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Intramuscular injection
Intramuscular injection
Intramuscular injection
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Intramuscular injection
A healthcare worker preparing to administer a vaccine via intramuscular injection
ICD-10-PCS3E023
ICD-9-CM99.1
MeSHD007273
CPT96372

Intramuscular injection, often abbreviated IM, is the injection of a substance into a muscle. In medicine, it is one of several methods for parenteral administration of medications. Intramuscular injection may be preferred because muscles have larger and more numerous blood vessels than subcutaneous tissue, leading to faster absorption than subcutaneous or intradermal injections. Medication administered via intramuscular injection is not subject to the first-pass metabolism effect which affects oral medications.

Common sites for intramuscular injections include the deltoid muscle of the upper arm and the gluteal muscle of the buttock. In infants, the vastus lateralis muscle of the thigh is commonly used. The injection site must be cleaned before administering the injection, and the injection is then administered in a fast, darting motion to decrease the discomfort to the individual. The volume to be injected in the muscle is usually limited to 2–5 milliliters, depending on injection site. A site with signs of infection or muscle atrophy should not be chosen. Intramuscular injections should not be used in people with myopathies or those with trouble clotting.

Intramuscular injections commonly result in pain, redness, and swelling or inflammation around the injection site. These side effects are generally mild and last no more than a few days at most. Rarely, nerves or blood vessels around the injection site can be damaged, resulting in severe pain or paralysis. If proper technique is not followed, intramuscular injections can result in localized infections such as abscesses and gangrene. While historically aspiration, or pulling back on the syringe before injection, was recommended to prevent inadvertent administration into a vein, it is no longer recommended for most injection sites by some countries.

Uses

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Intramuscular injection is commonly used for medication administration. Medication administered in the muscle is generally quickly absorbed in the bloodstream, and avoids the first pass metabolism which occurs with oral administration.[1] The medication may not be considered 100% bioavailable as it must still be absorbed from the muscle, which occurs over time.[2]: 102–103  An intramuscular injection is less invasive than an intravenous injection and also generally takes less time, as the site of injection (a muscle versus a vein) is much larger. Medications administered in the muscle may also be administered as depot injections, which provide slow, continuous release of medicine over a longer period of time.[3] Certain substances, including ketamine, may be injected intramuscularly for recreational purposes.[4] Disadvantages of intramuscular administration include skill and technique required, pain from injection, anxiety or fear (especially in children), and difficulty in self-administration which limits its use in outpatient medicine.[5]

Vaccines, especially inactivated vaccines, are commonly administered via intramuscular injection.[6] However, it has been estimated that for every vaccine injected intramuscularly, 20 injections are given to administer drugs or other therapy.[6] This can include medications such as antibiotics, immunoglobulin, and hormones such as testosterone and medroxyprogesterone.[5] In a case of severe allergic reaction, or anaphylaxis, a person may use an epinephrine autoinjector to self-administer epinephrine into the muscle.[7]

Contraindications

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Because an intramuscular injection can be used to administer many types of medications, specific contraindications depend in large part on the medication being administered.[8] Injections of medications are necessarily more invasive than other forms of administration such as by mouth or topical and require training to perform appropriately, without which complications can arise regardless of the medication being administered. For this reason, unless there are desired differences in rate of absorption, time to onset, or other pharmacokinetic parameters in the specific situation, a less invasive form of drug administration (usually by mouth) is preferred.[8]

Intramuscular injections are generally avoided in people with low platelet count or clotting problems, to prevent harm due to potential damage to blood vessels during the injection. They are also not recommended in people who are in hypovolemic shock, or have myopathy or muscle atrophy, as these conditions may alter the absorption of the medication.[5] The damage to the muscle caused by an intramuscular injections may interfere with the accuracy of certain cardiac tests for people with suspected myocardial infarction and for this reason other methods of administration are preferred in such instances.[5] In people with an active myocardial infarction, the decrease in circulation may result in slower absorption from an IM injection.[9]: 368–369  Specific sites of administration may also be contraindicated if the desired injection site has an infection, swelling, or inflammation.[9]: 368–369  Within a specific site of administration, the injection should not be given directly over irritation or redness, birthmarks or moles, or areas with scar tissue.[9]: 368–369 

Risks and complications

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Using safety needles that prevent more than one use and accidental injections ensures sterility and prevents needlestick injuries.

As an injection necessitates piercing the skin, there is a risk of infection from bacteria or other organisms present in the environment or on the skin before the injection. This risk is minimized by using proper aseptic technique in preparing the injection and sanitizing the injection site before administration.[9]: 369  Intramuscular injections may also cause an abscess or gangrene at the injection site, depending on the specific medication and amount administered. There is also a risk of nerve or vascular injury if a nerve or blood vessel is inadvertently hit during injection. If single-use or sterilized equipment is not used, there is the risk of transmission of infectious disease between users, or to a practitioner who inadvertently injures themselves with a used needle, termed a needlestick injury.[5][9]: 372 

Site-specific complications

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Injections into the deltoid site in the arm can result in unintentional damage to the radial and axillary nerves. In rare cases when not performed properly, the injection may result in shoulder dysfunction.[10] The most frequent complications of a deltoid injection include pain, redness, and inflammation around the injection site, which are almost always mild and last only a few days at most.[11]

