Systems of
measurement
The
proper administration of medication requires the ability to calculate the
medication doses accurately and measure medication correctly. Any errors in the
calculation may cause a fatal error. The health care industry uses Metric,
Apothecaries, Household system and solutions. Three systems of measurement are
used. The metric system, the apothecaries system, and the household system
which is similar to apothecaries system.
1. Metric system: The metric system is logically arranged in to units of
10; it is decimal system. Basic units
can be multiplied or divided by 10 to form secondary units. Multiples are
calculated by moving the decimal points to the right, and the division is
accomplished by moving the decimal point to the left.
·
Basic units of measurement are the meter,
the litre and the gram.
·
Prefixes derived from the Latin designate
subdivisions of the basic unit: deci (1/10 or 0.1) centi (1/100 or 0.01) and
milli (1/1000 or 0.001).
·
Multiples of the basic units are
designated by prefixes derived from greek: deka (10) hecto (100) and kilo
(1000)
·
Only the measurements of volume (the
litre) and of weight (gram) are the
measures used in medication administration. In nursing practice kilogram (kg)
is the only multiple of the gram (gm.) used and milligram (mg) and microgram
(mcg or Ug) are subdivisions.
·
Fractional part of the liter are usually
expressed in millilitres (mL) Multiples of the liter are usually expressed in
liters or milliliters for example 2.5 liters or 2500ml.
·
The basic units of measurement in metric
system are the meter (length). liter (volume), and gram (weight).
Designation of units:
Gram
|
g or Gm
|
Liter
|
l or L
|
Milligram
|
mg
|
Milliliter
|
ml
|
Greek prefix designate the multiples of the basic units
Deka
|
10
|
Hexa
|
100
|
Kilo
|
1000
|
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Apothecaries' system
·
The basic unit of the weight in
apothecaries system is grains (gr) likened to grain of the Wheat and the basic
unit of volume IS minim, a volume of water equal in weight to a grain of a
wheat. The word minim means "the least".
·
In ascending order the other units of
weight are the scruple, the dram, the ounce, and the pound,
·
The units of volume are in ascending order
the fluid dram, the fluid ounce, the pint, the quart and the gallon
·
The roman letters follows rather than
procedes the units of measure. For e.g., 2 ounces are written as 3 II.
·
Quantities less than I are expressed as
fraction form e.g. Gr 1/6.
·
This system is not used much.
Household system
Household
measures may be used when more accurate systems of measure are not required.
Included in household measures are drops, teaspoons, tablespoons, cups, and
glasses. Although pints and quarts are often found in home, they are defined as
apothecaries measures.
Metric
|
Apothecary
|
Household
|
1 ml
|
15-16
minims
|
15
drops
|
5 ml
|
1 drams
|
1
tablespoon
|
15 ml
|
4 drams
|
1
tablespoon
|
30 ml
|
1 ounce
|
2
tablespoon
|
240 ml
|
8 ounce
|
1 cup
|
480 ml
|
1 pint
|
1 pint
|
960 ml
|
1 quart
|
1 quart
|
3840 ml
|
1
gallon
|
1
gallon
|
Unit
|
Approximate
value
|
|||
1 dram
|
60
minims
|
60
grains
|
4 grams
|
4 ml (1
teaspoonful)
|
1 ounce
|
8 tsf
|
480
grains
|
30
grams
|
30 ml
|
1 liter
|
40
ounces
|
2 pints
|
1 quart
|
1000 ml
|
1 gram
|
15
grains
|
1000 mg
|
||
1 grain
|
60 mgms
|
|||
1 cc
|
15
minims
|
1 ml
|
||
1 minim
|
1 drop
|
|||
1 pint
|
20
ounces
|
500 ml
|
||
1 pound
|
16
ounces
|
480
grams
|
||
1 kg
|
2.2 lb
|
1000
grams
|
||
1 mgm
|
1000
mcg
|
|||
1
gallon
|
4000 ml
|
4
quarts
|
||
1 tsf
|
4 to 5
ml
|
60
drops
|
||
1
tablespoon
|
15 ml
|
4 drams
|
4 tsf
|
|
1 tea
cupful
|
150 ml
|
6
ounces
|
||
1
glassful
|
200 ml
|
8
ounces
|
||
1 cm
|
10 mm
|
|||
1 km
|
1000 m
|
|||
1 mile
|
1.6 km
|
|||
1 foot
|
12
inches
|
30 cm
|
||
1 inch
|
2.5cm
|
3. Household measurements
Household
measurements are familiar to most people as it is used in the home set up for measurement.
The main disadvantage is its inaccuracy and the .variation in the size and
shape of the utensils. Household measures include Drops, Teaspoon, Tablespoon
and cups for volume and Pints and quarter for weight.
4. Solutions
Solutions
of various concentrations are used for injections, irritations and infusions. A
solution is a given mass of solid substance dissolved in a known volume of
liquid.
The
concentration is in units of mass per units of volume. E.g. g/ml, g/ l, mg/ml.
We
can also express a concentration of solution in percentage.
e.g.:
10% solution = I0 g solid dissolved in 1000 ml of liquid.
1/1000
solution = 1 g of solid in 1000ml of liquid
OR
1
ml of liquid in 1000ml of liquid.
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System of
Measurement
Household:
tsp., gtt, T, c
Apothecary:
gr, dr, oz
Metric:
ml, mg, g, I
Converting weights within metric system.
