Spinal infusion involves placing a tiny catheter into the spinal sac or epidural space. These spaces around the spinal sac are separated only by a thin membrane called the dura mater which includes a layer called the arachnoid membrane. Spinal or subarachnoid infusions place medicine inside the spinal sac (inside the dura mater) where the medicine can spread out in the cerebral spinal fluid (CSF) and affect wide areas of pain. Epidural ( above the dura ) medicines flow into the epidural space where the fluid spread is much more restricted by the epidural contents (epidural fat and blood vessels). Where the catheter is placed, and the kind of medicine infused depends on the patient, the type of pain, the duration of infusion, and many other factors.