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 5-1A 3-mL syringe.
FIGURE 5-2A 1-mL syringe.
Calculating Liquid Injections
To solve liquid injection problems, use the same rule as for oral solids and liquids.
EXAMPLE
Order: Stelazine (trifluoperazine) 1.5 mg IM q6h prn
Supply: Read the label.
(Courtesy of GlaxoSmithKline.)
Desire: the amount ordered, here, 1.5 mg
Have: strength of the drug supplied, here, 2 mg
Supply: the unit form of the drug, here, 1 mL (For liquid calculations, the supply is usually 1 mL although there are exceptions).
Amount how much liquid to give by injection in milliliters; “x” is used to denote the unknown or answer.
Use a 3-mL syringe. Round to the nearest tenth. Give 0.8 mL IM.
EXAMPLE
Order: morphine 4 mg IM q4h prn
Supply: Read the label.
(Courtesy of Abbott Laboratories.)
Desired Dose: 4 mg
Supply: 1 mL
Have: 8 mg in 1 mL
Give 0.5 mL IM.
EXAMPLE
Order: heparin sodium 1500 units subcutaneous bid
Supply: Read the label.
Desired dose: 1500 units
Supply: 1 mL
Have: 5000 units in 1 mL
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
Order: Lanoxin (digoxin) 120 mcg IV every day. Use a 1-mL syringe.
Supply: Read the label. Use 0.25 mg per mL and convert.
(Used by permission Hospira, Inc.)
Conversion: 0.25 mg = 250 mcg
Desired dose: 120 mcg
Supply: 1 mL
Have: 250 mcg in 1 mL
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
Order: Lasix (furosemide) 20 mg IV q12h
Supply: Read the label.
(Used by permission of Hospira, Inc.)
No equivalent is needed.
Desired dose: 20 mg
Supply: 1 mL
Have: 10 mg in 1 mL
Give 2 mL IV.
Lasix IVP is given undiluted at a rate of 20 mg over 1 to 2 minutes.
EXAMPLE
Order: A-Methapred (methylprednisolone) 100 mg IV daily.
Supply: Read the label.
(This vial is mixed right before administration. Press on the top cap, which pushes the rubber stopper into the vial, mixing the diluent with the powder. Shake gently to mix the two.)
No equivalent needed.
Desired dose: 100 mg
Supply: 2 mL
Have: 125 mg in 2 mL
SELF-TEST 1
Calculation of Liquids for Injection
Practice calculations of injections from a liquid. Report your answer in milliliters; mark the syringe in milliliters. Answers appear at the end of the chapter.
|
1. Order: |
Cleocin (clindamycin) 0.3 g IM q6h |
|
Supply: |
liquid in a vial labeled 300 mg/2 mL |
|
2. Order: |
morphine 12 mg IV stat |
|
Supply: |
vial of liquid labeled 15 mg/mL |
|
3. Order: |
vitamin B12 1 mg IM every day |
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Supply: |
vial of liquid labeled 1000 mcg/mL |
|
gentamicin 9 mg IM q8h | |
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Supply: |
pediatric ampule labeled 20 mg/2 mL |
|
5. Order: |
Lanoxin (digoxin) 0.5 mg IV q6h × 3 doses |
|
Supply: |
vial labeled 0.25 mg/mL |
|
6. Order: |
gentamicin 50 mg IM q8h |
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Supply: |
vial labeled 40 mg/mL |
|
7. Order: |
phenobarbital 100 mg IM stat |
|
Supply: |
ampule labeled 130 mg/mL |
|
8. Order: |
Lanoxin (digoxin) 0.25 mg IV stat |
|
Supply: |
ampule labeled 0.5 mg/2 mL |
|
9. Order: |
heparin 6000 units subcutaneous q4h |
|
Supply: |
vial labeled 10,000 units/mL |
|
Brethine (terbutaline) 0.25 mg subcutaneous | |
|
Supply: |
ampule labeled 1 mg/1 mL |
|
11. Order: |
Normodyne (labetalol) 20 mg IV stat |
|
Supply: |
vial labeled 5 mg/mL |
|
12. Order: |
Haldol (haloperidol) 2.5 mg IM q 4–8 hr |
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Supply: |
vial labeled 5 mg/mL |
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13. Order: |
methadone 3 mg subcutaneous now |
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Supply: |
vial labeled 10 mg per mL |
|
14. Order: |
Endep (amitriptyline) 0.025 g IM tid |
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Supply: |
vial labeled 10 mg/mL |
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15. Order: |
Thorazine (chlorpromazine) 50 mg IM now |
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Supply: |
vial labeled 25 mg/mL |
|
16. Order: |
Fragmin (dalteparin) 2500 units subcutaneous every day |
|
Supply: |
syringe labeled 5000 units in 0.2 mL |
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
Adrenalin 1:1000
Ratios are always interpreted in the metric system as grams per milliliters. In the example given, 1:1000 means 1 g in 1000 mL. Ratios may be stated in three ways:
1 g per 1000 mL
1 g = 1000 mL
1 g/1000 mL
SELF-TEST
2 Ratios
Write the following ratios in three ways. Answers appear at the end of the chapter.
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 =
Order: epinephrine 1 mg subcutaneous stat
Supply: ampule labeled 1:1000
Equivalent: 1:1000 means
1 g in 1000 mL
1 g = 1000 mg
Therefore, 1000 mL contains 1000 mg and 1 mL contains 1 mg.
