How many mg in a gram? 1,000
How many micrograms in a mg? 1,000
How many mcgs in a gram? 1,000,000
How many mis in a litre? 1,000
How many decilitres in a litre? 10
How many mis in a decilitre? 100
How many microlitres in a ml? 1,000
Note
You need to be able to transpose from one metric unit to another:
e.g. 1mg is the same as 1000mcgs
e.g. 1gm/l is the same as 1mg/ml
How about the following?
How many mmols in a mg? Depends on the specific substance because mmols represent a quantity of the substance based on number of molecules. Mg represents mass.
How many international units (iu) in a mg?
Maintain consistency using units for a given drug - some units can't be readily transposed to metric units
e.g. 1 gram of K+ is VERY different to 1 mmol of K+
(1g of K+ = 13mmol of K+)
What is the concentration of a substance that contains 1mg of a drug in 10 mls of solution?
concentration = dose of drug divided by volume of solution
Conc = dose/volume
Concentration = 1 mg/10ml = 0.1mg/ml or 100mcg/ml
If 0.1mg of Adrenaline in 1 ml is mixed with 9mls of normal saline, what is the new concentration of Adrenaline?
10mcg/ml
How much dextrose is there in a 1 litre bag of 5% dextrose? (NB % = 1gram per 100mls)
50 grams
You are the psychiatry intern and a patient is fitting. The MET team are still 10 minutes away. A nurse has drawn up some diazepam in a syringe. She has used 2mls of 5mg/ml diazepam and 8mls of normal saline. You prescribe 5mgs of diazepam - the nurse asks you how many mls to inject. What is your answer?
First work out the new concentration
Conc = dose/vol = 10mg/2+8mls = 10mg/10mls = 1 mg/ml
The dose we want is 5mg therefore the volume is 5 mls.
You are treating a 60kg patient with a laceration that you will need to suture under local anaesthetic. Given that the maximum safe dose of lignocaine is 3mg/kg, what is the maximum volume of 1% lignocaine that can be administered safely?
First work out the maximum safe dose
60 x 3 = 180mg
Then transpose lignocaine conc into metric units
1% = 1 gram/100ml = 1000mg/100ml = 10mg/ml
Then calculate maximum volume
If conc = dose/vol then vol = dose/conc =
180mg divided by 10mg/ml = 18ml
A 12kg 2-year-old boy has tonsillitis and you would like to give him paracetamol syrup 120mg/5mls. The correct dose is 15mg/kg. How many mis do you give?
Method 1
First calculate the dose required
15mg X 12 = 180mg
Then transpose concentration into numbers easy to do sums with
120mg/5ml = 24mg/ml (approx)
Then calculate required volume
If conc = dose/vol then vol = dose/conc
Vol = 180mg divided by 24mg/ml = 7.5ml
Method 2
An alternate way to solve this problem is by using ratios.
Start with the dose you want over the dose you have 180mg/120mg in a given volume 5ml. Times this ratio (1.5) by the given volume (5ml) to give the volume you want = 7.5ml
Which drugs need their levels monitored?
• Vancomycin
• Gentamicin
• Phenytoin
• Sodium valproate
• Carbamazepine
• Digoxin
• Warfarin
• Clozapine
• Lithium
What is the difference between an IV bolus, slow IV and an infusion?
Why are some intravenous drugs given by infusion?
• Very short half-life
• Tight control - titratable
Which drugs may require a loading dose?
• Digoxin
• Phenytoin
• Lignocaine
• Amiodarone
• Clopidogrel
• Warfarin