The dorsogluteal site of injection is associated with a higher risk of skin and tissue trauma, muscle fibrosis or contracture, hematoma, nerve palsy, paralysis, and infections such as abscesses and gangrene.[12] Furthermore, injection in the gluteal muscle poses a risk for damage to the sciatic nerve, which may cause shooting pain or a sensation of burning. Sciatic nerve damage can also affect a person's ability to move their foot on the affected side, and other parts of the body controlled by the nerve. Damage to the sciatic nerve can be prevented by using the ventrogluteal site instead, and by selecting an appropriate size and length of needle for the injection.[13]

Technique

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An intramuscular injection is performed perpendicular to the skin, close to a 90 degree angle.
Comparison of angle of intramuscular injection with other injection types

An intramuscular injection can be administered in multiple different muscles of the body. Common sites for intramuscular injection include: deltoid, dorsogluteal, rectus femoris, vastus lateralis and ventrogluteal muscles.[12][14] Sites that are bruised, tender, red, swollen, inflamed or scarred are generally avoided.[15] The specific medication and amount being administered will influence the decision of the specific muscle chosen for injection.

The injection site is first cleaned using an antimicrobial and allowed to dry. The injection is performed in a quick, darting motion perpendicular to the skin, at an angle between 72 and 90 degrees. The practitioner will stabilize the needle with one hand while using their other hand to depress the plunger to slowly inject the medication – a rapid injection causes more discomfort. The needle is withdrawn at the same angle inserted. Gentle pressure may be applied with gauze if bleeding occurs.[16] Pressure or gentle massage of the muscle following injection may reduce the risk of pain.[17]

Aspiration

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Aspirating for blood to rule out injecting into a blood vessel is not recommended by the US CDC, Public Health Agency of Canada, or Norway Institute of Public Health, as the injection sites do not contain large blood vessels and aspiration results in greater pain.[18][19][20] There is no evidence that aspiration is useful to increase safety of intramuscular injections when injecting in a site other than the dorsogluteal site.[6]

Aspiration was recommended by the Danish Health Authority for COVID-19 vaccines for a time to investigate the potential rare risk of blood clotting and bleeding, but it is no longer a recommendation.[21][22]

Z-track method

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The Z-track method is a method of administering an IM injection that prevents the medication being tracked through the subcutaneous tissue, sealing the medication in the muscle, and minimizing irritation from the medication. Using the Z-track technique, the skin is pulled laterally, away from the injection site, before the injection; then the medication is injected, the needle is withdrawn, and the skin is released. This method can be used if the overlying tissue can be displaced.[23]

Injection sites

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The deltoid muscle in the outer portion of the upper arm is used for injections of small volume, usually equal to or less than 1 mL. This includes most intramuscular vaccinations.[12] It is not recommended to use the deltoid for repeated injections due to its small area, which makes it difficult to space out injections from each other.[12] The deltoid site is located by locating the lower edge of the acromion process, and injecting in the area which forms an upside down triangle with its base at the acromion process and its midpoint in line with the armpit.[15] An injection into the deltoid muscle is commonly administered using a 1-inch long needle, but may use a 5⁄8-inch long needle for younger people or very frail elderly people.[11]

The ventrogluteal site on the hip is used for injections which require a larger volume to be administered, greater than 1 mL, and for medications which are known to be irritating, viscous, or oily. It is also used to administer narcotic medications, antibiotics, sedatives and anti-emetics.[12] The ventrogluteal site is located in a triangle formed by the anterior superior iliac spine and the iliac crest, and may be located using a hand as a guide.[15] The ventrogluteal site is less painful for injection than other sites such as the deltoid site.[17]

The vastus lateralis site is used for infants less than 7 months old and people who are unable to walk or who have loss of muscular tone.[12] The site is located by dividing the front thigh into thirds vertically and horizontally to form nine squares; the injection is administered in the outer middle square.[15] This site is also the usual site of administration for epinephrine autoinjectors, which are used in the outer thigh, corresponding to the location of the vastus lateralis muscle.[24]

The dorsogluteal site of the buttock site is not routinely used due to its location near major blood vessels and nerves, as well as having inconsistent depth of adipose tissue.[25] Many injections in this site do not penetrate deep enough under the skin to be correctly administered in the muscle.[12][26] While current evidence-based practice recommends against using this site, many healthcare providers still use this site, often due to a lack of knowledge about alternative sites for injection.[27] This site is located by dividing the buttock into four using a cross shape, and administering the injection in the upper outer quadrant. This is the only intramuscular injection site for which aspiration is recommended of the syringe before injection, due to higher likelihood of accidental intravenous administration in this area.[12] However, aspiration is not recommended by the Centers for Disease Control and Prevention, which considers it outdated for any intramuscular injection.[16]

Sites for administration of intramuscular injections
Deltoid site
Diagram showing the deltoid site for intramuscular injection
Ventrogluteal site
Ventrogluteal and rectus femoris sites for intramuscular injection
Vastus lateralis site in adult
Vastus lateralis site of administration in an adult
Vastus lateralis site in child
Vastus lateralis site of administration in an infant or young child

