Injectable Medication Calculations
Introduction
This chapter offers a variety of practice problems involving subcutaneous and intramuscular dose calculations. Work out all the problems. This practice will improve your calculations in the clinical setting so that you can focus on injection technique. Selected injectable drugs are mixed in one syringe to spare the patient two injections. Technical competence in preparing parenteral medications requires supervised clinical practice.
Intradermal Injections
Small-volume injections usually administered as skin tests are injected intradermally just under the epidermis at a very shallow angle. A small fluid-filled wheal or bleb like a mosquito bite forms. The area is examined daily for a few days to see if there is an antibody reaction to the antigen injected. The observations are measured and documented.
Medications are not delivered via the ID route because this route offers poor absorption capability.
0.1 mL is the usual dose for a skin test administered with a 1-mL syringe and a 26- to 29-gauge needle (Figure 8-1).

Subcutaneous Injections
Nonirritating substances up to 1 mL may be injected into subcutaneous fatty tissue sites, usually with a 25- to 29-gauge needle. Insulin and anticoagulants, such as heparin and Fragmin, are medications that are delivered through the subcutaneous route. The fluid volume needle gauge, needle length (average ½ inch), and angle of injection depend on the patient’s size, skin thickness, and condition.
The most common sites for subcutaneous injection are the subcutaneous fatty areas of the upper posterior arm, the abdomen, and the anterior thigh (Figure 8-2).

A 2-inch zone around the umbilicus is to be avoided. As with all injections, the sites must be rotated systematically to avoid tissue injury.
It is best to write out subcutaneous or subcut on medical records to avoid misinterpretation. Subq and subc can be misinterpreted. Printed materials do use abbreviations such as subc or subQ but are less likely to be misinterpreted than is handwriting.
Intramuscular Injections
Intramuscular injection sites are selected to deliver medications for faster absorption and tolerate more concentrated substances than do subcutaneous sites. Solutions up to 3 mL may be injected with a 20- to 23-gauge needle into a single muscle site, depending on the patient’s skin integrity and muscle size. The volume of fluid and the length, gauge, and angle of needle are scaled down for smaller adults, children, and infants.
The most common sites for intramuscular injection are the deltoid, the ventrogluteal muscle, and the vastus lateralis (Figure 8-3).

Administering Injections
Mastery of the technique of delivering injections to the correct layer of tissue requires supervised laboratory and clinical practice with anatomical models and a variety of different-sized patients.
Medications for subcutaneous and intramuscular injections are administered with a variety of equipment, including vials, ampules, prefilled syringes, and syringe cartridges, as illustrated below. Ampules are single-dose glass containers containing liquid medications and solutions. Vials are supplied as singledose or multidose glass containers and may contain liquid or dry medication forms (Figure 8-4).

Prefilled syringes reduce the chance of contamination during preparation of the medication.
However, they do not entirely eliminate that risk because dose adjustments may be required with prefilled syringes. They may contain 0.1 to 0.2 mL of extra medication in case of loss during preparation.
There are many variations on the sizes and amounts shown in Table 8-1. These figures are just averages.
Note that the 3-mL syringe for intramuscular injections corresponds to the maximum 3-mL amount preferred for intramuscular injections in one site.
1. How many times does 0.05 go into 0.1? How many times does 0.25 go into 0.5? How many times does 0.5 go into 1? How many times does 0.025 go into 0.05?

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