Mechanisms of drug absorption
The entry of molecules into the systemic circulation after injection occurs at the level of the microvasculature. The rate and pathway of absorption depend on the physicochemical properties of the specific medicinal substance:
- Filtration: this is the main pathway for water-soluble (hydrophilic) substances. They penetrate into the blood through specialized intercellular clefts (pores) located in the endothelium of blood vessels.
- Passive diffusion: typical for fat-soluble (lipophilic) compounds. Due to their properties, such molecules are able to penetrate directly through the lipid membranes of capillary endothelial cells.
Anatomy and pharmacokinetics of intramuscular administration
The optimal anatomical localization for intramuscular injections is the upper outer quadrant of the gluteal muscle (m. gluteus). This topography is chosen to prevent dangerous damage to major nerve trunks.
Muscle tissue is characterized by very good vascularization—it contains a dense network of capillaries (vasa capillaria). The drug passes from the intercellular space of the muscle through the capillary wall and enters the venous circulation (Vena). Due to intensive blood supply, the absorption of aqueous solutions (up to 10 ml in volume) occurs rapidly, and the peak concentration ($C_{max}$) in blood plasma is reached in just 10–30 minutes.
However, there are exceptions. Substances that have the ability to specifically bind to muscle tissue proteins are absorbed with a delay. Classical examples of such drugs are diazepam (Diazepamum) and phenytoin (Phenytoinum).
Subcutaneous administration and methods of absorption regulation
Subcutaneous adipose tissue (Tela subcutanea) has less intensive vascularization compared to muscles, so the transport of substances proceeds more slowly here. The volume of injected aqueous solutions is usually limited to 2 ml. In clinical practice, a physician can purposefully regulate the rate of drug entry into the blood:
- Accelerated absorption: achieved by physical methods (warming compresses, local massage to stimulate blood flow) or pharmacologically. For example, the simultaneous administration of the enzyme hyaluronidase causes depolymerization of connective tissue mucopolysaccharides, which dramatically increases permeability and the absorption area.
- Delayed absorption: used when systemic action is undesirable (for example, when using local anesthetics). For this purpose, vasoconstricting substances (adrenaline) are added to the drug, which reduce local blood flow and localize the effect.
Creation of a tissue depot and implantation
Special dosage forms are used to prolong (extend) the therapeutic effect:
- Oily solutions and suspensions: when introduced into tissues, they form a local "depot". The kinetics of the process changes—a slow, gradual release of the active substance into the blood occurs. When administered subcutaneously, they are used with extreme caution.
- Implantation: used subcutaneously to create a constant concentration of a substance for a long period (from several weeks to months). Sterile tablet forms or special silicone containers are sewn into the tissue. Most often, contraceptives and testosterone preparations are administered this way.
Contraindications and potential complications
Parenteral administration is associated with a number of risks (Contraindicationes et Complicationes). It is strictly forbidden to inject hypertonic solutions (they draw out water, causing cellular dehydration and necrosis) and irritating substances (provoke severe chemical inflammation) into muscles and subcutaneously.
Specific complications include:
- The development of an infectious process and abscesses (abscessus)—localized purulent foci in the muscle, which can occur even with strict adherence to sterility rules (asepsis).
- Tissue necrosis upon subcutaneous administration of irritating salts (e.g., calcium chloride) or powerful vasoconstrictors (norepinephrine), which cause critical ischemia of the area.
Briefly: answers to topical questions
Which nerves can be damaged during an incorrect intramuscular injection into the gluteal region?
During an incorrect intramuscular injection into the gluteal region, the sciatic nerve is most often damaged. This complication occurs when the drug is administered outside the safe upper-outer zone (into the lower-outer or inner quadrants).
- The sciatic nerve (n. ischiadicus) is the largest nerve in the body, damage to which is one of the most frequent iatrogenic complications in this area.
What post-injection complications are possible with intramuscular and subcutaneous administration?
With intramuscular and subcutaneous administration, infectious, inflammatory, necrotic, and other local complications are possible.
- Abscesses (abscessus)—localized purulent inflammations, possible even with adherence to asepsis rules.
- Tissue necrosis—possible with the subcutaneous administration of irritating salts, hypertonic solutions, and powerful vasoconstrictors.
- Lipodystrophies—disruption of adipose tissue at injection sites.
- Infiltrates—indurations, to prevent which it is necessary to alternate injection sites.
- Microhematomas—local reactions in the injection zone.
Which muscles, other than the gluteus maximus, are used for intramuscular injections?
In addition to the gluteus maximus muscle, the triceps brachii muscle (m. triceps brachii) can be used for intramuscular injections. Injections into this muscle are often painful due to the abundance of cutaneous nerves. Intramuscular administration is also performed into the lateral surface of the thigh.
Into which anatomical regions are subcutaneous injections usually given?
Subcutaneous injections are performed in the following anatomical regions:
- Middle third of the outer surface of the shoulder—a common site of administration.
- Anterior abdominal wall—at a distance of 2–4 cm from the linea alba of the abdomen; do not inject above the navel and below the navel.
- Anterolateral surface of the thigh—an additional injection site.
- Subscapular region—used less frequently.
- Buttocks—can be used as an alternative injection site for insulin.
Volume rule: up to 10 ml of an aqueous solution can be safely injected into a muscle, and no more than 2 ml subcutaneously.
Class and exam questions
Why is an intramuscular injection given specifically in the upper outer quadrant of the buttock?
This anatomical zone is chosen specifically to prevent damage to major nerve trunks (in particular, the sciatic nerve) when the needle is inserted.
How does hyaluronidase accelerate absorption during subcutaneous administration?
The enzyme destroys (depolymerizes) connective tissue mucopolysaccharides. This increases its permeability and expands the area of drug absorption into capillaries.
Is a drug always rapidly absorbed from muscle tissue?
No, drugs that specifically bind to muscle tissue proteins (such as diazepam or phenytoin) are released into the systemic circulation extremely slowly.
Why is it forbidden to administer hypertonic solutions subcutaneously or intramuscularly?
Hypertonic solutions cause acute dehydration of surrounding cells, which leads to tissue death (necrosis).
What else to study on this topic
- Dependence of pharmacokinetics on the lipophilicity and hydrophilicity of drugs
- Mechanisms of aseptic abscess formation in muscle tissue
- Comparative characteristics of skin, adipose tissue, and muscle microcirculation
- Pharmacology of drugs that form tissue depots
- Chemical inflammation and necrosis: pathophysiological mechanisms
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