Res 246 ch 2

Metric system andDrug Dose Calculations

Ch 2

Base Units

Kilo Hecto deca Liter, meter, or gram

Deci Centi milli

Common units of measurements

Volume 1 L = 1000 mL1 mL = 0.001 L

Weights

1 Kg = 1000 g1 g = 0.000 Kg

1 g = 1000 mg1 mg = 0.001 g

500 mls is equal to how many liters?

30%

13%27%

30%

1 2 3. 4.

1. 502. 53. 0.54. 2.5

450 mls is equal to now many liters?

1 2. 3. 4

20%

30%

23%

27%

1. 45 2. 0.453. 4.54. 2

How many grams are equal to 50 kg’s?

13%

33%

30%

23% 1. 5002. 50003. 500004. 0.05

How many mls in 2 liters?

25%

25%25%

25%

1 2. 3 4

1. 200 2. 0.2 3. 24. 2000

How many kg’s in 3500 gms?

25%

25%

25%

25%

1. 350

2. 35

3. 3.5

4. 0.35

• Drops – only use dropper calibrated specifically for that particular medicine (one from manufacturer)

16 gtts = 1 ml = 1 cc

• Only use measuring spoons not “eating” spoons

• 1 tsp = 5 mL• 1 Tbs = 15 ml

• 1 inch = 2.54 cm• 2.2 lb = 1 kg• 1.06 qt = 1 liter

• 1 ml = 1 cc (JCAHO recommends using ml though)

Conversion of unitsFraction method

• Step 1– Put down starting value, with its units,

as a fraction with 1 in denominatori.e. How many kg in 150 lbs?150 lbs 1

• Step 2– Place units you started with in the

denominator and put units you want to convert to in numerator with corresponding equivalent values

150 lbs= 1 kg 1 2.2 lbs

cancel out lbs; do math150 = 68.2 kg2.2

How many kg’s does a 180 lb male weigh?

25%

25%

25%

25%

1. 82

2. 360

3. 100

4. 60

Your patient is 5 ft 8 in. tall. Convert to cm’s

25%

25%

25%

25%

10

1. 34

2. 150

3. 80

4. 173

Calculating doses from percentage-strength

solutions• A solution contains a solute, which is

dissolved in a solvent. • The strength of a solution is expressed in

percentage of solute to total solution. – The % is the part of the active ingredient

(solute) in a 100 mL of the preparation. • We usually use a weight to volume type of

preparation for respiratory drugs.

• A 20% saline solution contains 20 g of salt in enough water to make 100 ml of solution

• A 5% albuteral solution contains 5 ml of albuteral in enough water to make 100 ml of solution

Calculating with proportions (ratio/proportion method)

1. Set up a proportion

Original dose = desired dose per amount per amount

2. Convert to consistent units of measurements– Don’t use g and mg at same time, make

both g or both mg– Units on top should be the same – Units on bottom should be the same

How much salt is needed to make 1000 ml of a

20% soln?

20 g = X g100 ml 1000 ml

Solve by cross-multiplying20 g = X g100 ml 1000 ml

20g x 1000ml = 100mlX20,000g/ml= 100mlX20,000g/ml = 100mlX100 ml 100ml200g = X

• Flexeril comes as 5 mg per tablet. You have an order to give 2.5 mg. How many tablets will you need?

• This is how I think it through• What have I GOT? 5 mg/1 tab.• What do I want to GIVE? 2.5 mg• What do I GET? ????

• Set up proportion5 mg = 2.5 mg1 tab X (or ?)

Now solve for unknown (you can make this an X or ?)5 mg = 2.5 mg1 tab X tab

5 mg x X = 2.5 mg x 1 tab

X = 2.5 mg x 1 tab5 mg

X = 0.5 mg/mg x 1 tab (mg/mg will cancel themselves out and leave you with just tab)

X = 0.5 tab or ½ tab

• So I would go GET ½ tablet to GIVE 2.5 mg

• Set up ALL drug calculations like this and it is easy, just make sure same measurements are on top and same measurements are on bottom.

• In above example, mg’s both on top and tab’s on bottom.

• Doesn’t matter which is on top or bottom as long as the same.

1 tab = ?5 mg 2.5 mg would work also

1 tab x 2.5 mg = ? x 5 mg

1 tab x 2.5 mg = ?5 mg

0.5 tab = ?

Dimensional AnalysisWhat have I GOT? 5 mg in 1 tabletWhat do I want to GIVE? 2.5 mgWhat do I GET? ????

? = 2.5 mg x 1 tab 5 mg? = 2.5 mg x 1 tab 5 mg? = 0.5 mg tab

mg? = 0.5 tab

Ratio Solutions

• May see isoproterenol in a 1:200 solution

• Means 1ml of isoproterenol per 200 ml of solution

• Set up as 1 ml 200 ml

How many mls of 1:200 Proventil soln will you give

with 2.5 mg Proventil?

• Given: 1:200 soln. = 1 g of Proventil200 ml of soln.

1 g Proventil = 2.5 mg Proventil200 ml soln. X ml soln.

Convert g to mg1000 mg = 2.5 mg 200 ml x ml

1000 mg = 2.5 mg 200 ml x ml

1000mg x = 500 mg/ml

1000mg x = 500 mg/ml1000 mg 1000 mg

x = 0.5 ml

You have epi 1:100. How many mls for 30 mg?

25%

25%

25%

25%

10

1. 30 ml2. 15 ml3. 0.3 ml4. 3 ml

Calculations with a dosage schedule

• Sometimes the dose of a drug must be obtained from a schedule, which is usually based on the size of the patient.

• Most common is weight but sometimes BSA (body surface area). (p42)

• The body wt. must be obtained first, usually in kg then the dose can be calculated.

The schedule for albuterol syrup in children 2 to 6 yrs old is 0.1 mg/kg of body wt.

You have a prepared-strength mixture of 2 mg/ 5ml; how much is needed for a 20 kg child?

1. Calculate the dose needed:Dose = 0.1 mg x 20 kg = 2.0 mg

Kg(kg’s cancel themselves out and just left with mg’s)

2. Calculate the amount of preparation needed

Use ratio/proportion setup2 mg x 2 mg (from calculation above)5 ml X

2 mg (X) = 2 mg (5 ml)

2 mg (X) = 10 mg/mL

X = 10 mg/mL 2 mg

X = 5 mL

A dosage schedule for surfactant calls for 2.5 ml/kg birth weight. How much drug will you need for

a 800 g baby?

25%

25%

25%

25%

10 SecondsRemaining

1. 2 ml

2. 0.2 ml

3. 200 ml

4. 20 ml