Special populations

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Some populations require a different injection site, needle length, or technique. In very young or weak elderly patients, a normal-length needle may be too long to inject properly. In these patients, a shorter needle is indicated to avoid injecting too deeply.[28] It is also recommended to consider using the anterolateral thigh as an injection site in infants under one year old.[28]

To help infants and children cooperate with injection administration, the Advisory Committee on Immunization Practices in the United States recommends using distractions, giving something sweet, and rocking the baby side to side. In people who are overweight, a 1.5-inch needle may be used to ensure the injection is given below the subcutaneous layer of skin, while a 5⁄8-inch needle may be used for people who weigh under 60 kilograms (130 lb). In any case, the skin does not need to be pinched up before injecting when the appropriate length needle is used.[29]

History

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Injections into muscular tissue may have taken place as early as the year 500 AD. Beginning in the late 1800s, the procedure began to be described in more detail and techniques began to be developed by physicians. In the early days of intramuscular injections, the procedure was performed almost exclusively by physicians.[8] After the introduction of antibiotics in the middle of the 20th century, nurses began preparing equipment for intramuscular injections as part of their delegated duties from physicians, and by 1961 they had "essentially taken over the procedure".[8] Until this delegation became virtually universal, there were no uniform procedures or education for nurses in proper administration of intramuscular injections, and complications from improper injection were common.[8]

Intramuscular injections began to be used for administration of vaccines for diphtheria in 1923, whooping cough in 1926, and tetanus in 1927.[30] By the 1970s, researchers and instructors began forming guidance on injection site and technique to reduce the risk of injection complications and side effects such as pain.[8] Also in the early 1970s, botulinum toxin began to be injected into muscles to intentionally paralyze them for therapeutic reasons, and later for cosmetic reasons.[31] Until the 2000s, aspiration after inserting the needle was recommended as a safety measure, to ensure the injection was being administered in a muscle and not inadvertently in a vein. However, this is no longer recommended as evidence shows no safety benefit and it lengthens the time taken for injection, which causes more pain.[29]

Veterinary medicine

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In animals common sites for intramuscular injection include the quadriceps, the lumbodorsal muscles, and the triceps muscle.[32]

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Intramuscular injection

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from Grokipedia
An intramuscular injection (IM) is a parenteral technique for delivering into the depth of a specifically selected muscle, where the rich promotes rapid absorption into the bloodstream. This method is widely used in clinical settings for administering vaccines, antibiotics, hormones, and other therapeutics that require efficient delivery or avoidance of first-pass metabolism in the liver. The choice of injection site depends on factors such as the patient's age, body mass, medication volume (typically up to 5 mL in adults), and the need to avoid major nerves or blood vessels. Common sites include the in the upper arm (suitable for volumes up to 2 mL and often used for in adolescents and adults), the vastus lateralis in the anterolateral thigh (preferred for infants and self-administration), and the ventrogluteal region in the (considered the safest for larger volumes due to minimal neurovascular structures). The dorsogluteal site in the is less favored owing to the risk of injury. While effective, IM injections carry risks including localized pain, hematoma, infection, abscess formation, or rare nerve palsies if landmarks are misidentified. Proper training and anatomical knowledge are essential to ensure safety and efficacy, particularly in pediatric or obese patients where needle length adjustments may be needed.

Fundamentals

Definition and Mechanism

An intramuscular injection is a technique for delivering into the depth of specifically selected skeletal muscles using a , penetrating beyond the subcutaneous fat and dermal layers to reach the muscular tissue directly. This method leverages the anatomical structure of muscles, which consist of layered fibers surrounded by , allowing the needle to deposit the drug in a region with minimal interference from overlying . The primary mechanism of intramuscular injection involves the absorption of the injected substance through the extensive capillary network and rich of , enabling rapid into the bloodstream for systemic distribution. possess a high density of blood vessels compared to , which promotes efficient uptake of water-soluble drugs via passive across concentration gradients maintained by local blood flow. This vascular architecture results in faster and more uniform absorption than routes with lower , such as . Intramuscular injections are particularly suited for medications requiring quick onset or controlled release, including vaccines, antibiotics, hormones, and analgesics, as the muscle's blood supply supports both immediate systemic effects and potential depot formation for sustained delivery.

