Nutrition Support: Enteral and Parenteral Nutrition
This domain tests the mechanics of delivery: route selection, initiation and advancement, formula choice, access-device care, and the transition between enteral nutrition, parenteral nutrition, and oral intake. Answers hinge on whether the gut is usable and whether the patient is hemodynamically safe to feed.
Enteral nutrition rules
- Begin enteral nutrition within 24–48 hours in critically ill adults unable to eat; advance toward goal over 48–72 hours.
- Routine gastric residual volume checks are not recommended; when checked, a 500 mL threshold with clinical assessment guides holding feeds.
- Keep the head of bed 30–45 degrees and provide daily oral care; use a post-pyloric tube for gastroparesis, high aspiration risk, or severe pancreatitis when gastric feeding is failing.
- Standard polymeric formulas suit most patients; elemental or semi-elemental formulas are reserved for malabsorption, short bowel, or chyle leak.
Parenteral nutrition rules
- Use parenteral nutrition when enteral feeding is contraindicated or cannot meet needs. In previously well-nourished patients, delay until about day 7; in high-risk or severely malnourished patients, start within 3–7 days.
- Peripheral parenteral nutrition is limited by osmolarity, roughly ≤900 mOsm/L; lipid emulsion helps buffer tonicity but volume limits still apply.
- Three-in-one admixtures carry instability risks: calcium-phosphate precipitation and lipid destabilization. Never add medications directly to a parenteral nutrition bag.
- Dedicate a single lumen for parenteral nutrition, use a standardized bundle, and monitor for catheter-related bloodstream infection.
Worked example
A 70-year-old man after Whipple resection has an ileus on postoperative day 3 and a body mass index of 18.5 kg/m². Because he is already malnourished, parenteral nutrition starts rather than waiting a week. Energy is set at 25 kcal/kg with 1.3 g/kg protein, glucose infusion kept below 5 mg/kg/min, and 20% lipid provided twice weekly. Feeds are delivered over 24 hours initially; once ileus resolves, enteral feeding restarts at 20 mL/h and the parenteral rate is reduced proportionally to avoid overfeeding.
Traps
Continuing parenteral nutrition at full rate while enteral feeds advance; forgetting to add lipid-soluble vitamins and trace elements; using strict residual checks that interrupt feeding unnecessarily; and stopping enteral nutrition for diarrhea without investigating formula osmolality, sorbitol-containing medications, or Clostridioides difficile.