Intraperitoneal (IP) injection is a method of introducing a substance directly into the peritoneal cavity. This cavity is the space within the abdomen that contains the stomach, intestines, liver, and other organs. The technique involves puncturing the abdominal wall with a needle to deposit the material into this space. While IP injection is a common route of administration in non-human animal research, its application in human medicine is limited to specialized treatments. The goal of an IP injection is to achieve rapid absorption into the bloodstream or to deliver a high concentration of the substance directly to the organs within the abdomen.
Understanding the Injection Site
The peritoneal cavity is the space enclosed by the peritoneum. The peritoneum is a thin membrane that lines the inner wall of the abdomen and covers most abdominal organs. This membrane is highly vascularized, meaning it is richly supplied with blood vessels.
The cavity contains a small amount of lubricating fluid. When a substance is injected, it mixes with this fluid and is exposed to the vast surface area of the peritoneum. This large surface area, which is roughly equivalent to the total surface area of the skin, facilitates the swift transfer of the injected substance across the membrane and into the underlying blood and lymphatic vessels. This extensive absorptive surface allows for the quick uptake of fluids and drugs.
Why Use Intraperitoneal Delivery
IP delivery is often selected for its ability to deliver relatively large volumes of fluid compared to other injection methods. This is an advantage in laboratory settings, particularly with small animal models, where the IP route is the most frequently used method for administering compounds. It is also considered less technically challenging than directly injecting into a small vein.
In human medicine, the use of IP injection is highly specialized and focuses on delivering treatment directly to the abdominal area. One established application is peritoneal dialysis, where fluid is introduced into the cavity to filter waste products from the blood in patients with kidney failure. The route is also utilized for targeted chemotherapy to treat abdominal cancers, such as ovarian cancer, that have spread to the peritoneal lining. By confining the drug to the abdominal cavity, a much higher concentration can be delivered directly to the tumor sites, which reduces systemic side effects.
Comparing Absorption Rates
The speed at which a substance enters the systemic circulation is a factor when choosing an injection route, and the IP route offers an intermediate rate of absorption. Substances injected intraperitoneally are absorbed faster than those administered subcutaneously (under the skin) or intramuscularly (into the muscle) because of the peritoneum’s large, vascularized surface. Some compounds are detectable in the bloodstream within seconds of an IP injection, which is significantly faster than subcutaneous absorption.
However, IP absorption is slower than intravenous (IV) injection, which places the substance directly into the bloodstream for immediate effect. The absorbed substance is drained into the mesenteric blood vessels, which feed directly into the hepatic portal vein and the liver. This means the drug must pass through the liver before reaching the general circulation, a process known as first-pass metabolism. This hepatic processing can alter or reduce the concentration of the drug, resulting in a lower systemic bioavailability compared to an IV injection.
Administering the Injection Safely
The safe administration of an intraperitoneal injection relies on proper technique to avoid injury to internal organs. The procedure requires careful needle placement, typically in a lower quadrant of the abdomen, to avoid puncturing the bladder or the intestines. In many animal models, the injection is specifically directed to the lower right side to reduce the risk of hitting the large pouch known as the cecum.
Complications can arise if the needle inadvertently damages an organ or blood vessel, leading to internal bleeding or the release of gut contents. A puncture of the bowel can introduce bacteria into the peritoneal cavity, resulting in a serious infection called peritonitis. Other risks include the formation of visceral adhesions or the sequestration of the injected material in the abdominal fat or subcutaneous tissue, known as a misinjection. For these reasons, the procedure is performed exclusively by trained professionals in controlled settings. Careful monitoring is necessary to ensure the substance is delivered correctly and the recipient does not suffer adverse effects.