Peripheral edema is a type of swelling in which excess fluids fall to the lowest part of the body, usually the legs and feet, according to eMedicineHealth. Since peripheral edema is gravity dependent, it may produce swelling in the sacrum if the patient is lying on his back.
The swelling observed in cases of peripheral edema may either be caused by too much fluid moving from the blood vessels into the tissues, or not enough fluid moving from the tissues back into the blood vessels, according to the Gale Encyclopedia of Medicine. Peripheral edema may be caused by a host of possible underlying factors, notes the Mayo Clinic. Mild cases of peripheral edema may be caused by factors such as eating too much salt, sitting for too long, or being pregnant, while more severe cases of peripheral edema may be caused by conditions such as kidney failure and congestive heart failure.