Mechanisms of Drug Absorption
The entry of drug molecules into the systemic circulation following an injection occurs at the level of the microvasculature. The rate and pathway of absorption depend on the physicochemical properties of the specific pharmacological agent:
- Filtration: This is the primary pathway for water-soluble (hydrophilic) substances. They cross into the blood through specialized intercellular clefts (pores) located in the endothelium of blood vessels.
- Passive diffusion: This is characteristic of fat-soluble (lipophilic) compounds. Due to their properties, such molecules can penetrate directly through the lipid membranes of capillary endothelial cells.
Anatomy and Pharmacokinetics of Intramuscular Injection
The optimal anatomical location for intramuscular injections is the upper outer quadrant of the gluteal muscle (m. gluteus). This topography is chosen to prevent dangerous injury to major nerve trunks.
Muscle tissue features a very rich vascular supply containing a dense capillary network (vasa capillaria). The drug moves from the interstitial space of the muscle through the capillary wall into the venous circulation (Vena). Due to this intensive blood supply, absorption of aqueous solutions (in volumes up to 10 mL) occurs rapidly, and peak concentration ($C_{max}$) in blood plasma is reached within 10–30 minutes.
However, exceptions exist. Substances that exhibit a high capacity for specific binding to muscle tissue proteins are absorbed more slowly. Classic examples of such drugs include diazepam (Diazepamum) and phenytoin (Phenytoinum).
Subcutaneous Administration and Methods of Absorption Regulation
Subcutaneous adipose tissue (Tela subcutanea) has a less intensive vascular supply compared to skeletal muscle, meaning substance transport occurs more slowly here. The volume of injected aqueous solutions is typically restricted to 2 mL. In clinical practice, physicians can intentionally modulate the rate of drug entry into the bloodstream:
- Accelerating absorption: Achieved via physical methods (warming compresses, local massage to stimulate blood flow) or pharmacologically. For instance, the simultaneous administration of the enzyme hyaluronidase causes depolymerization of connective tissue mucopolysaccharides, drastically increasing tissue permeability and the absorptive surface area.
- Slowing absorption: Utilized when a systemic effect is undesirable (e.g., when administering local anesthetics). Vasoconstrictors (such as epinephrine) are added to the drug formulation to reduce local blood flow and localize the effect.
Creation of Tissue Depots and Implantation
To prolong the therapeutic effect, special dosage forms are employed:
- Oily solutions and suspensions: When injected into tissues, they form a local "depot." Kinetics shift, resulting in a slow, gradual release of the active substance into the bloodstream. They are used with extreme caution subcutaneously.
- Implantation: Employed subcutaneously to maintain a steady drug concentration over an extended period (ranging from weeks to months). Sterile tablet forms or specialized silicone containers are surgically placed into the tissues. Contraceptivos and testosterone formulations are frequently administered this way.
Contraindications and Potential Complications
Parenteral administration is associated with specific risks (Contraindicationes et Complicationes). It is strictly prohibited to inject hypertonic solutions (they draw out water, causing cellular dehydration and necrosis) and irritant substances (which provoke severe chemical inflammation) intramuscularly or subcutaneously.
Specific complications include:
- The development of infectious processes and abscesses (abscessus) — localized purulent foci within the muscle that may occur despite strict adherence to aseptic protocols.
- Tissue necrosis resulting from the subcutaneous injection of irritating salts (e.g., calcium chloride) or potent vasoconstrictors (norepinephrine), which provoke critical local ischemia.