Topic 3Math/Drugs Calculations

IV Giving Sets

Generally there are 2 types of giving sets in use and they deliver drops of different sizes

Blood giving sets 15 or 20 drips/ml. these are known as macrodrips

The more commonly used metrisets deliver 60/drips/ml. These are known as microdrips.

IV calculation

For drops per minute the rule is:

Rate (drops/minute) = Volume (in ml) x Drip factor Time (minutes)

Example 1

• A 1L bag of N/saline is to run over 6 hours. You are using a giving set which delivers 20 drops/ml. What should the flow rate be set at?

Rate = 1000 x 20 convert the 1L to 1000ml 6 x 60 1 = 1000 x 20 common factor 10 & 4 360 1 = 2000 36 = 500 Long division 9

= 55.55

= 56 drops/ minute Rounded up to nearest whole number

Example 2

A 500ml bag of Hartman’s solution is to run over 4 hours. The giving set delivers 20 drops/ml. What should the flow rate be?

Solution – Example 2

Rate = 500 x 20 4 x 60 1 = 500 x 20 240 1 = 500 12 = 125 3 = 42 drops/minute

Example 3

A patient is to receive 500ml of 5% Dextrose over 2 hours using a giving set which delivers 20 drops/ml. Calculate the drip rate in drops/minute.

Solution - Example 3

Rate = 500 x 20 2 x 60 = 10000 120 = 250 3 = 83 drops/minute

Example 4

A patient has to have an IV set up to deliver 1L of fluid over 4 hours, using a metriset (60 drops/ml). What should the flow rate be?

Solution - Example 4

Rate = 1000 x 60 4 x 60 = 60000 240 = 2000 8 = 250 drops/minute

Example 5

A dehydrated adult is ordered 1L of normal saline over 2 hours, using a macrodrip (20 drops/ml) IV giving set. Calculate the drip rate in drops/minute.

Solution - Example 5

Rate = 1000 x 20 2 x 60 = 20000 120 = 1000 6 = 167 drops/ minute

IV calculations

You made need to calculate how long an IV infusion will take to run through, if you know the drip rate re-organise the previous rule

For the time for a drip to run the rule is:

Time (in minutes) = Volume (in ml) x Drip factor Rate (drops/min)

Example 1

There is 500ml left to run through the IV metriset. The drip rate has been set at 50 drops/minute. How long will it take to run through?

Time (in minutes) = 500 x 60 50 1 = 10 x 60 1 1 = 600 minutes = 10 hours

Example 2

There is 700ml of N/saline left to run through the IV metriset. The drip rate is set at 40 drops/minute. How long will it take to complete?

Solution - Example 2

Time (in minutes) = 700 x 60 40 1 = 700 x 6 4 1 = 700 x 3 2 1 = 2100 2 = 1050 =17 ½ hours = 17 hours 30 minutes

Example 3

How long will an IV take to run through if the drip rate is set at 80 drops/minute and there are 300ml left using a drip factor of 20 drops/ml?

Solution - Example 3

Time (in minutes) = 300 x 20 80 = 6000 80 = 600 8 = 75 minutes OR = 1 ¼ hr

Example 4

How long will 300ml of IV packed cells take to run through at 30 drops/minute using a blood giving set (15 drops/ml)?

Solution - Example 4

Time (in minutes) = 300 x 15 30 = 4500 30 = 450 3 = 150 minutes OR = 2 ½ hr

Example 5

How long will 2l of Normal Saline take if it is running at 100ml/hour using a metriset?

Solution - Example 5

Time (in minutes) = 2000 x 60 100 = 120000 100 = 1200 minutes OR = 20 hours

IV Calculations

Sometimes you made need to calculate the hourly volume of an IV. To do this reorganise the previous rule

For volume per hour the rule is

Volume per hour = Total Volume Time (hours)

Example 1

The doctor has ordered 1500 ml of fluid to be given over 5 hours. What should the hourly volume be in ml per hour?

Volume = 1500 5 = 300 ml/hr

Example 2

The client is ordered 1.2L of Hartmann’s solution over the next 6 hours. What should be the hourly volume in ml/hour?

Solution - Example 2

Volume = 1200 6 = 200 ml/hr

Example 3

An IV containing 600ml has to run through in the next 4 hours. What volume needs to run through every hour?

Solution - Example 3

Volume = 600 4 = 150 ml/hr

Example 4

A client’s IV is commenced at 1000HR. It contains 1L and is to run through by 2000HR. What should the hourly volume be (in ml per hour)?

Solution - Example 4

Volume = 1000 10 = 100 ml/hr

Example 5

What volume is needed every hour if a client must have 700 ml in 10 hours?

Solution - Example 5

Volume = 700 10 = 70 ml/hr

Paediatric Drug Doses

Clarke’s Body Weight Rule (used for children aged 1 and over)

Child’s dose = Weight of child (kg) x Adult dose

Average adult weight (70kg)

Example 1

A child weighing 7kg is to be given ampicillin. The adult dose is 500mg. How much should the child be given?

Child’s dose = 7 x 500 70 = 1 x 500 10 = 50 mg

Example 2

A child weighing 10 kg is to have penicillin and the adult dose is 500mg. How much would you give?

Solution - Example 2

Child’s dose = 10 x 500 70 = 1 x 500 7 = 500 7 = 71.4 = 71 mg

Paediatric Drug Doses

Clark’s Body Surface Area (BSA) Rule

Child’s dose = Surface area of child (m²) x Adult dose average surface area of adult (1.7 m²)

Example 1

A child whose BSA is 0.7 m², is ordered penicillin. The adult dose is 1g. Calculate how much to give.

Child’s dose = 0.7 x 1000 1.7 = 7 x 1000 17 = 7000 17 = 412 mg

Example 2

A child whose BSA is 0.6 is ordered pethidine. The adult dose is 100 mg. How much would you give?

Solution - Example 2

Child’s dose = 0.6 x 100 1.7 = 6 x 100 17 = 600 17 = 35 mg

Paediatric Drug Doses

Freid’s Rule (used for infants under 1yr old)

Child’s dose = Age in months x Adult dose 150

Example 1

A six month old child is to be given amoxycillin with clavulanic acid (Augmentin). The adult dose is 500mg. How much should the child be given?

Child’s dose = 6 x 500 150 = 6 x 10 3 = 60 3 = 20 mg

Example 2

A 4 month old is to be given tetracycline. The adult dose is 250mg. How much would you give?

Solution - Example 2

Child’s dose = 4 x 250 150 = 4 x 5 3 = 20 3 = 6 .6666 = 7 mg

Paediatric Drug Doses

Young’s Rule (used for children aged 2 to 12)

Child’s dose = Age in years x Adult dose age + 12

Example 1

The adult dose for metronidazole (Flagyl) is 500mg. How many milligrams should you give a 10 year old?

Child’s dose = 10 x 500 10 + 12 = 10 x 500 22 = 5000 22 = 227 6/22 mg = 227 mg

Example 2

An 8 year old is ordered ampicillin. The adult dose is 500mg. How much should you give the client?

Solution - Example 2

Child’s dose = 8 x 500 8 + 12 = 8 x 500 20 = 8 x 50 2 = 4 x 50 = 200mg