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An intraperitoneal (IP) injection is an injection given directly into the intraperitoneal cavity, the area of the body that surrounds the abdominal organs. It is a form of parenteral administration and is more common in veterinary medicine than human medical practice. There are a number of settings where a care provider may recommend or prefer this method, if possible, usually in the case of handling small animals who cannot receive medications and fluids by other methods. Technicians need special training to give an IP injection accurately.
One concern with the IP injection is the risk of puncturing an abdominal organ. In veterinary medicine, care providers may hold the animal in a slightly tilted, head down position to encourage the organs to move up toward the head, freeing up space for the injection. The care provider selects a needle of appropriate size and carefully inserts it. He can aspirate it to determine if it is in a bad location; if it fills with yellow fluid, for example, it is in the bladder, not the peritoneal cavity.
With animals, an IP injection may be the only way to reliably deliver medication. Organisms like mice and rats are so small that intramuscular injections are not suitable because they don't have enough muscle mass to absorb the material, and a subcutaneous injection might have the same problem. The IP injection provides a mechanism for introducing high volume medications with limited distress and discomfort for the animal.
In animal research, the IP injection may be part of the treatment protocol. Scientists and technicians must be careful, as errors with the injections may throw off the study results. They need to make sure their needles are properly placed, and may rely on help from an assistant to restrain the animal during the procedure so they can concentrate on needle placement. Lab personnel also receive general training in humane animal handling and lab protocols to minimize trauma to study subjects.
In human medicine, the IP injection is recommended and used in some forms of chemotherapy. For ovarian cancer, for instance, the chemotherapy medications can bathe the entire abdominal cavity to eliminate any metastases, including growths too small to identify and remove. Patients appear to have a better outcome with intraperitoneal chemotherapy in these cases. One delivery option involves administering the medications in surgery, so the patient does not need to be awake or conscious during the intraperitoneal chemotherapy session.
I had to do IP injections on mice when I was working at a research laboratory. A mouse IP injection is not the easiest procedure, especially because mice are so small. There were times where I caused a hematoma in the mice's abdomen because the needle would accidentally puncture a vein.
Bleeding was a problem, not just because it would affect the research results but because it could be detrimental to the mouse as well. Even a small amount of blood could cost a mice its life, so that was always a worry. Thankfully, I got better at it with experience and had no problems 99% of the time.