Dosages of medications for injection

3 Dosages of medications for injection

Correct measurement of dosages of medications for injection is essential. An overdose can be dangerous; too low a dose may result in a medication being ineffective.

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.

• Give answers greater than 1 mL correct to one decimal place

• Give answers less than 1 mL correct to two decimal places

• If the next decimal place is 5 or more, add one to the previous digit

• Check that stock strength and the strength required are given in the same units in a particular calculation (i.e. both strengths in grams or milligrams or micrograms).

If in any doubt about the answer to a calculation, then ask a supervisor to check your work.

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.

Volume of ampoule = 2 mL

75 mg (prescribed) is less than 100 mg (ampoule).

Therefore, volume to be drawn up is less than 2 mL.

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.

Volume of ampoule = 1 mL

12 000 units (prescribed) is less than 25 000 units (ampoule).

Therefore, volume to be drawn up is less than 1 mL.

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.

1 An injection of morphine 9 mg is prescribed. A stock ampoule contains morphine 15 mg in 1 mL. The volume to be drawn up for injection will be equal to 1 mL/less than 1 mL/more than 1 mL.

2 A patient is to receive an injection of ondansetron 6 mg. Stock ampoules contain ondansetron 4 mg in 2 mL. The volume to be drawn up for injection is equal to 2 mL/less than 2 mL/more than 2 mL.

3 Furosemide (frusemide) 80 mg is prescribed. Ampoules contain furosemide 250 mg/5 mL. The volume required for injection is equal to 5 mL/less than 5 mL/more than 5 mL.

4 Benzylpenicillin 1.2 g is prescribed. Stock vials contain 600 mg in 2 mL, when diluted. The volume of stock required is equal to 2 mL/less than 2 mL/more than 2 mL.

5 A patient is prescribed flucloxacillin 1000 mg, IV. If stock ampoules contain 1 g in 10 mL, once diluted, then the amount of stock solution to be drawn up will be equal to 10 mL/less than 10 mL/more than 10 mL.

6 On hand are digoxin ampoules containing 500 micrograms in 2 mL. An injection of 225 micrograms is prescribed. The volume required is equal to 2 mL/less than 2 mL/more than 2 mL.

7 Heparin is available at a strength of 1000 units per mL. The volume needed to give 1250 units is equal to 1 mL/less than 1 mL/more than 1 mL.

8 Diazepam 15 mg is to be given by IV injection. Stock ampoules contain 10 mg in 2 mL. The volume to be drawn up is equal to 2 mL/less than 2 mL/more than 2 mL.

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?

Check your answers on page 148.

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:

If you are having difficulty simplifying the fractions in these examples then refer to Review exercises 1H and 1I in Chapter 1.

Tags: Nursing Calculations

Dec 3, 2016 | Posted by admin in NURSING | Comments Off

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