Only
three metric units of weight are used for drug dosages, the gram (gm.)
milligram (mg) and microgram (mcg or Ug) 1000mg or l, 00,000 mcg, equals 1 gm.
Equalents are computed by dividing or multiplying; for e.g. to change
milligrams to grams divide the number of
milligrams by 1000, simplest way to divide by 1000 is to move the
decimal point three places to the left.
E.g.; 500mg=? gm
Move
the decimal point three places to the left.
Answer
is 0.5 gm
Conversely,
to convert grams to milligrams, multiply the number of grams by 1,000, or move
the decimal point three places to the right: 0.006 g = ? mg. Move the decimal
point three places to the tight:
Answer
= 6 mg
Converting units of volume
Approximate
volume equivalents:
I ml = 15 minims / 15
drops
15 ml = 4 fluid drams / I
tablespoon
30ml = I fluid ounce/ I
ounce
500ml = 1 pint (pt)
1000ml= Iquart (qt)
4000ml = 1 gallon (gal)
·
ml dosages need to be fractionalized. The
nurse can fractionalize ml dosages by
remembering that I ml contain 15 drops or minims.
·
Fluid drams and ounces are commonly used
in prescribing liquid medications, such as cough syrups, laxatives, antacids,
and antibiotics for children.
·
The fluid ounce is usually converted to ml
when measuring a client's fluid intake or output.
·
Liters and milliliters are the volumes
commonly used in preparing solutions for enemas, irrigating solutions for
bladder irrigations, and solutions for cleaning open wounds.
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Calculating units of weight
Calculating units of weight
The
units of weight commonly used in nursing practice are the gram, milligram and kilogram, and the gain and the pound.
1 mg 1/60 grain
60 mg= 1 grain
1 g = 15 grains
4 g = 1 dram
30gm = 1 ounce
500gm= 1.1 pound (1b)
1000 gm = 2.2 lb
·
Converting milligrams to grains and vice
versa, for example, when preparing medications
·
Converting pounds to kilograms and vice
versa, for example, a person's weight
·
When converting units of weight from the
metric system to the apothecaries' system,
the nurse should keep in mind that a milligram is smaller than a grain
·
Small units (mg) to large units conversion
( grains) = smaller number
·
large units ( grains) to Small units (mg)
to conversion= larger number
3000mg
----------------- = 50 grains
60
50
grains x 60 = 3000mg
When converting pounds to kg pound is
smaller unit than the kg
2.2 lb = 1 kg
1 10 lb = x kg
110 x 1
X =
---------------- = 50 kg
2.2
50 kg = x lb
2.2 x 50
X =
--------------- = 110 lb
1
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Calculating dosages
Several
formulas can be used to calculate the drug dosage.
Amount
to administer (X) = desired dose x quantity on hand
--------------------------------------------
Dose on hand
E.g.
Erythromycin 500 mg is ordered. It is supplied in a liquid form containing 250
mg in 5m to calculate the dosage
500
x 5ml
------------------ = 10 ml
250
Dose ordered: Amount of medication prepared.
Dose on hand: weight or volume of medication available in units
supplied from pharmacy.
Amount on hands: Is the basic unit or quantity of the medication that
contains the dose on hands.
Amount to administer: Is the actual amount of available medication that will
administer.
The
amount to administer is always expressed in the same unit as the amount on
hands.
E.g.
the dose ordered is Inj Heparin 5000 units. It is supplied in a liquid form
containing 10,000 units per ml.
5000 units x I ml
------------------------ = ½ ml = 0.5 ml
10000 units
Tab
Lanoxin 0.125 mg is ordered. It is supplied is 0.25 mg tablets
0.125 x I
---------------- = ½ tablet
0.25
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2. Conversion between systems
Frequently
you will determine the correct dose of medication by converting weight or
volume from one system to another. e.g.: Metric units are converted in to
equalent household measures for ease administration at home.
To
convert from one measurement system to other, it is mandatory to use equalent
measurement. E.g. A health care provider orders 30ml of Robittusin for your
patient. To provide 9roper instruction to the client, you will convert 'ml' to
common household measurement. From the
table, determine that 30ml = 2 tablespoon. Therefore instruct to take 2 table
spoon of Robittusin.
Dosages for children
Although
dosage is stated in the medication order, nurses must understand something
about the safe dosage for children. Body size significantly affect dosage for
children.
Body
surface area is determined by using a nomogram and the child's height and
weight. This is considered to be the most accurate method of calculating a
child's dose.
Child's dose = surface area of child
(m2)
-------------------------------------- X normal adult dox
1.7 m2
For
example, a child who weighs 10 kg and is 50 cm tall has a body surface area of
0.4 m2. Therefore, the child's dose of tetracycline corresponding to an adult
dose of 250 mg would be as follows:
Child's
dose = 0 4 m2 X 250 mg
= 0.23 X 250 = 58.82 mg
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Dimensional Analysis
Given Quantity: the beginning point of the problem
Conversion Factors- Equivalents necessary to
convert between systems of measurement and
to allow unwanted units to be cancelled from the problem
Unit Path: the series of conversions necessary to achieve the
Wanted Quantity
Wanted Quantity: the answer to the problem
Drip Rates
You
have started an IV of 0.9% Sodium Chloride and a physician request that you
give 150 mL/ hr. You are using a 20gtt tubing set. How many drops/ minute will
you give?
·
50drops / minute
·
(mL * drop factor) / minutes
·
Example: 500 ml, using 15 gtt set over I
hour.
·
(500 x 15) / 60 = 125 drops/min
COMMENTS