Mathematical Aspects of Intravenous Therapy

Simon ShahPharmacy Department

General Points

Use S.I. units at all times Grams (g) Milligrams (mg) Micrograms Nanograms Litres (l) Millilitres (ml)

Units

1kg contains 1000g 1g contains 1000mg 1mg contains 1000 micrograms 1 microgram contains 1000 nanograms 1l contains 1000ml

Concentrations

Different ways of expressing concentration:

Weight in volume Units in volume Percentages Molar Ratios

Weight in Volume

Expressed as - grams/litre (g/l)

mg/ml e.g. dobutamine 250mg/5ml digoxin 0.5mg/2ml morphine 10mg/ml pethidine 50mg/ml or 100mg/2ml

Units in Volume

Unit is a measure of biological activity i.e. 1 unit heparin is not equal to 1 unit

of insulin. Insulin 100 units/ml Heparin 1000 units/ml 5000 units/ml 5000 units/0.2ml

Percentages

1%w/v = 1g in 100ml (1000mg in 100ml)

5%w/v = 5g in 100ml (or 50g/l) 0.9%w/v = 900mg in 100ml (or 9g/l)

Potassium Chloride15%w/v = 15g/100ml = 1.5g/10ml10ml also contains 20mmol

Molar

Rarely used 1 molar = 1mole/litre 1 mole contains a certain number of

molecules, electrolytes etc. 1 molar solution = 1000mmol/lOften used by Biochemistry to report

results e.g. Na 139 mmol/l

Ratio

Concentrations expressed as 1 in..... means 1g in however many mls

1 : 1 solution contains 1g/ml 1 : 1000 is 1/1000th of this

Adrenaline 1 : 1000 = 1g/1000ml or 1mg/ml 1 : 10,000 = 1g/10,000ml or 1mg/10ml

Exception –Tuberculin -ratio based on units

Other points

Always make sure decimal points are preceded by a number e.g. 0.5mg not .5mg

Always think carefully about calculations

Always get a check on calculations

Calculations - Volumes

Volume required is based on proportions.

What you want (Dose) x Volume you have(ml)

What you have (Amount in vial)

e.g. morphine 10mg/ml. Dose required = 15mg Volume required = 15mg x 1ml = 1.5ml 10mg

Gentamicin 80mg in 2ml

Dose prescribed is 140mg What volume do you need?

You want 140mg x It is in 2mls You have 80mg

= 3.5ml

Aminophylline 250mg in 10ml

Dose prescribed is 210mg What volume is required?

You want 210mg x it is in 10ml You have 250mg

= 8.4ml

Infusions

Giving IV drugs by infusion requires two separate stages:

Preparation Administration

Infusions - Doses

Dose of a drug given will depend on two factors:

1. Concentration of infusion (mg/ml)2. Rate of administration (ml/hr)BOTH must be known for the dose to

be calculated. e.g. 2ml/hr on a prescription means

nothing.

Infusion Preparation

Two main methods:

1. Amount of drug added individualised to patient

2. Standardised solutions

Individualised Infusions

AMINOPHYLLINE INFUSION(Vials available as 250mg/10ml)

A 60kg patient is to receive an aminophylline infusion at a rate of 0.5mg/kg/hr for 24 hours

How much aminophylline injection (250mg/10ml) would you add to 1L of fluid and what rate would you set the pump at?

0.5mg/kg/hr = 30mg/hr = 720mg in 24 hrs

720 x 10 = 28.8ml 250 Add 28.8ml to 1L infusion fluid 1 litre over 24 hours = 42ml/hr

Standard Solution Infusions

AMINOPHYLLINE INFUSION-Standardised solution. e.g. 500mg in 500ml, i.e. 1mg/ml Dose is 30mg/hr therefore rate is 30ml/hr

Easier to adjust rate if dose changes Standardisation of preparation between

nurses

Dobutamine 250mg in 5ml

A 60kg patient is currently receiving dobutamine at a rate of 8 micrograms/kg/min...

How would you prepare the solution at what rate would you set the

syringe pump? Use a standard solution =

250mg/50ml

Standard solution preparation 250mg/50ml

Dose= 60 x 8 = 480 micrograms/minute= 480 x 60 = 28,800 micrograms/hour= 28,800/1000mg/hour= 28.8mg/hr

Dose = 28.8mg/hour

Infusion Concentration = 250mg in 50ml

Rate = 28.8mg/hour x 50ml = 5.8ml/hr 250mg

i.e. the rate you want (mg/hr) x volume it’s in (ml) the amount you have (mg)

Adrenaline 1:1000

How would you prepare a 50ml infusion of adrenaline 0.2mg/ml

it needs to be added to a 50ml syringe

Amount = Concentration x Volume= 0.2mg/ml x 50ml = 10mg in 50ml= 10ml of 1:1000 (1mg/ml)

Glyceryl Trinitrate 50mg/10ml

How would you prepare a 0.1mg/ml infusion of GTN?

needs to be added to a 500ml polyfusor Amount = Concentration x Volume

= 0.1mg/ml x 500ml = 50mg in 500ml polyfusor = 1 x 10ml ampoule

Heparin

Heparin is to be given at a dose of 30,000 units/day

How would you prepare the solution and what rate would you use?

Use a 1000unit/ml solution e.g. Pump-

Hep

Use a 1000unit/ml solution Draw up 30ml into syringe. Don’t dilute. 30,000 units/24 hours = 1250 units/hour 1250 units = 1.25ml therefore rate =

1.25 or 1.3ml/hr

What if the dose is increased to 40,000 units/day?

Summary Different units may be used Different uses of same calculation Doses depend on concentration and

rate Use standard solutions if possible Calculate carefully Get a check - redo calculation If not sure - Ask!