ICU Nutrition and Refeeding

In depth

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Metabolism of critical illness: why less is more early

Critical illness runs through an early acute phase (roughly days 1–2), a late acute phase (to about day 7) and then recovery or persistent critical illness (ESPEN 2019 and 2023). Cortisol, catecholamines, glucagon and cytokines drive gluconeogenesis and muscle breakdown while insulin resistance blocks glucose uptake. Endogenous energy production supplies 500–1,400 kcal/day that feeding does not switch off (ESPEN 2023), so a full prescription in the first days lands on top of energy the patient is already producing.

Overfeeding shows as hyperglycemia, extra CO₂ to clear, high triglycerides and fatty liver. Early full feeding may also blunt autophagy, the cell's recycling of damaged proteins: in an EPaNIC substudy of 600 patients (Hermans 2013), delaying supplemental PN to day 8 cut ICU-acquired weakness from 43% to 34%.

Muscle is the protein store being spent: rectus femoris cross-sectional area fell 17.7% by day 10 in 63 ventilated patients, faster with multiorgan failure, and muscle protein synthesis was depressed on day 1 (Puthucheary, JAMA 2013). Anabolic resistance may explain why extra early protein has not improved outcomes in trials; protein and mobilization likely matter more in recovery.

Enteral nutrients maintain the mucosa, tight junctions and gut immune tissue, the case for using the gut. Digestion also raises splanchnic oxygen demand: in low-flow shock a full feed can outstrip supply and cause nonocclusive bowel ischemia (NUTRIREA-2, below).

Screening, energy and protein targets

ESPEN 2023 counts every patient in the ICU beyond 48 h as at risk of malnutrition. NRS 2002 ≥5 marks high nutrition risk; the modified NUTRIC score (age, APACHE II, SOFA, comorbidities, hospital days before ICU) uses ≥5 of 9. Malnutrition is diagnosed from weight loss, low BMI or low muscle mass plus a cause (GLIM). Resuscitation fluid hides weight loss, so ask the family about intake and weight before admission.

Indirect calorimetry is the reference (ESPEN 2023); equations can miss by up to 60%, and ventilator VCO₂ × 8.19 does better than equations when calorimetry is unavailable. Full weight-based targets are 20–25 kcal/kg/day (ESPEN) or 25–30 (ASPEN/SCCM 2016). In the first 7–10 days ASPEN 2022 suggests 12–25 kcal/kg/day; ESPEN 2023 gives at most 70% of expenditure early, then up to 80–100% of measured expenditure after day 3. Protein: 1.2–2.0 g/kg/day (ASPEN/SCCM 2016, kept in 2022) or 1.3 g/kg/day reached progressively (ESPEN 2023).

Calories from drugs count (ASPEN 2022, ESPEN 2023). Propofol is a 10% lipid emulsion with 1.1 kcal and 0.1 g of fat per mL (label): 30 mcg/kg/min in a 70 kg patient runs 12.6 mL/h, about 330 kcal and 30 g of fat a day. Clevidipine carries 2.0 kcal/mL (label limit 1,000 mL per 24 h), D5W 170 kcal/L and CRRT citrate a few hundred kcal/day. ESPEN caps IV lipid, propofol included, at 1.5 g/kg/day, and the propofol label calls for less lipid in PN.

Enteral access: placement and verification

Large-bore sump tubes (14–18 Fr) decompress the stomach and can feed short term; small-bore 8–12 Fr tubes placed with a stylet suit longer feeding but collapse under suction, so aspirate is often hard to obtain and residual volumes through them are unreliable. Use the oral route with basilar skull or severe midface fractures. Post-pyloric tubes go in blind, with electromagnetic guidance, under fluoroscopy or by endoscopy; a gastrostomy or jejunostomy is considered when tube feeding will last beyond about 4–6 weeks.

A blind small-bore tube can pass into the trachea and lung, often without coughing in a sedated patient and despite a cuffed endotracheal tube. AACN 2016 asks for two or more bedside checks during insertion (signs of respiratory distress, capnography, aspirate pH, aspirate appearance), never the air-bolus or bubbling test, and a radiograph showing the whole course of every blindly placed tube, electromagnetically guided ones included, before first use.

• Mark the exit point at the nose or mouth and record it once the x-ray is read (AACN 2016).
• Check the mark every 4 h during feeding; re-image if it has moved, after vomiting or violent coughing, or whenever position is in doubt.
• Aspirate pH ≤5.5 supports gastric placement; acid suppression and continuous feeding raise pH, so a higher value does not prove misplacement.
• Never reinsert the stylet into a tube in the patient; unclog with warm water and gentle push-pull, then an enzyme declogger per protocol.

Delivering feeds and daily nursing care

Standard polymeric formulas carry about 1.0–1.2 kcal/mL; energy-dense 1.5–2.0 kcal/mL formulas suit fluid restriction, though delivering more calories that way did not lower deaths (TARGET 2018). Peptide formulas are for malabsorption and mixed-fiber formulas for persistent diarrhea (ASPEN care pathway). Continuous infusion is preferred to bolus, with less diarrhea (RR 0.42, ESPEN 2023). Start at 10–20 mL/h (ESICM 2017) and reach goal progressively, not before 48–72 h (ESPEN 2023).

