Who needs to have total parenteral nutrition (tpn)?

Bowel rest. If swallowing is an issue, we can give food via peg tube thru skin directly into stomach. But some patients need bowel rest so we give total parenteral nutrition (TPN) through veins so nothing is in stomach/intestine. These are major veins, not tiny ones on back of hand, usually via picc (peripherally inserted central catheter) line since it's expected that you'll need TPN for a long time.
Intestine failure. Tpn is the primary therapy for intestinal failure - a condition that exists when your intestine cannot provide enough nutrition and fluid to support life. You may be malnourished or dehydrated and require TPN for a short time. However, in certain situations, TPN can be required long term and may be permanent. In those instances, you may consider intestine transplantation.