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Parenteral Medications
The same methods, ratio-proportion or formula, are used to determine the amount to be
given.
 Injectable medication guidelines:
- Intradermal - the volume to be administered is 0.1 ml or less
- Subcutaneous - the volume to be administer is 1.0 ml or less
- Intramuscular - depends upon the size of the person
a. A healthy well developed person can tolerate 3.0 ml in large muscles - this does
NOT include the deltoid.
b. For elderly, thin clients or children the total amount should not exceed 2.0 ml.
c. No more than 1.0 ml should be given to young children and older infants.
Calculating dosages in units (insulin,
heparin, pitocin, vitamins, some antibiotics)
Example: Ordered:
Heparin 8000 units subcutaneous q12h
Available: Heparin 10,000 units per ml
How much will you administer?
Formula: 8000 units X 1 ml =
0.8 ml
10,000 units
Ratio: 10,000 units : 1 ml = 8000 units : x
8000 units X 1 ml = 10,000 units x
8000 / 10,000 = x
0.8 ml = x
If the answer is greater than 1, you probably calculated the problem incorrectly.
Rarely, the desired dose is large and you will have to administer it in more than one
site.
Reconstituting powdered drugs:
Read the label for the amount of fluid to add, the type of fluid and the final
concentration of the reconstituted fluid. The label will also tell you how long the
mixture may be stored and what conditions are required for storage. The final volume
will be larger than the amount of fluid you add because the powder will take up some room
when diluted. If you are not given a final volume calculate the concentration based on the
amount of fluid you added. You will calculate the amount to administer from the final
concentration.
Insulin
Insulin is administered only using an insulin syringe. Most insulin vials contain 100
units/ml. Insulin may be administered subcutaneously, intramuscularly (rarely used) and
intravenously.
Regular insulin is the only type that may be given IV since it does not contain any
additives to prolong the action. Regular insulin is clear. If the vial is cloudy, it has
been contaminated and should be discarded.
Longer acting insulin is cloudy and may have a precipitate on the bottom of the vial.
Be sure to mix the vial well by rotating it between the hands.
Mixing regular insulin and a longer acting insulin in the same syringe.
- Inject air into the longer acting insulin vial first - don't let the tip of the needle
touch the surface of the fluid.
- Using the same syringe, inject air into the regular insulin then invert the bottle and
withdraw the correct amount of regular insulin. Remove air bubbles.
- Still using the same syringe, withdraw the correct amount of the longer acting insulin.
You cannot return any extra fluid withdrawn. If you withdraw too much, you must start
over.
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Intravenous calculations
Drop factor - IV tubing has a
drip chamber that is used to count drops (gtts) per minute. Each tubing is labeled
with the number of drops per milliliter (drop factor).
Macrodrop tubing - has a drop factor of 10, 15 or 20 gtts/ml (drops per milliliter).
Microdrip tubing - has a drop factor of 60 gtts/ml.
Formula for calculating IV flow
rate:
V1 X V2 = gtt/min
V1 - volume to infuse
T1 T2
T1 - time to infuse (hours or minutes)
V2 - drop factor
T2 - time in minutes - this number is
always 60 (minutes/hour) unless
you are going to infuse for less
than
60 minutes
Example: Ordered: D5W 1000 ml to
infuse in 6 hours.
The drop factor of your tubing is 10 gtts/ml.
How many gtts/min will you infuse?
V1=1000 ml V2=10 gtts/ml T1= 6 hr T2
= 60 min/hr
1000 ml X
10 gtt/ml = 1000 X 1 = 1000 = 27.8
= 28 gtts/min
6 hr
60 min/hr 6
6 36
The same formula may be used to calculate drop rates for fluids administered in less
than 1 hour:
Example: Ordered:
Gentamycin 40 mg/100 ml IVPB q 6h
Drop factor 15 gtts/ml
Your drug book says you can give this in 45 min
How many gtts/min will you infuse the Gentamycin
100 ml X 15 gtts/ml = 100 X 1 = 33 gtts/min
1 hr 45 min/hr
3
You may only give 32 gtts/min because it is easier to count for 15 seconds (8
gtts every 15 seconds = 32 gtts/min).
When using an IV pump, the rate is in ml/hr. Therefore, you do not need to
determine a drop factor.
Example: Infuse Ancef 1
g/50 ml IVPB q6h. The IV handbook states this can be given in 20 minutes. What rate will
you set on the IV pump?
50 ml X 60 min = 50
X 3 = 150 ml/hr
20 min
1 hr
You need to give 50 ml in 20 minutes. But you have to
convert the minutes to hours.
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Pediatric calculations
Accurate doses are especially important in giving medications to infants and children
because even small errors can be dangerous due to their small body size.
Two methods are used to calculate pediatric dosages:
According to the weight in
kilograms (kg)
According to the child's body
surface area (BSA)
Calculations
based on body weight
- The first step is to convert the child's body weight into kg. The formula is 2.2 lb. = 1
kg.
- The second step is to calculate the medication dose.
a. Calculate the daily dose
b. Divide the daily dose by the number of doses to
be administered.
c. Use either the ratio-proportion or formula
method to calculate the number
of tablets/
capsules or volume to be administered with
each dose.
Example: A child weighing 76
lbs. is ordered to receive 150 mg of Clindamycin q6h. The pediatric drug handbook
states the recommended dose is 8-20 mg/kg/day in four divided doses. The Clindamycin is
supplied in 100 mg scored tablets.
1. What is the weight in kg?
76 lbs. ¸ 2.2 kg/lb. = 34.5 kg
2. What is the safe total daily dose?
Minimum: 8 mg/kg/day
X 34.5 kg = 276 mg/day
Maximum: 20 mg/kg/day X 34.5
kg = 690 mg/day
3. Is this a safe dose?
150 mg/dose X 4 doses/day = 600 mg/day
Yes this is within the recommended safe range.
4. Calculate the number of tablets to give.
100 mg : 1 tablet = 150 mg : x (multiply
means
100 x = 150
& extremes)
x = 1.5 tablets
Calculations
based on Body Surface Area (BSA)
BSA is determined from a nomogram using the child's height and weight.
When you know the child's BSA the dosage is determined by multiplying the BSA by the
recommended dose.
To determine whether the dose is safe, compare the ordered dose and the calculation based
upon the BSA.
The formula for calculating child's dosage is
Child's BSA X adult
dosage
1.7 M2
Example: The child has a BSA of
0.67 M2. the adult dose is 40 mg. The physician ordered 8 mg. Is the
dosage correct?
0.67 X 40 = 26.8 =
15.8 mg
No, the dose is
1.7
1.7
too small.
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Now you are ready for some practice problems.
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