3 Dosages of medications for injection
In this chapter you’ll be shown how to calculate the volume of medication to be drawn up in a syringe, to be given by injection.
What you need to know
The number of decimal places in each answer should relate to the graduations on the syringe being used. Syringes with a capacity of more than 1 mL are usually graduated in tenths or fifths of a millilitre: so for volumes greater than 1 mL calculate answers to one decimal place. Syringes with a capacity of 1 mL or less are often graduated in hundredths of a millilitre: so for volumes less than 1 mL calculate answers to two decimal places.
Refer to Inside front cover page for explanations of abbreviations.
Estimating the volume for injection
It is important to learn how to estimate an answer before beginning to work out the actual answer.
Example 1
Pethidine 75 mg is to be given by IM injection. Stock ampoules of pethidine contain 100 mg in 2 mL. Is the volume to be drawn up for injection equal to 2 mL, less than 2 mL, or more than 2 mL?
A stock ampoule contains 100 mg of pethidine.
75 mg (prescribed) is less than 100 mg (ampoule).
Example 2
Vancomycin 1200 mg is prescribed. Stock vials contain vancomycin 1 g in 10 mL (once diluted). Is the volume of stock required for injection equal to 10 mL, less than 10 mL, or more than 10 mL?
A stock vial contains 1 g of vancomycin.
1200 mg (prescribed) is more than 1000 mg (vial).
Example 3
A patient is to be given 12 000 units of Calciparine. Available ampoules contain 25 000 units in 1 mL. Should the volume to be drawn up for injection be equal to 1 mL, less than 1 mL, or more than 1 mL?
A stock ampoule contains 25 000 units.
12 000 units (prescribed) is less than 25 000 units (ampoule).
Exercise 3A
Choose the correct answer to each problem. The answer will be equal to, less than, or more than the volume of the stock ampoule or vial.
Think carefully about each answer in the exercises that follow. Should the volume to be drawn up for injection be equal to, less than, or more than the volume of the stock ampoule?
Calculating volumes of dosages for injection
Example 1
A patient is prescribed furosemide (frusemide) 60 mg, IV. Ampoules contain furosemide 80 mg in 2 mL. Calculate the volume required for injection.
Example 2
An injection of digoxin 175 micrograms is prescribed. Stock on hand is digoxin 500 micrograms in 2 mL. What volume of stock solution should be given?
The volume required formula can be abbreviated to:
Exercise 3B
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