Pharmacokinetics and Rationale

Intramuscular injections facilitate absorption primarily through the rich vascular network of , where the injected diffuses into surrounding capillaries for rapid uptake into the systemic circulation. This process typically involves an initial phase of quick absorption for aqueous solutions, often achieving significant plasma levels within minutes to hours, followed by a distribution phase influenced by the drug's physicochemical properties. For depot formulations, absorption is biphasic: an initial slow release from the injection site forms a in the muscle tissue, enabling prolonged systemic exposure over days to weeks. Bioavailability of intramuscularly administered drugs is generally high, ranging from 90% to 100% for most compounds, as the route bypasses gastrointestinal barriers and hepatic first-pass metabolism. Key factors affecting this include the drug's solubility—aqueous formulations absorb faster than oil-based ones—muscle blood flow, which varies by site and physiological state, and the injected volume or dispersion within the tissue. For instance, reduced blood flow in certain muscles can delay absorption, while highly soluble drugs achieve near-complete bioavailability. The intramuscular route is clinically rationalized for scenarios requiring faster onset than oral or , particularly for emergency treatments or drugs prone to erratic gastrointestinal absorption. It avoids first-pass hepatic metabolism, preserving drug potency for compounds like certain antibiotics or hormones that undergo significant oral inactivation. Additionally, it suits poorly water-soluble agents by allowing larger volumes (up to 5 mL in adults) and providing a depot for sustained delivery, reducing dosing frequency. Pharmacokinetic parameters for intramuscular injections often include a time to peak plasma concentration (T_max) of 30 to for rapidly absorbed drugs, enabling quicker therapeutic effects compared to subcutaneous routes. The elimination remains drug-specific but informs dosing intervals; for example, standard formulations may require repeat injections every 4-12 hours, while depots extend this to weeks. Depot intramuscular injections, such as those using oil-based testosterone esters like enanthate or undecanoate, exemplify sustained-release formulations that form an intramuscular reservoir for gradual and absorption. These achieve peak testosterone levels within 1-3 days for enanthate (maintaining therapeutic concentrations for about 2 weeks) or around 7 days for undecanoate (with effects lasting 10-12 weeks), supporting less frequent dosing in .

Indications and Contraindications

Medical Indications

Intramuscular () injections are primarily indicated for administering medications that require rapid absorption, reliable , or when oral or intravenous routes are impractical or unavailable. This route is particularly suitable for patients who are noncompliant with oral medications, uncooperative, or intolerant to gastrointestinal administration. Additionally, IM injections are used in scenarios where intravenous access is not feasible, such as in settings or outpatient care, leveraging the muscle's for quicker onset compared to subcutaneous routes. A key prophylactic application of IM injections is in immunizations, accounting for approximately 5% of their use, as recommended by health authorities for optimal . Vaccines routinely administered via IM include (inactivated influenza vaccine), , diphtheria-tetanus-pertussis (DTaP, Tdap), type b (Hib), human papillomavirus (HPV), pneumococcal conjugate (PCV13), and meningococcal conjugate (MenACWY) vaccines. The and Centers for Disease Control and Prevention endorse IM administration for these vaccines to ensure effective delivery and minimize local reactions. For therapeutic purposes, which comprise over 95% of IM injections, common categories include agents for treating infections where sustained release is beneficial. Examples are penicillin G benzathine for or streptococcal infections and for . Analgesics such as are indicated IM for moderate to severe pain, including acute attacks in settings, providing rapid relief when oral intake is limited. Antiemetics like metoclopramide are used IM for and , particularly in postoperative or migraine-related cases, to facilitate faster symptom control. Hormone therapies also frequently employ the IM route for depot formulations that enable prolonged action. Medroxyprogesterone acetate (Depo-Provera) is administered IM as a long-acting contraceptive, while testosterone esters are used IM for replacement therapy. In emergencies, IM epinephrine is the first-line treatment for , injected into the anterolateral thigh for swift systemic absorption to reverse life-threatening allergic reactions. Biologicals such as immunoglobulins and toxoids are likewise delivered IM for or booster protection.

Contraindications and Precautions

Absolute contraindications for intramuscular () injection include known or to the medication or its excipients, as this can precipitate severe allergic reactions such as . Active bleeding disorders or severe also constitute absolute contraindications due to the high risk of uncontrolled hemorrhage at the injection site. Relative contraindications encompass conditions where IM injection may proceed with heightened caution or alternative routes preferred. These include local infection, , or at the intended injection site, which could exacerbate the infection or lead to poor absorption. Muscle atrophy or in the target area is another relative contraindication, as atrophied muscles absorb medications inadequately and increase the risk of subcutaneous deposition. Neuromuscular disorders, such as , represent a relative contraindication particularly when the injected may worsen neuromuscular transmission, necessitating evaluation of the specific medication. Anticoagulation therapy is a relative contraindication, as it elevates the risk of formation, though IM injection may be performed if benefits outweigh risks and with additional hemostatic measures. Precautions prior to IM injection involve a thorough assessment of the patient's for allergies or prior adverse reactions to ensure no risks. Monitoring for potential is essential, especially with large-volume injections or in patients with compromised tissue integrity, by selecting appropriate sites and volumes to avoid excessive pressure buildup. Dose adjustments are required in cases of renal or hepatic impairment, as these conditions can alter and clearance regardless of the administration route. Special alerts include avoiding IM injections in patients with or those on anticoagulation without prior reversal or consultation, to mitigate bleeding risks.

