Nutrition in Critical Illness
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Direct answer
Enteral nutrition started within 24 to 48 hours of ICU admission, advanced as tolerated toward 20 to 25 kcal per kg per day and 1.2 to 2 g per kg per day of protein, is the standard of care in critical illness — trophic feeds for the first few days in unstable shock, full targets within the first week, and supplemental parenteral nutrition only after about day seven if enteral delivery fails. The gut is used early because it maintains barrier function, and feeding is deliberately restrained early because overfeeding a catabolic patient causes harm.
What you must remember
- Timing: initiate enteral feeds within 24 to 48 hours in patients expected not to eat for over 3 days; hold only for uncontrolled shock, uncontrolled hypoxaemia or active bowel ischaemia.
- NUTRIREA-2 is the cautionary trial — early full-calorie enteral feeds in patients on vasopressors increased digestive ischaemia and colitis; trophic feeds of about 10 to 20 mL per hour while shock resolves are the compromise.
- Targets: 20 to 25 kcal per kg per day in the first week (roughly 70 to 80 per cent of requirements), 25 to 30 kcal per kg later; protein 1.2 to 2 g per kg per day, higher end for burns and continuous renal replacement therapy, which strips amino acids.
- Gastric residual volumes below 500 mL in 4 to 6 hours are not a reason to stop feeds — the number exists to prevent blind overfeeding, not to justify starvation; use prokinetics and post-pyloric delivery for intolerance.
- Supplemental parenteral nutrition is added after about day 7 if enteral delivery remains under 60 per cent of target — not earlier, as trials show early combination feeding adds infective risk.
- Refeeding syndrome: hypokalaemia, hypomagnesaemia, hypophosphataemia with thiamine depletion in the chronically malnourished; give thiamine 200 to 300 mg daily and replace electrolytes while feeding at quarter to half targets.
- Glutamine is not supplemented in shock, multi-organ failure or burns (REDOXS showed excess mortality); propofol carries 1.1 kcal per mL of lipid and must be counted.
Feeding a ventilated patient, day by day
Take a 55-year-old on day two of ventilation for septic shock, norepinephrine now at 0.1 micrograms per kg per minute, abdomen soft. Day two: trophic nasogastric feeds at 20 mL per hour of a standard 1 kcal per mL formula — roughly 300 to 400 kcal — while vasopressors taper. Day three: advance by 20 to 25 mL per hour each shift as tolerated, head of bed at 30 degrees, checking gastric residuals 4 to 6 hourly with an action threshold at 500 mL; add metoclopramide and erythromycin if residuals linger high, and move the tube post-pyloric before conceding to parenteral nutrition. Day five: she should be near 1400 to 1600 kcal and 90 to 100 g protein; if indirect calorimetry exists, prefer measured energy targets — 25 kcal per kg per day is the fallback arithmetic on actual or adjusted weight. Day eight, still under half delivered by gut: add supplemental parenteral nutrition to close the gap rather than forcing the gut. Throughout, glucose targets sit at 140 to 180 mg per dL, and every prescribing error — propofol lipid forgotten, "NPO for a morning scan" stacking into repeated 24-hour fasts — is a day of deficit that is never recovered.
Where students slip
Two reflexes betray candidates. The first is the old "keep nil by mouth until fully stable" — the evidence runs the other way, and NUTRIREA-2's lesson is titration, not abstinence. The second is quoting gastric residual cut-offs of 150 or 200 mL from decade-old practice; the modern threshold is 500 mL in 4 to 6 hours with feeds continued. Expect the viva to test edge cases: refeeding risk in the cachectic patient who must start slow on thiamine; glutamine harm in shock; the CRRT patient who needs extra protein, not less; and post-pyloric feeding, which improves delivery and reduces aspiration risk in proven intolerance but does not magically prevent ventilator-associated pneumonia at large.
Frequently asked questions
When should enteral feeding start in the ICU?
Within 24 to 48 hours of admission in patients not expected to eat for three or more days, as trophic feeds in resolving shock and full feeds as stability permits.
What energy and protein targets apply?
20 to 25 kcal per kg per day initially, rising to 25 to 30 kcal; protein 1.2 to 2 g per kg per day, higher with burns or continuous renal replacement therapy.
What gastric residual volume mandates action?
Above roughly 500 mL in a 4 to 6-hour period — with prokinetics and post-pyloric feeding, not automatic cessation; below that, feeds continue.
When is parenteral nutrition justified?
Supplementally after about day seven if enteral delivery remains under 60 per cent of target, or primary when the gut is unusable — obstruction, mesenteric ischaemia, high-output fistula.
How is refeeding syndrome prevented and treated?
Identify at-risk patients, feed at 25 to 50 per cent of target, give thiamine 200 to 300 mg daily, and correct potassium, magnesium and phosphate before advancing.
Why is glutamine avoided in shock and burns?
REDOXS found increased mortality with high-dose intravenous glutamine in multi-organ failure and burns; it is simply not prescribed in these groups.