Clinical Calculations: Module 5: Parenteral Medications

Module 5 – Calculating Doses of Parenteral Medications

What’s in this module?

Parenteral medications are administered by injection, not through the gastrointestinal tract.  The medication may be injected by the following routes:

1. Intramuscular (into a muscle, abbreviated as IM)
2. Subcutaneous (below the dermis and epidermis, abbreviated as subcut or SC)
3. Intradermal (a shallow injection under the dermis, abbreviated as ID)
4. Intravenous (into a vein, abbreviated as IV)
5. Intrathecal (into the cerebrospinal fluid)
6. Intracardiac (into the heart muscles or ventricles)
7. Intra-articular (into a joint)

We will focus primarily on IM, SC, and IV injections in this module.  You will learn more about the other parenteral routes in your more advanced nursing courses.

In this module, you will learn how to read information on manufacturer’s medication labels for parenteral medications.  You will learn to calculate the correct amount of injectable medications to give a client.  

Summary of problem types in this module

Problems will involve reading medication labels and healthcare providers' orders.  You will calculate the correct amount of injectable medications to give a client.  Assume all questions ask for the amount per dose unless instructed otherwise.

Equivalents to know

You should now know all your commonly used equivalents.

Rounding rules to know

You will continue to use the rounding rules for numbers >1 and <1.

There are no new rounding rules for this module, but if there are specific instructions in a problem please follow those instructions.

Starting factors and answer units

The SF in most of the problems is the amount of medication the healthcare provider has ordered.  The dose is usually stated in mg, mcg, or g.  Insulin and heparin doses will be stated in units.

Weight-based medications will use the client’s weight as the SF.  This will be illustrated for you later in this module.

Types of Injections

slideplayer.com/slide/2422396/8/images/2/Types+of+Injections+Intramuscular+Subcutaneous+Intradermal.jpg   Retrieved 1/5/19

This illustration shows the administration of injections by the IM, SC, and ID routes.  The blue area is a cross-section of skin with the layers of skin and the muscle labeled.  The angles of administration are also shown.  Needles and syringes are very small for the ID route, a bit larger for the SC route, and still larger for the IM route.

The ID route is most often used for TB tests.  SC injections include insulin and heparin.  IM and IV injections are used for other medications.

Syringes and Needles

https://is.alicdn.com/img/pb/628/425/367/367425628_793.JPG   Retrieved 1/5/19

Syringes vary considerably in size.  The nurse will select the correct syringe depending on the amount of fluid to be injected.  The smallest syringe shown measures up to 2 ml.  The largest syringe measures up to 20 ml, and would not be used for IM, SC, and ID injections.  Syringes used for TB tests, insulin, and heparin are even smaller than the smallest syringe shown.

Needle sizes are smallest for ID injections, a bit larger for SC injections, and larger for IM injections. 

https://slideplayer.com/slide/10867983/39/images/11/Syringes+The+larger+the+gauge+number%2C+the+smaller+the+diameter+of+the+lumen.+The+smaller+the+gauge+number%2C+the+larger+the+diameter+of+the+lumen..jpg   Retrieved 1/5/19

The sizes of syringes and needles are included here just so you will have a better understanding of giving parenteral medications.  In this module, the focus will be on calculating the amount of medication to give.  You will learn more about giving parenteral medications later in your nursing program.

IV injections may not use a needle at all.  IV access is established first with tubing and a port for syringe access.  There will be a further discussion of IV access in a later module.

slideplayer.com/slide/12221989/72/images/8/Reading+a+Syringe+1.+2.+3.+Always+use+the+plunger+ring+closest+to+the+tip+to+read+the+volume+contained+in+a+syringe..jpg   Retrieved 1/5/19

Vials and Ampules

s3.amazonaws.com/static.hivequal.org/files/posts/747/facebook_images/large.jpg?1453904827  Retrieved 1/5/19

Parenteral medications are packaged in vials and ampules.  Vials are glass containers with a rubber stopper in the top of the container.  The vial may contain more than one dose of medication and any excess is saved for later use.  The medication is accessed by pushing a needle attached to a syringe through the rubber stopper. 

