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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