Intraperitoneal Injection: Anatomy, Steps, and Uses

Intraperitoneal (IP) injection delivers substances directly into the peritoneal cavity and is widely used in laboratory research and veterinary medicine. It is favored for its efficiency in drug absorption and ease compared to intravenous administration. Proper technique ensures accuracy, minimizes complications, and improves reliability.

Anatomy Of The Peritoneal Cavity

The peritoneal cavity is a fluid-filled space in the abdomen, enclosed by the peritoneum, a serous membrane lining the abdominal wall and covering the visceral organs. This membrane has two layers: the parietal peritoneum, which lines the abdominal wall, and the visceral peritoneum, which covers the organs. Between them, a small volume of peritoneal fluid reduces friction and allows organ movement.

The peritoneum’s extensive vascularization enables rapid absorption of injected substances. Its permeability allows compounds to diffuse into capillary networks. Factors such as molecular weight, lipophilicity, and carrier proteins influence absorption. Additionally, proximity to the portal circulation means some drugs undergo first-pass metabolism in the liver before entering systemic circulation.

Gravity and fluid dynamics affect how injected substances distribute within the cavity. Positioning the subject can help direct substances toward more vascularized areas, optimizing absorption. Peritoneal folds, such as the greater omentum and mesentery, can also influence fluid movement and drug bioavailability.

Instruments And Preparation

Choosing the right syringe and needle is crucial. For small rodents like mice and rats, a 1 mL or 3 mL syringe with a 25- to 30-gauge needle minimizes tissue trauma. Larger animals require bigger syringes and needles. Needle length is also important; a 0.5-inch needle suits mice, while rats and larger animals may need a 1-inch needle to ensure proper penetration without risking organ puncture.

Injection solutions must be sterile and physiologically compatible. Filtering through a 0.22-micron filter removes particulates. The pH and osmolarity should prevent irritation or adverse reactions. Hypertonic or hypotonic solutions can cause cellular stress and inflammation. Injecting cold solutions may induce vasoconstriction and slow absorption, while pre-warming enhances comfort and uptake.

Proper handling and restraint are essential. In rodents, gentle but firm restraint prevents sudden movements that could misplace the needle. Holding the animal in a supine position with slight dorsal extension provides clear access to the injection site. The lower right or left quadrant of the abdomen is preferred to avoid major organs and blood vessels. In larger animals, sedation or local anesthesia may be necessary for precise administration.

Steps Of The Procedure

Correct positioning is key to accuracy and safety. In small animals like rodents, a supine position with slight dorsal extension allows clear access to the ventral abdomen. Conscious subjects require restraint to prevent movement, while larger animals may need sedation. The lower left or right quadrant of the abdomen is the preferred injection site to avoid major organs.

The needle should be inserted at a 20- to 30-degree angle in rodents, directed slightly caudally to reduce the risk of organ puncture. Superficial insertion can result in subcutaneous or intramuscular deposition, delaying absorption. Gentle aspiration can confirm correct placement; blood or intestinal contents indicate improper positioning and necessitate repositioning.

Injection should be smooth and controlled to prevent pressure buildup and discomfort. Rapid injection increases leakage risk. Volume must be within species-specific tolerances—rodents should not receive more than 10 mL/kg to prevent abdominal distension. After injection, withdrawing the needle swiftly and applying slight pressure with sterile gauze minimizes leakage or irritation.

Common Use In Animal Models

Intraperitoneal injection is widely used in preclinical research for delivering pharmaceuticals, anesthetics, and experimental compounds. It is particularly useful in small laboratory animals where intravenous access is challenging. Researchers use this method for drug testing, pharmacokinetics, and disease modeling due to its reliability and minimal invasiveness.

In oncology, intraperitoneal administration allows direct exposure of tumor cells to chemotherapeutic agents, particularly in peritoneal carcinomatosis models. This approach is valuable in studying ovarian and gastrointestinal cancers where malignant cells spread within the peritoneal cavity. It is also used in infectious disease research for introducing pathogens or antigens to study immune responses.

Mechanism Of Drug Absorption

Once injected, substances are absorbed through the peritoneal membrane’s capillary network, primarily via passive diffusion. Small, lipophilic compounds cross the membrane more readily, while larger hydrophilic molecules take longer. Absorption is influenced by peritoneal blood flow, molecular weight, and carrier proteins. Compounds under 500 Da absorb quickly, whereas larger molecules, like monoclonal antibodies, diffuse more slowly.

Peritoneal fluid distribution and subject positioning affect absorption. Fluid movement helps disperse the substance, but gravitational effects can lead to uneven distribution, especially in larger animals. Some drugs localize in specific regions, impacting bioavailability. The peritoneal cavity’s proximity to portal circulation means certain drugs undergo first-pass metabolism in the liver, which can influence efficacy and toxicity. Understanding these pharmacokinetics helps optimize dosing strategies for better therapeutic outcomes.