www.groundreport.com/wp-content/uploads/archived/01359189955_ARTICLE_IMAGE_syringeandvialjpg.jpg  Retrieved 1/5/19

Ampules are glass containers with a top designed to be snapped or broken off.  The correct dose of medication is removed from the ampule and any excess is discarded.  Special filtered needles are used to withdraw the medication to prevent glass from being withdrawn into the medication.  A different needle (not filtered) is used to give the medication. 

https://skillsmodules.atitesting.com/SkillsModulesContent/v2/medication-administration-3/images/Meds3_ampule4.jpg 

Retrieved 1/5/19

Problem Type 1 – Reading Medication Labels

https://www.drugs.com/pro/images/c56598d5-9cd1-4cce-b172-ac338775aec7/bentyl-ampule.jpg    Retrieved 1/5/19

If the medication is generic, it will have no trade name.  The generic name is always required.  The generic name may be in lower-case type, but sometimes may be printed in upper-case type.

A trade name is always marked with the symbol  for trademark.  This symbol may be small and difficult to read, but it always follows the trade name.  The trade name usually begins with a capital letter, but this feature depends on the manufacturer.

 
The medication label shown above contains the following information:

1. Trade name of the medication:  Bentyl (note trademark symbol, capitalized name)
2. Generic name of the medication:  dicyclomine HCl (in lower case letters here but may sometimes be capitalized)
3. Route:  IM only
4. Strength:  20 mg per 2 ml (or 10 mg per ml)
5. Total in the container:  2 ml
6. Manufacturer:  Aptalis Pharma US, Inc.
7. Lot number: not shown in this illustration
8. Expiration date: not shown in this illustration

Make certain that you can identify all this information from a label.  Note that the strength of the medication must be stated per ml.  On an actual label, the lot number and expiration date would be shown.

Here’s another sample label:

drugline.org/img/drug/ondansetron-injection-17054_3.jpg   Retrieved 1/5/19

The medication label shown above contains the following information:

1. Trade name of the medication:  none
2. Generic name of the medication:  ondansetron
3. Route:  IV or IM
4. Strength:  40 mg per 20 ml (or 2 mg per ml)
5. Total in the container:  20 ml
6. Manufacturer:  Novaplus
7. Lot number: not shown in this illustration
8. Expiration date: not shown in this illustration
9. Storage instructions
10. Details about the formulation of the medication

Make certain that you can identify all this information from a label. 

Problem Type 2 – Injections

Example 1:

https://d2ch1jyy91788s.cloudfront.net/buyemp/images/product/Hospira-Furosemide-Injection-USP-33773522-1200_1200.png  Retrieved 1/5/19

A healthcare provider has prescribed furosemide 30 mg IV every 2 hours for your client.  How many ml will you give?

Here’s the problem set up in the dimensional analysis format:

SF = 30 mg

AU = ml

Equivalents:

10 mg = 1 ml

Equation for the dose in ml:

You will need more than one vial to give this dose.  This vial only contains 2 ml.

Example 2:

www.clintpharmaceuticals.com/mm5/graphics/00000001/Chronic%20Pain%20Treatment%20Agents%20-01-0379601%20-%20Ketorolac%2060mg%20-%202ml%20vial%20-%20Box-25_03.jpg      Retrieved 1/5/19

A healthcare provider has prescribed ketorolac tromethamine 50 mg IM now for your client.  How many ml will you give?

SF = 50 mg

AU = ml

Equivalents:

60 mg = 2 ml

Equation for the dose in ml:

Problem Type 3 – Medication with Doses in Units

Doses in units are always stated in whole numbers.  When the dose is converted to ml, use the rounding rules for numbers <1 and >1.

https://images.mooremedical.com/450x450/87992.jpg   Retrieved 1/5/19

Heparin is an anticoagulant used to prevent blood clots after surgery, for an immobile client, or for a cardiac client.  Doses are stated in units.

