Parenteral Nutrition (TPN) Basics

Parenteral nutrition (PN) supplies dextrose, amino acids, lipid injectable emulsion, electrolytes, vitamins, and trace elements directly into the venous circulation. Because it bypasses the gastrointestinal tract entirely, it is used when the gut is unavailable, inaccessible, or unable to absorb enough nutrition. Examples include bowel obstruction, prolonged ileus, mesenteric ischemia, short bowel syndrome, severe malabsorption, and a high-output enterocutaneous fistula. Enteral nutrition remains the preferred route whenever the gastrointestinal tract can safely receive and absorb nutrition. The preference is physiologic: enteral feeding preserves intestinal structure and stimulation without exposing the patient to central-line risks. Timing in critical illness is not an automatic “start PN immediately” decision. ASPEN found no meaningful difference in outcomes or harm between exclusive PN and exclusive enteral nutrition during the first week of critical illness when adequate enteral delivery was not possible. Supplemental PN added to inadequate enteral feeding is not recommended before ICU day 7 because earlier supplementation has not demonstrated clinically important benefit. A patient at low nutrition risk who cannot tolerate enteral nutrition can generally wait about 7 days before PN is started....