Technique

Preparation and Equipment

Intramuscular injections require meticulous preparation to ensure , , and minimization of risk, involving the assembly of sterile , proper handling, readiness, and adherence to aseptic principles. Essential includes a sterile typically ranging from 1 to 5 mL in capacity, selected based on the volume of to be administered, and a needle of 22-25 gauge with a length of 5/8 to 1.5 inches for adults, chosen based on body weight, site, and muscle depth to ensure intramuscular placement. Additional items encompass alcohol swabs or alcohol-based solutions for disinfection, nonsterile gloves to maintain barrier protection, and a sharps for safe needle disposal after use. A filter needle may be used when drawing from vials or ampules containing potential particulates to prevent contamination of the . Medication preparation begins with verifying the correct against the prescription, inspecting the or ampule for , cloudiness, discoloration, or visible particulates, and discarding any compromised product to avoid adverse effects. If the medication is in powder form, reconstitution is performed by adding the appropriate , such as sterile or saline, according to manufacturer instructions, followed by gentle mixing until fully dissolved without vigorous shaking to preserve integrity. The dose is then drawn into the using aseptic handling, ensuring no air bubbles are introduced, and the needle is changed to a fresh one for injection if a filter needle was used initially. Patient preparation involves explaining the procedure to the individual, addressing any concerns to reduce anxiety and obtain , and positioning them comfortably to expose the injection site while promoting muscle relaxation, such as lying or on their side depending on the selected area. considers the patient's body habitus, age, and muscle mass to ensure proper intramuscular placement, briefly referencing anatomical landmarks like the deltoid or vastus lateralis for optimal access. Aseptic techniques are paramount throughout, starting with thorough hand hygiene using soap and water if visibly soiled or alcohol-based rub otherwise, performed immediately before handling equipment. Work surfaces must be disinfected with an appropriate agent, such as 70% , and all manipulations conducted in a clean environment to prevent microbial introduction, including avoiding touch contamination of syringe tips, needle caps, and vial septa. Gloves are donned after hand hygiene and changed if torn or contaminated, reinforcing the barrier against pathogens.

Standard Procedure

The standard procedure for an intramuscular () injection involves a series of evidence-based steps to ensure safe and effective administration of into the muscle tissue, promoting optimal absorption while minimizing discomfort and complications. This technique assumes that preparation and equipment, such as a sterile , needle of appropriate length (5/8 to 1.5 inches for adults based on weight and site), and the , have already been assembled. To begin, perform hand hygiene using soap and water or an alcohol-based hand rub to maintain aseptic conditions. Select and expose the injection site, then clean the skin with an alcohol swab in a circular motion from the center outward, allowing it to air dry completely to reduce the risk of introducing contaminants. Next, stretch the skin taut at the site using your non-dominant hand to stabilize the area and facilitate needle insertion. Insert the needle swiftly and smoothly into the muscle at a 90-degree angle to the skin surface, using a dart-like motion to penetrate the muscle belly perpendicularly; this angle ensures the reaches the intended depth, which varies by site and patient but for the deltoid in adults is achieved with needles of 5/8 to 1.5 inches based on weight (e.g., 5/8-1 inch for <152 lbs, up to 1.5 inches for heavier individuals). Once inserted to the hub of the needle, inject the slowly at a rate of approximately 1 mL every 10 seconds to allow muscle fibers to stretch gradually, reducing pain and the risk of leakage. The maximum volume per injection varies by site and muscle mass (e.g., up to 1 mL for deltoid, 5 mL for vastus lateralis or ventrogluteal), to prevent overflow into . After injection, withdraw the needle quickly and steadily at the same 90-degree angle, without recapping it, and immediately dispose of it in a sharps container. Apply gentle pressure to the site with a clean or for 10-30 seconds to promote and absorption, but avoid massaging unless specified for the , as this can increase . Monitor the patient for immediate adverse reactions, such as excessive or . Finally, document the procedure in the patient's medical record, including the medication name and dose, injection site, date and time, needle gauge and length used, patient tolerance, and any observations. This documentation ensures continuity of care and legal compliance with healthcare standards.

Aspiration and Z-Track Method

The aspiration technique in intramuscular injection involves inserting the needle fully into the muscle and then gently pulling back on the syringe plunger for 5 to 10 seconds to observe for the presence of blood, which would indicate inadvertent entry into a blood vessel. Dark blood aspirated during this step typically indicates that the needle has entered a vein, as venous blood is deoxygenated and appears dark red, in contrast to bright red oxygenated arterial blood. Aspirating blood of any color signifies intravascular placement rather than in muscle tissue. If blood appears, the needle should be withdrawn immediately, the syringe and needle discarded, and the injection attempted at a new site to avoid unintended intravenous or intra-arterial administration, which is inappropriate for most medications and vaccines intended for intramuscular delivery. If no blood appears, the injection proceeds. This step aims to enhance safety by preventing unintended vascular injection, though it is associated with increased patient discomfort and procedural time. Current guidelines reflect a debate on the necessity of aspiration, particularly for non-intravenous medications. The Centers for Disease Control and Prevention (CDC) and (WHO) do not recommend routine aspiration for intramuscular vaccine administration, citing the low likelihood of puncturing major blood vessels at standard sites like the deltoid or vastus lateralis. For non-vaccine medications, some protocols still advise aspiration in high-risk sites or with certain drugs to minimize complications, though evidence suggests it may not significantly reduce risks in most cases and can be omitted to improve efficiency and reduce pain. The Z-track method is an advanced intramuscular injection modification designed to seal the medication within the muscle and prevent leakage into , which can cause irritation or staining. To perform it, the provider displaces the patient's laterally (about 1-2 inches) with the non-dominant hand, inserts the needle at a 90-degree angle into the muscle, injects the slowly, withdraws the needle while maintaining skin displacement, and then releases the skin to form a zigzag tract that closes over the injection path. This technique is particularly indicated for administering irritating, viscous, or oily medications, such as or iron dextran, where leakage could lead to local reactions or reduced efficacy. It is commonly used in the ventrogluteal or deltoid sites to further minimize subcutaneous tracking and is supported by guidelines for reducing post-injection complications in these scenarios.