Example 1:

A healthcare provider has prescribed 6,000 units of heparin SC every 12 hours for your client.  Please refer to the vial above.  How many ml will you give?

SF = 6,000 units

AU = ml

Equivalents:

5,000 units = 1 ml

Equation for the dose in ml:  

Example 2:

Insulin is used to lower blood glucose in diabetic clients.  The standard strength of insulin is U 100, meaning 100 units per ml. Always check your label because there are other strengths of insulin.  Special syringes are used for insulin with the dose marked in units.  The syringes in the illustration are a typical size for insulin syringes and hold only 1 ml (100 units).  The measurements on the side of the syringes are units.  Insulin injections are given SC.

c-hit.org/wp-content/uploads/2016/04/Insulin-2-1170x780.jpg   Retrieved 1/5/19

A healthcare provider has prescribed 85 units of Humalog insulin lispro SC every morning.  How many ml of insulin would equal the dose in units?

SF = 85 units

AU = ml

Equivalents:

100 units = 1 ml

Equation for the dose in ml:

Most insulin injections are less than 1 ml (100 units).  Doses more than 100 units should be verified with the healthcare provider before the first dose is given. 

A second nurse always checks the syringe and verifies the healthcare provider’s prescription before any dose of insulin is given.

Problem Type 4 –Doses Based on Body Weight

New SF: the client’s weight.  Most weight based medication prescriptions are stated in kilograms, so if your client’s weight is in pounds a conversion factor must be included in your equation.

Example 1: Let’s use the Humalog insulin lispro from the example above  

The healthcare provider has set a total daily insulin requirement at 0.75 units per kg per day SC.  What is the total daily requirement in units if your client weighs 215 pounds?  (Round your answer to a whole number.)

Here’s the problem set up in the dimensional analysis format:

SF = 215 lb

AU = units

Equivalents:

0.75 units = 1 kg

1 kg = 2.2 lb

Equation for the dose in units:

Example 2:

Your client has a bacterial infection. The healthcare provider has prescribed gentamicin 1.5 mg per kg IV every 8 hours.  Your client weighs 81.8 kg. You have on hand gentamicin solution 40 mg/ml.  How many ml will you give?

SF = 81.8 kg

AU = ml

Equivalents:

1.5 mg = 1 kg

40 mg = 1 ml

Equation for the dose in ml:

Note that you have the client’s weight in kg, so you do not need to do a weight conversion in this problem.

Example 3:

Your client, who weighs 175 lb, has pneumonia.  The healthcare provider has prescribed amikacin 20mg/kg IV every 24 hours for 7 days.  You have on hand amikacin 250mg/ml.  How many ml will you give?

SF = 175 lb

AU = ml

Equivalents:

20 mg = 1 kg

1 kg = 2.2 lb

250 mg = 1 ml

Equation for the dose in ml:

Note that even a complicated problem like this one can easily be set up with dimensional analysis.  Just make certain that all your units of measurement except the AU cancel out in the equation. The other units of measurement must appear in both the numerator of a fraction and the denominator of a fraction in the equation to cancel out.

Problem Type 5 – Prefilled Syringes

www.innovationessence.com/wp-content/uploads/2016/11/syringes2.jpg   Retrieved 1/5/19

Some medications or solutions come in a prefilled syringe like those pictured above.  In this case you need to calculate both the dose you will give and the amount you need to waste (or discharge) from the syringe before delivering the dose to the client.

Example 1:

https://4.imimg.com/data4/KL/QW/MY-10335408/etanercept-injection-250x250.jpg   Retrieved 2//19

Your client needs 35 mg of Enbrel (etanercept) SC for arthritis.  You have on hand a prefilled syringe containing 1 ml with a strength of 50 mg per ml.  How much must you discard from the prefilled syringe to give the client the correct dose?

SF = 35 mg

AU = ml

Equivalents:

50 mg = 1 ml

Equation for the dose in ml:

Next you need to determine the amount of medication to discard. To do so, subtract the amount to give to the client from the 1 ml total in the syringe.

Make certain that you notice the requirements of the problem.  Does the problem ask for the amount to give or the amount to discard?