4Concentrations Different ways of expressing concentration: Weight in volumeUnits in volumePercentagesMolarRatios
5Weight in Volume Expressed as - grams/litre (g/l) mg/ml e.g. dobutamine 250mg/5mldigoxin 0.5mg/2mlmorphine 10mg/mlpethidine 50mg/ml or 100mg/2ml
6Units 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/mlHeparin units/ml5000 units/ml5000 units/0.2ml
7Percentages 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
8Molar 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
9RatioConcentrations expressed as 1 in..... means 1g in however many mls1 : 1 solution contains 1g/ml1 : 1000 is 1/1000th of thisAdrenaline1 : 1000 = 1g/1000ml or 1mg/ml1 : 10,000 = 1g/10,000ml or 1mg/10mlException –Tuberculin -ratio based on units
10Other points Always make sure decimal points are preceded by a number e.g. 0.5mg not .5mgAlways think carefully about calculationsAlways get a check on calculations
11Calculations - 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 = 15mgVolume required = 15mg x 1ml = 1.5ml10mg
12Gentamicin 80mg in 2ml Dose prescribed is 140mg What volume do you need?You want 140mg x It is in 2mlsYou have 80mg= 3.5ml
13Aminophylline 250mg in 10ml Dose prescribed is 210mg What volume is required?You want 210mg x it is in 10mlYou have 250mg= 8.4ml
14Infusions Giving IV drugs by infusion requires two separate stages: PreparationAdministration
15Infusions - Doses Dose of a drug given will depend on two factors: Concentration of infusion (mg/ml)Rate of administration (ml/hr)BOTH must be known for the dose to be calculated.e.g. 2ml/hr on a prescription means nothing.
16Infusion Preparation Two main methods: Amount of drug added individualised to patientStandardised solutions
17Individualised 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 hoursHow much aminophylline injection (250mg/10ml) would you add to 1L of fluid and what rate would you set the pump at?
180.5mg/kg/hr = 30mg/hr = 720mg in 24 hrs 720 x 10 = 28.8ml250Add 28.8ml to 1L infusion fluid1 litre over 24 hours = 42ml/hr
19Standard Solution Infusions AMINOPHYLLINE INFUSION-Standardised solution.e.g. 500mg in 500ml, i.e. 1mg/mlDose is 30mg/hr therefore rate is 30ml/hrEasier to adjust rate if dose changesStandardisation of preparation between nurses
20Dobutamine 250mg in 5mlA 60kg patient is currently receiving dobutamine at a rate of8 micrograms/kg/min...How would you prepare the solutionat what rate would you set the syringe pump?Use a standard solution = 250mg/50ml
21Standard 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
22Infusion Concentration = 250mg in 50ml Dose = 28.8mg/hourInfusion Concentration = 250mg in 50mlRate = 28.8mg/hour x 50ml = 5.8ml/hr250mgi.e. the rate you want (mg/hr) x volume it’s in (ml)the amount you have (mg)
23Adrenaline 1:1000How would you prepare a 50ml infusion of adrenaline 0.2mg/mlit needs to be added to a 50ml syringeAmount = Concentration x Volume= 0.2mg/ml x 50ml = 10mg in 50ml= 10ml of 1:1000 (1mg/ml)
24Glyceryl Trinitrate 50mg/10ml How would you prepare a 0.1mg/ml infusion of GTN?needs to be added to a 500ml polyfusorAmount = Concentration x Volume= 0.1mg/ml x 500ml= 50mg in 500ml polyfusor= 1 x 10ml ampoule
25Heparin 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
26Use a 1000unit/ml solutionDraw up 30ml into syringe. Don’t dilute.30,000 units/24 hours = 1250 units/hour1250 units = 1.25ml therefore rate = 1.25 or 1.3ml/hrWhat if the dose is increased to40,000 units/day?
27Summary Different units may be used Different uses of same calculation Doses depend on concentration and rateUse standard solutions if possibleCalculate carefullyGet a check - redo calculationIf not sure - Ask!