Injection Sites and Anatomy

Intramuscular injections are administered into specific muscle groups to ensure effective absorption while minimizing risks to underlying structures such as nerves and blood vessels. The primary sites include the deltoid, vastus lateralis, ventrogluteal, and dorsogluteal muscles, each selected based on anatomical features that provide adequate muscle mass and relative safety. These sites are identified through of bony landmarks to locate the thickest portion of the muscle, avoiding areas near major neurovascular bundles like the in the deltoid or the in gluteal regions. The , located in the upper arm, is a common site for small-volume injections, particularly , due to its accessibility and moderate muscle bulk. To identify the site, palpate the acromion process of the and extend the fingers 2–3 fingerbreadths (approximately 5 cm) downward toward the ; the injection is given in the center of this triangular area, which corresponds to the thickest part of the muscle. This site accommodates a maximum volume of 1 mL in most adults, as larger amounts may lead to discomfort or suboptimal absorption, and care must be taken to avoid the and axillary vessels located inferomedially. The in the anterolateral is preferred for infants, young children, and some adults requiring larger volumes, owing to its large size and low risk of damaging major nerves or vessels. Landmarks are determined by dividing the into thirds from the to the lateral femoral condyle; the injection is administered in the middle third, approximately 7–13 cm below the , targeting the vastus lateralis without encroaching on the or . This site supports volumes up to 5 mL in adults and is particularly suitable for patients with limited arm mobility. The ventrogluteal site, situated in the lateral hip over the and minimus, is widely regarded as the safest for intramuscular injections because of its thick muscle layer, minimal subcutaneous fat, and distance from major nerves and vessels. To palpate, place the palm of the hand on the , extend the index finger along the , and position the middle finger posteriorly toward the to form a ; the injection is given in the center of this area, ensuring avoidance of the . It accommodates volumes up to 4–5 mL and is recommended for adults and children. Although historically used, the dorsogluteal site in the upper outer quadrant of the buttock is generally avoided due to its proximity to the , which increases the risk of injury even with proper landmarking. The site is located by drawing an imaginary line from the to the and injecting 5–7.5 cm lateral and superior to this line in the , but this area has variable fat thickness and potential overlap with the nerve in up to 10–20% of cases depending on body habitus. Site selection depends on patient-specific factors, including age (e.g., vastus lateralis for neonates and infants under 12 months), (deeper sites like ventrogluteal for obese individuals), mobility (deltoid for self-administration), and medication volume (deltoid limited to ≤1 mL, gluteal sites for >2 mL). Sites should be rotated across injections to prevent localized , formation, or , with the insertion angle typically at 90 degrees to reach the muscle belly.

Risks and Complications

General Complications

Intramuscular injections commonly cause at the site of administration, with studies reporting incidences ranging from 21% to 80% depending on the and technique used. This pain can be attributed to factors such as the solvents in the formulation (e.g., due to their acidity and high osmolarity), the concentration of the drug, or the body's reaction to oil carriers in some formulations. This discomfort typically peaks immediately after injection and resolves within 1-2 days, often described as moderate in severity. Infections, such as abscesses or , represent a significant but infrequent , occurring in less than 2% of cases when proper aseptic technique is followed. These complications arise from bacterial contamination during the procedure and can lead to localized swelling, redness, and fever if untreated. Allergic reactions to the injected or excipients may manifest as urticaria, itching, or more severe responses, though they are uncommon in routine administrations. In rare instances, can occur, with reported rates as low as 11 per million doses for certain vaccines administered intramuscularly. Hematoma formation results from inadvertent puncture of blood vessels, leading to localized bleeding and bruising; this complication is generally rare, with an incidence of approximately 0.06%. Systemic issues can arise from procedural errors or medication properties, including inadvertent intravenous administration, which accelerates drug absorption and may cause or overdose effects such as cardiovascular . Anaphylaxis, as a systemic allergic response, can also progress to , , and shock if not promptly managed. Prevention of these general complications emphasizes strict , including hand hygiene, skin disinfection with alcohol or , and use of sterile equipment to minimize risk. Accurate dosing and verification of the injection path help avoid overdose or vascular puncture, while post-injection monitoring of enables early detection of systemic reactions.

