The catheter is inserted into the spine by an anesthesiologist. The pump and catheter are completely under the skin. They both are secured in place and should not change their position. You will be able to feel the pump under your skin, but it will not be painful. The bottom chamber holds a charging fluid.
Body temperature causes the charging fluid to expand.
The pump is filled by injecting the medication through the skin and through a rubber stopper leading into the top chamber. This is done with a special needle. You should feel only a pin prick sensation as the needle passes through the skin. Since the pump provides a constant flow of medication, it will need to be refilled on a regular schedule. Know the drug that is in your pump.