0
31
Q

what are the equivalent doses to 5mg of diazepam

  • chlordiazepoxide
  • lorazepam
  • nitrazepam
  • temazepam
A
  • chlordiazepoxide = 12.5mg
  • lorazepam = 500 micrograms
  • nitrazepam = 5mg
  • temazepam = 10mg
32
Q

why are corticosteroids difficult to do the dose on

A
  • more complex as they have both glucocorticoid and mineralocorticoid activity
33
Q

what are the equivalents to 5mg of prednisolone

  • betamethasone
  • dexamethasone
  • hydrocortisone
  • methylprednisolone
A
  • betamethasone= 750 micrograms
  • dexamethasone = 750 micrograms
  • hydrocortisone = 20mg
  • methylprednisolone = 4mg
34
Q

prednisolone 1mg is equivalent to…

A

hydrocortisone 4mg

35
Q

when switching oral morphine to fentanyl patches how do you do it

A
  • need to change the oral morphine to a patch that is either slightly less or slightly more potent that the morphine dose
  • it is normal practise to use the lowest dose in the conversion Ange and then adjust according to a response and tolerance
36
Q

what is the conversion to oral and subcutaneous morphine

A
  • oral morphine sulfate and subcutaneous morphine sulphate are not equivalent
  • the subcutaneous dose of morphine is half the oral dose
37
Q

what should you communicate with the health care professional administering the infusion

A
  • the name of the active drug
  • total quantity of the drug to be added to the infusion
  • the name the concentration of the diluent
  • the total volume of the syringe or infusion bag
  • how long the infusion is to be administered over
  • the infusion rat e
  • the infusion range
  • the intended dose
  • the route of administration
38
Q

many IV drugs need…

A

further dilution before they are administered - usually because they are too concentrated to be given through a peripheral vein

39
Q

what is used in the treatment of paracetamol overdose

A
  • acetylcysteine
40
Q

How do you calculate dosage in acetylcysteine

A
  • using the patients actual body weight

- maximum body weight you should use is 110kg even if the patient is heavier

41
Q

what do you dilute the acetylcystiene in

A
  • it is preferable diluted in glucose 5%
42
Q

what are the three parts of administration of acetylcystiene

A
  • the first is given over 1 hour in 200ml of glucose 5%
  • the second is given over the next 4 hours in 500ml of glucose 5%
  • the third and final is given over the next 16 hours in 1L of glucose 5%
43
Q

describe how the dose of acetylcysteine varies at each step

A
  • first = 150mg/kg
  • second = 50mg/kg
  • third = 100mg/kg
  • total dose of 300mg/kg over 21 hours
44
Q

how do you calculate the volume of a parenteral drug required to be administered or added to an infusion

A

volume required = (dose prescribed x volume of solution)/ amount of drug in solution

same as
- Volume required = (what you want/what you’ve got) x what’s in it

45
Q

how do you calculate percentage change

A
  • first we convert the percentage to a decimal by dividing it by 100 (X/100)
  • Step 2 - X=25 (a 25% decrease in dose, 25/100 = 0.25
  • Step 3 - then we multiply this by the number we need to affect 0.25 x 2300 mg = 575mg
  • step 4 = 2300-575 = 1725mg twice a day

Year 3 - Script (11 decks)

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