Site-Specific Complications

Intramuscular injections carry risks that vary by anatomical site due to differences in underlying structures such as nerves, blood vessels, and bones. Complications arising from improper site selection or technique can lead to localized injuries, with the deltoid, gluteal, and thigh regions presenting distinct vulnerabilities based on their proximity to critical tissues. In the deltoid muscle of the upper arm, shoulder injury related to vaccine administration (SIRVA) is a recognized complication, typically resulting from injections placed too high or too deep, causing inflammation of the subacromial bursa, rotator cuff tendons, or surrounding soft tissues. This leads to persistent shoulder pain, reduced range of motion, and potential long-term dysfunction, often requiring conservative management like physical therapy or corticosteroid injections. Additionally, the axillary nerve, which innervates the deltoid and lies beneath the muscle, is susceptible to direct trauma from needles inserted too low or posteriorly, resulting in axillary neuropathy with symptoms such as deltoid weakness, sensory loss over the shoulder, and muscle atrophy in severe cases. Gluteal site injections, particularly in the dorsogluteal region, pose significant risks to the due to its superficial course through the lower buttock, rare, with an overall incidence of nerve injuries from IM injections estimated at 3.38 per 1,000,000 injections, though higher risk at the dorsogluteal site due to proximity. leading to acute pain, , sensory deficits in the lower leg, and possible chronic neuropathy. In the same area, the proximity of the injection site to the ilium or increases the rare but serious risk of , where bacterial contamination from the injection spreads to adjacent bone, causing localized bone destruction, formation, and systemic that may necessitate surgical and prolonged antibiotics. For thigh sites like the vastus lateralis, femoral nerve irritation can occur if the needle deviates medially toward the nerve's path along the , resulting in transient or persistent quadriceps weakness, knee buckling, and down the anterior . Bruising is also more prevalent in this mobile region due to the site's exposure to mechanical stress from leg movement, which can exacerbate formation or vessel trauma, leading to visible ecchymosis and discomfort that resolves slowly over days to weeks. To mitigate these site-specific risks, modern clinical guidelines recommend the ventrogluteal site over the dorsogluteal for gluteal injections, as it avoids the and major vessels while providing adequate muscle mass, thereby reducing incidence by up to several fold compared to traditional approaches. Proper landmarking and technique remain essential across all sites to minimize anatomical vulnerabilities.

Special Populations and Considerations

Pediatric and Geriatric Use

In pediatric patients, intramuscular injections require careful due to smaller muscle mass and developmental variations. The in the anterolateral thigh is the preferred site for infants and young children because of its larger size and lower risk of nerve or vascular injury compared to other areas. For older children, the may be used, but volumes should be appropriate to the and patient size to avoid overflow into . These guidelines align with childhood vaccine schedules, where many immunizations, such as those for and DTaP, are given intramuscularly to ensure optimal . Pain management is essential in to improve cooperation and reduce distress during injections. Techniques such as —using toys, videos, or verbal engagement—have been shown to significantly lower perceived scores in children undergoing vaccinations. Topical anesthetics, like eutectic mixture of local anesthetics (EMLA) cream applied prior, provide effective numbing for needle insertion, particularly in infants and toddlers, though application time must be balanced against procedural delays. Challenges include securing cooperation from fearful children, which may necessitate parental involvement or positioning aids to minimize movement. In geriatric patients, age-related changes such as reduced and mass increase the risk of inadvertent subcutaneous deposition rather than true intramuscular delivery. To mitigate this, the bunching technique—squeezing the muscle tissue before injection—is recommended for the deltoid site, as studies in cadavers demonstrate it achieves proper depth in over 75% of cases, compared to less than 15% with the flattening method. Frailty in older adults elevates the potential for complications like at the injection site due to thinner and slower healing. Site rotation is advised for repeated injections to prevent localized . Pharmacokinetic adjustments are often necessary in , as intramuscular absorption can be delayed, leading to slower peak concentrations; for example, tobramycin peaks occur later in elderly patients, potentially requiring dose modifications to avoid under- or overdosing. Skin fragility further complicates procedures, increasing bruising risks, while overall reduced lean mass may alter distribution and necessitate lower volumes or alternative routes in frail individuals. These considerations emphasize tailored approaches to enhance safety and efficacy.

Patient Education and Aftercare

Patients should be informed that intramuscular injections commonly cause mild , soreness, or swelling at the injection site, which typically resolves within 1-2 days. These effects occur due to the needle insertion and delivery into the muscle tissue. materials should explain that over-the-counter pain relievers like acetaminophen can help manage discomfort if needed, but patients should follow dosage instructions from their healthcare provider. Key education topics include recognizing signs of potential complications, such as increasing redness, warmth, or swelling at the site, persistent pain beyond 48 hours, fever, or drainage, which may indicate or formation. Other serious indicators include severe localized pain, numbness, or difficulty moving the affected limb, possibly signaling or tissue damage. Patients must be advised to seek immediate medical attention if these symptoms appear or if systemic signs like high fever or chills develop, as delays can lead to more severe issues like . Aftercare instructions emphasize monitoring the injection site for 24-48 hours and applying a cold compress or wrapped in cloth for 10-20 minutes several times a day to reduce pain and swelling. Patients should avoid rubbing or massaging the area to prevent leakage or , and refrain from strenuous exercise or heavy lifting involving the injected limb for at least 24 hours. Any adverse effects, even mild ones, should be reported to the healthcare provider promptly to ensure proper follow-up. For patients on chronic therapies requiring self-administration, such as certain treatments, emphasis should be placed on strict practices: washing hands thoroughly with soap and water before preparation, cleaning the vial top and injection site with an alcohol swab, and using a new needle and for each dose to minimize risk. Medications must be stored according to manufacturer guidelines, typically at away from direct sunlight and out of reach of children, with specified for some formulations. of injection sites is crucial to avoid tissue damage from repeated use. To enhance accessibility, should use clear, simple language free of medical jargon, with materials available in multiple languages where possible to accommodate diverse populations. Resources like illustrated guides or videos from reputable health organizations can support , ensuring understanding across cultural and literacy levels.