Desired dose: 1 mg
Supply: 1000 mL
Have: 1000 mg in 1000 mL
FIGURE 5-3Epinephrine 1:1000. (Courtesy of American Regent Laboratories, Inc.)
Give 1 mL subcutaneously.
Order: Isuprel (isoproterenol) HCl 0.2 mg IM stat
Supply: ampule labeled 1:5000
Equivalent: 1:5000 means
1 g in 5000 mL
1 g = 1000 mg
Therefore, 5000 mL contains 1000 mg.
Desired dose: 0.2 mg
Supply: 5000 mL
Have: 1000 mg in 5000 mL
Give 1 mL IM.
Using Ratios With Liquids for Injection
Solve these problems involving ratios. Answers appear at the end of the chapter.
|
1. Order: |
neostigmine 0.5 mg subcutaneous |
|
Supply: |
ampule labeled 1:2000 |
|
2. Order: |
Isuprel (isoproterenol) 1 mg: add to IV |
|
Supply: |
vial labeled 1:5000 |
A 12-mL syringe is available.
|
3. Order: |
neostigmine methylsulfate 0.75 mg subcutaneous |
|
Supply: |
ampule labeled 1:1000 |
|
4. Order: |
epinephrine 0.5 mg subcutaneous stat |
|
Supply: |
ampule labeled 1:1000 |
|
5. Order: |
neostigmine methylsulfate 1.5 mg IM tid |
|
Supply: |
ampule labeled 1:2000 |
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
Lidocaine 2% = 2 g in 100 mL
Percents may be stated in three ways:
2 g per 100 mL
2 g = 100 mL
2 g/100 mL
SELF-TEST 4
Percentages
Write the following percentage in three ways. Answers appear at the end of the chapter.
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.)
Order: lidocaine 30 mg for injection before suturing wound
Supply: Read the label.
(Courtesy of American Regent Laboratories, Inc.)
Equivalent: 2% means
2 g in 100 mL
1 g = 1000 mg
2 g = 2000 mg
Therefore, 100 mL contains 2000 mg.
Desired dose: 30 mg
Supply: 100 mL
Have: 2000 mg in 100 mL
Prepare 1.5 mL or 1 ½ mL.
Order: magnesium sulfate 1 g; add to IV stat
Supply: Read the label.
(Courtesy of American Regent Laboratories, Inc.)
Equivalent: 50% means 50 g in 100 mL
Desired dose: 1 g
Supply: 100 mL
Have: 50 g in 100 mL
Add 2 mL to IV
Using Percentages With Liquids for Injection
Solve these problems involving percentages. Answers appear at the end of the chapter. Answers are in milliliters (mL).
|
1. Order: |
epinephrine 5 mg subcutaneous stat |
|
Supply: |
ampule labeled 1% |
|
2. Order: |
lidocaine 15 mg subcutaneous |
|
Supply: |
ampule labeled 1% |
|
3. Order: |
Neo-Synephrine (phenylephrine HCl) 3 mg subcutaneous stat |
|
Supply: |
ampule labeled 1% |
|
4. Order: |
Prepare for IV use calcium gluconate 0.3 g |
|
Supply: |
ampule labeled 10% |
|
5. Order: |
dextrose 5 g IV × 1 |
|
Supply: |
syringe labeled 50% |
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 5-4Label of Monocid (cefonicid sodium). (Courtesy of GlaxoSmithKline Pharmaceuticals.)
EXAMPLE
Order: Monocid (cefonicid sodium) 0.65 g IM every day (Fig. 5-4).
Label directions: Add 2.5 mL sterile water for injection. Shake well. Provides an approximate volume of 3.1 mL (325 mg/mL). Stable 24 hours at room temperature or 72 hours if refrigerated (5°C).
Equivalent: 0.65 g = 650 mg
Desire: The order in the example is 0.65 g.
Supply: The fluid portion of the solution made. In this example, it is 1 mL = 325 mg.
Have: The strength of the drug supplied. The example is 1 g as a dry powder; when reconstituted, it is 325 mg/mL. Remember that the manufacturer gives the strength of the solution; you do not have to determine it.
Answer: How much liquid to give, stated as mL.
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)
1.
2.
3.
4.
5.
6.
7.
STEPS FOR RECONSTITUTING POWDERS WITH DIRECTIONS
Read the order.
Identify the supply and directions for dilution.
Dilute the fluid.
Identify the solution and new supply.
Calculate the amount to give and prepare the amount.
Write on the label the solution made, date, time, your initials, and expiration date.
Store according to directions.
EXAMPLE
The package insert information concerning the dilution of Cefobid (cefoperazone) injection is reproduced in Figure 5-5. Examine the directions with the intention of solving the following problem, then read the explanation:
Order: Cefobid (cefoperazone) 0.5 g IM q12h
Supply: 1-g vial of powder
Search the directions for three pieces of information to dissolve your supply, which is 1 g:
1. Type of fluid needed to dissolve the powder
2. Amount of fluid to add
3. Solution made
FIGURE 5-5Reconstitution directions for Cefobid (cefoperazone). (Courtesy of Pfizer Laboratories.)
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.
Order: Ancef (cefazolin) 0.3 g IM (Fig. 5-6)
Supply: 500 mg powder
Diluting fluid: 2.0 mL sterile water for injection
Solution and new supply: 225 mg/mL
Equivalent: 0.3 g = 300 mg
Desired dose: 300 mg
Supply: 1 mL
Have: 225 mg in 1 mL
FIGURE 5-6Reconstitution directions for Ancef (cefazolin). (Courtesy of GlaxoSmithKline.)
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