An intramuscular injection is an injection given directly into the central area of a specific muscle. In this way, the blood vessels supplying that muscle distribute the injected medication via the cardiovascular system.
Intramuscular injection is used for the delivery of certain drugs not recommended for other routes of administration, for instance intravenous, oral, or subcutaneous. The intramuscular route offers a faster rate of absorption than the subcutaneous route, and muscle tissue can often hold a larger volume of fluid without discomfort. In contrast, medication injected into muscle tissues is absorbed less rapidly and takes effect more slowly that medication that is injected intravenously. This is favorable for some medications.
Careful consideration in deciding which injectable route is to be used for the prescribed medication is essential. The intramuscular route should not be used in cases where muscle size and condition is not adequate to support sufficient uptake of the drug. Intramuscular injection should be avoided if other routes of administration, especially oral, can be used to provide a comparable level of absorption and effect in any given individual's situation and condition. Intramuscular injections should not be given at a site where there is any indication of pain.
Intramuscular (IM) injections are given directly into the central area of selected muscles. There are a number of sites on the human body that are suitable for IM injections; however, there are three sites that are most commonly used in this procedure.
The deltoid muscle located laterally on the upper arm can be used for intramuscular injections. Originating from the Acromion process of the scapula and inserting approximately one-third of the way down the humerus, the deltoid muscle can be used readily for IM injections if there is sufficient muscle mass to justify use of this site. The deltoid's close proximity to the radial nerve and radial artery means that careful consideration and palpation of the muscle is required to find a safe site for penetration of the needle. There are various methods for defining the boundaries of this muscle.
The vastus lateralis muscle forms part of the quadriceps muscle group of the upper leg and can be found on the anteriolateral aspect of the thigh. This muscle is more commonly used as the site for IM injections as it is generally thick and well formed in individuals of all ages and is not located close to any major arteries or nerves. It is also readily accessed. The middle third of the muscle is used to define the injection site. This third can be determined by visually dividing the length of the muscle that originates on the greater trochanter of the femur and inserts on the upper border of the patella and tibial tuberosity through the patella ligament into thirds. Palpation of the muscle is required to determine if sufficient body and mass is present to undertake the procedure.
The gluteus medius muscle, which is also known as the ventrogluteal site, is the third commonly used site for IM injections. The correct area for injection can be determined in the following manner. Place the heel of the hand of the greater trochanter of the femur with fingers pointing towards the patient's head. The left hand is used for the right hip and vice versa. While keeping the palm of the hand over the greater trochanter and placing the index finger on the anterior superior iliac spine, stretch the middle finger dorsally palpating for the iliac crest and then press lightly below this point. The triangle formed by the iliac crest, the third finger and index finger forms the area suitable for intramuscular injection.
Determining which site is most appropriate will depend upon the patient's muscle density at each site, the type and nature of medication you wish to administer, and of course the patient's preferred site for injections.
Examples of medications that are sometimes administered intramuscularly are:
Sex hormones, such as Testosterone, Estradiol Valerate, and Depo Provera
Quinine, in its gluconate form
Vitamin B12, also known as cobalamin
In addition, some vaccines are administered intramuscularly:
Hepatitis A vaccine
Influenza vaccines based on inactivated viruses are commonly administered intramuscularly (although there is active research being conducted as to the best route of administration).
Platelet-rich plasma injections can be administered intramuscularly.
Certain substances (e.g. Ketamine) are injected intramuscularly for recreational purposes.
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