Syringes and Rounding

These are general acceptable guidelines. Always follow any institution-specific guidelines.

3-mL Syringe

When you’re calculating injection answers, the degree of accuracy depends on the syringe you use. Figure 5-1 shows a 3-mL syringe marked in milliliters to the nearest tenth. To calculate milliliter answers for this 3-mL syringe, carry out the arithmetic to the hundredth place and then round off the answer to the nearest tenth (follow standard rounding rules).

1.25 mL becomes 1.3 mL

1-mL Syringe

Figure 5-2 shows a 1-mL syringe (also called a 1-mL precision syringe or a 1-mL tuberculin syringe) marked in milliliters to the nearest hundredth. To calculate milliliters when the 1-mL syringe is used, carry out the arithmetic to the thousandth place and then round off the answer to the nearest hundredth (follow standard rounding rules).

0.978 mL becomes 0.98 mL

Each of the following examples provides a syringe. (Syringes will be shown without needles.) Calculate milliliters to the degree of accuracy required by the syringe markings. (Some syringes still have minim markings -M or m-however this unit is rarely used.)

FIGURE 5-1
A 3-mL syringe.

FIGURE 5-2
A 1-mL syringe.

Calculating Liquid Injections

To solve liquid injection problems, use the same rule as for oral solids and liquids.

EXAMPLE

Use a 3-mL syringe. Round to the nearest tenth. Give 0.8 mL IM.

EXAMPLE

Give 0.5 mL IM.

EXAMPLE

Give 0.3 mL.

Heparin can also be administered intravenously—this will be covered in Chapter 7.

IV Medications

IV push (IVP) medications must be diluted and administered either according to directions in a nursing drug book or according to hospital policy. (Hospital policies often use a greater dilution of certain medications than suggested in drug books. It is safest to follow hospital policy). First, calculate the correct dose.

EXAMPLE

Give 0.48 mL IV.

Digoxin IVP is given either undiluted or diluted in 4 mL of sterile water and administered over 5 minutes.

EXAMPLE

Give 2 mL IV.

Lasix IVP is given undiluted at a rate of 20 mg over 1 to 2 minutes.

EXAMPLE

Special Types of Problems in Injections From a Liquid

When Supply Is a Ratio

Labels may state the strength of a drug as a ratio.

EXAMPLE

Figure 5-3 shows epinephrine that is labeled 30 mL and is a 1:1000 solution. 1:1000 means 1 g in 1000 mL. 1 g is equivalent to 1000 mg. Therefore, you can interpret the solution as 1000 mg = 1000 mL. If 1000 mL contains 1000 mg, then 1 mL contains 1 mg.

Since the ampule contains 30 mL, the ampule contains 30 mg of the drug. When reading and writing milligram (mg) and milliliter (mL), remember that milligram is the solid measure; milliliter is the liquid measure. 1000 1 mg 1000 mL mg 1mL =

EXAMPLE

FIGURE 5-3
Epinephrine 1:1000. (Courtesy of American Regent Laboratories, Inc.)

Give 1 mL subcutaneously.

EXAMPLE

Give 1 mL IM.

When Supply Is a Percent

Labels may state the strength of a drug as a percent. Percent means parts per hundred. Percentages are always interpreted in the metric system as grams per 100 mL.

EXAMPLE

To solve percent problems, state the percent as the number of grams per mL. (In the following examples, do not include the total number of mL on the label in the dosage calculation.)

EXAMPLE

Prepare 1.5 mL or 1 ½ mL.

EXAMPLE

Add 2 mL to IV

Injections From Powders

Some medications are prepared in a dry form, powder, or crystal. As liquids, they are unstable and lose potency over time. The drug must be reconstituted according to the manufacturer’s directions, which will give the type and amount of diluent to use. Most drugs are premixed, and reconstitution is rare. However, in some hospitals and other healthcare settings, the pharmacy is responsible for reconstituting medications. Sometimes, this task becomes the your responsibility as a nurse; that’s why this book includes these kinds of dosage calculations. Many drugs are reconstituted using a special reconstitution device within an IV bag. (See Chapter 6 for more information.)

To solve injection-from-powder problems, you use the same rule as for oral medications and for injection from a liquid. This is because once the powder is dissolved, the powdered drug takes liquid form.

FIGURE 5-4
Label of Monocid (cefonicid sodium). (Courtesy of GlaxoSmithKline Pharmaceuticals.)

EXAMPLE

Give 2 mL. Store the remaining solution in the refrigerator. Label the vial with the solution made (325 mg/mL), the date and time solution reconstituted, initial of the nurse who dissolve the powder and the expiration date.

Distinctive Features of Injections From Powders

Aseptic technique is used to prepare and administer the medication, which is given parenterally (usually IM, IV, or intravenous piggyback [IVPB]). The dry drug is supplied in vials of powder or crystals and may come in different strengths. Because powders deteriorate in solution, choose the strength closest to the amount ordered. The powder is usually diluted with one of the following:

Sterile water for injection

Bacteriostatic water for injection with a preservative added

Normal saline for injection (0.9% sodium chloride)

Directions will state which fluids may be used. Read this information carefully, because some fluids may be incompatible (i.e., unsuitable) as diluents. When the powder goes into solution, displacement occurs. This means that as the powder dissolves, it increases the volume added to the vial. There is no uniformity in the way powders go into solution.

Refer to the label in Figure 5-4 again. The manufacturer tells the nurse to add 2.5 mL of sterile water to provide an approximate volume of 3.1 mL. In this example, 0.6 mL is the displacement volume. Injections-from-powder problems are solved by using the solution made, not the displacement volume. The manufacturer will give the solution.

Where to Find Information About Reconstitution of Powders

Information about reconstitution of powders may be found from the following:

The label on the vial of powder

The package insert that comes with the vial of powder

Nursing drug handbooks

Other references such as the Physicians’ Desk Reference (PDR)

EXAMPLE

Explanation

1. Figure 5-5 gives Solutions for Initial Reconstitution: sterile water for injection, bacteriostatic water for injection, and 0.9% sodium chloride injection. Choose one.

2. The heading Preparation for Intramuscular Injection states that when a concentration of 250 mg or more is to be administered, a 2% lidocaine solution should be used together with sterile water for injection in a two-step dilution.

3. Two tables are given. The upper table has the two-step dilution; the lower one does not. Because the order requires two steps, use the directions in the top table.

4. Two strengths of powder are listed in the upper table. Look at the extreme left. They are for a 1-g vial and a 2-g vial. Our supply is a 1-g vial. Follow directions for 1 g.

5. The next heading is Final Cefoperazone Concentration. Two possibilities are given for the dilution: 333 mg/mL and 250 mg/mL. Because the order calls for 0.5 g, choose 250 mg/mL. Since 0.5 grams = 500 mg, you will need 2 mL of solution.

6. To make the solution of 250 mg/mL, add the following: 2.8 mL sterile water and 1.0 mL 2% lidocaine.

7. The last column on the right, headed Withdrawable Volume, lists 4 mL. Ignore this column; it does not affect the answer: When you add 2.8 mL and 1.0 mL, you expect to have 3.8 mL. However, the package insert states that you will end up with 4 mL. The manufacturer is giving the displacement.

8. You now have all of the information needed to prepare the dose ordered. Your solution is 250 mg/mL. Equivalent: 0.5 g = 500 mg. 500 mg 250 mg

Give 2 mL IM.

9. Write on the label the solution you made, the date, time, your initials, and expiration date.

10. Note the storage directions and stability expiration.

EXAMPLE

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