Historical and Veterinary Aspects

Historical Development

The development of intramuscular (IM) injection traces its origins to the mid-19th century, coinciding with the invention of the hypodermic . In 1853, Scottish physician Alexander Wood independently developed a practical hypodermic , inspired by the stinger of a , which he used to administer subcutaneously for pain relief, marking a pivotal advancement in parenteral . Concurrently, French surgeon Charles Gabriel Pravaz created a similar piston that year, initially designed for injecting coagulants into aneurysms, further enabling precise subcutaneous and emerging deeper tissue injections. These innovations laid the groundwork for IM administration, as early uses of and other agents like for treatment in the 1850s and 1860s began to explore muscle layers for faster absorption and sustained effects. By the early 20th century, IM injections gained prominence in during , where they facilitated mass vaccinations against diseases such as typhoid and . antitoxin, derived from horse serum, was routinely administered via IM route to prevent wound infections among soldiers, significantly reducing mortality rates despite logistical challenges in battlefield settings. This era highlighted IM's utility for rapid, large-scale immunization, with millions of doses of tetanus prophylaxis distributed by Allied forces. Post-, the 1930s saw advancements in sustained-release formulations, including protamine-zinc-insulin suspensions for subcutaneous diabetes management, alongside the development of IM depot preparations for hormones like progesterone, allowing prolonged therapeutic effects without frequent dosing. Influential figures shaped subsequent standardization efforts. Alexander Wood's work not only popularized hypodermic delivery but also influenced global adoption of injection therapies. Following , the (WHO) played a key role in establishing uniform protocols for IM injections in vaccination campaigns, emphasizing safe sites and techniques to combat infectious diseases in developing regions. In the mid-20th century, anatomist Hochstetter first described the ventrogluteal site in the 1950s as a safer alternative to the dorsogluteal area, citing reduced risk of injury; this recommendation gained traction in the 1970s through nursing and pharmacology literature advocating the shift for adult injections. Safety practices evolved significantly in the late , particularly regarding aspiration—the technique of pulling back the plunger to check for blood, once routine to avoid intravascular injection. Studies from the and found no instances of blood return in most IM injections, concluding aspiration unnecessary and potentially increasing patient pain without benefit. This led to guideline changes by organizations like the WHO and CDC in the and that deemed it obsolete for most IM sites except dorsogluteal. These evidence-based shifts prioritized patient comfort and efficiency in modern protocols.

Veterinary Applications

Intramuscular (IM) injections are widely used in to administer vaccines, antibiotics, and sedatives to both companion animals and , providing rapid absorption due to the vascularity of muscle tissue. For example, penicillin G is commonly given IM to for treating bacterial infections such as , with doses up to 20,000 units per kilogram of body weight. In pets like dogs and cats, IM injections of sedatives such as are routine for pre-anesthetic procedures, achieving onset within 5-15 minutes. agents, including barbiturates like , are also frequently administered IM in small animals when intravenous access is challenging, as recommended by the (AVMA). Techniques in veterinary practice adapt to animal size and behavior, often involving restraint methods like halters for or squeeze chutes for to ensure safety. varies by to minimize tissue damage and maximize efficacy; in , the semimembranosus muscle in the hindquarter or cervical muscles in the neck are preferred for volumes up to 10 mL, using 1.5-inch, 18- to 20-gauge needles. For dogs, the cervical epaxial muscles or are common sites, limited to 2 mL per site to avoid discomfort. Larger volumes, such as 15 mL in adult , are feasible in neck muscles to prevent carcass blemishes in valuable cuts, with injections spaced at least 4 inches apart. Species-specific challenges arise in exotic animals, where IM sites must account for unique ; in reptiles like snakes, epaxial muscles along the spine are used for antibiotics or fluids, with volumes under 0.5 mL to prevent organ compression. receive IM injections in forelimb muscles, while chelonians () use hindlimb sites, often requiring due to stress responses. In farming, mass IM administration of , such as clostridial toxoids in sheep, occurs via handling systems for efficiency, emphasizing needle hygiene to reduce risk. Regulatory frameworks differ from human medicine, with the FDA's Center for Veterinary Medicine approving drugs for IM use based on target animal safety and efficacy studies, unlike the human-focused Center for Drug Evaluation and Research. AVMA guidelines stress proper and volume limits to avoid residues in food animals, aligning with Quality Assurance programs that prohibit IM injections in high-value muscle areas like the hindquarter. Animal pharmacokinetics vary from humans, often showing faster absorption in species like horses due to greater muscle blood flow.

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