Procedures, imaging, extubation and tube problems routinely cut delivery below the order. Volume-based protocols (ASPEN care pathway) set a 24 h volume and let the nurse raise the rate after an interruption, up to a protocol maximum. Hang closed systems up to 24–48 h, open-system formula 8 h and reconstituted powder 4 h (ASPEN).

Give drugs one at a time with water flushes before and after, never into the feed bag; do not crush extended-release or enteric-coated tablets, and remember that sorbitol in liquid drugs adds to diarrhea. Phenytoin, levothyroxine, fluoroquinolones and warfarin interact with feeds; follow pharmacy timing.

• Neuro and airway: sedation, cough and gag; swallow screen before eating after extubation (dysphagia in 10–67% across studies; ESPEN 2023).
• GI: abdomen, vomiting, flatus, stool count and form, last bowel movement; bladder pressure if intra-abdominal hypertension is a risk.
• Metabolic: glucose at least every 4 h for 2 days; K, Mg and phos daily in week 1 (ESPEN 2023), every 12 h with refeeding risk.
• Fluid: intake including flushes and drug diluents, output, daily weight.
• Skin: nares, perineum with loose stools, gastrostomy site.
• Delivery: prescribed vs delivered volume, the reason for every hold, calories and protein from all sources.
• Connections: trace the line from patient to bag before connecting; ENFit enteral connectors do not fit IV ports.

Shock, prone position, ECMO and other special cases

ASPEN/SCCM 2016 withholds EN while MAP is below 50 mm Hg or catecholamines are being started or escalated, and allows cautious feeding as pressors come down. ASPEN's 2022 consensus calls a vasopressor no contraindication with careful monitoring: gastric EN within 48 h if MAP is 60 or more, held below 50, trophic-only or held at high vasopressor doses, and held for residuals over 300 mL if measured (low-quality evidence). ESICM 2017 and ESPEN 2023 delay EN only while shock is uncontrolled, then start low-dose EN once fluids and vasopressors have controlled it; for septic shock ESPEN suggests 20–50% of full nutrition at first. NUTRIREA-2 shows why full doses wait: isocaloric EN in ventilated shock caused more vomiting (34% vs 20%), bowel ischemia (2% vs <1%) and colonic pseudo-obstruction (1% vs <1%) than PN, with no fewer infections.

Suspect bowel ischemia with distension, pain, rising lactate or pressor need, new GI bleeding, ileus or large gastric output: stop the feed and call the team.

Prone: ESICM 2017 advises against delaying EN for prone positioning. A protocol of 25° head-up tilt, faster advancement and erythromycin raised delivered volume from 774 to 1,170 mL/day without more vomiting or VAP (Reignier 2010). Units commonly pause feeds and vent the stomach before each turn (Prone card), then restart promptly.

Residuals, aspiration, intolerance and diarrhea

A residual mixes formula with saliva and gastric juice; the volume drawn back depends on tube tip, position and technique. REGANE (2010, 329 patients) raised the limit from 200 to 500 mL: more of the prescription was delivered in week 1 (88% vs 84%) with no more pneumonia. NUTRIREA-1 (2013, 452 patients) stopped measuring altogether against a 250 mL limit: VAP 16.7% vs 15.8%, more patients reached their calorie goal, and vomiting was more common.

ASPEN/SCCM 2016 therefore advises against routine residuals and, where they are still used, against holding feeds for less than 500 mL without other signs. ESPEN 2023 finds them useful while feeds start, delays EN above 500 mL/6 h and adds a prokinetic, and moves to post-pyloric feeding if large volumes persist.

Aspiration risk rises with an unprotected airway, ventilation, age over 70, reduced consciousness, neurologic deficits, reflux, supine position, poor oral care, low staffing, transport and bolus feeding (ESPEN 2023). Prokinetics (ESPEN 2023): IV erythromycin 100–250 mg three times daily first, metoclopramide 10 mg two or three times daily as the alternative, or both; benefit falls to a third by 72 h, so stop after 3 days. Erythromycin prolongs QT, interacts through CYP3A4 and is an antibiotic; metoclopramide carries a boxed warning for tardive dyskinesia.

Diarrhea is 3 or more loose or liquid stools a day with a stool weight over 200–250 g (or over 250 mL) a day (ESICM working group 2012). Keep feeding (ESICM 2017) while working through causes: laxatives and stool softeners, sorbitol or magnesium in liquid drugs, antibiotics, C. difficile (test for ≥3 unformed stools in 24 h without laxatives in 48 h; IDSA/SHEA 2017), bolus or contaminated feeds, malabsorption, ischemia and overflow around impaction. If it persists, try a mixed-fiber formula, then a peptide formula (ASPEN). Replace K, Mg and bicarbonate losses and protect the skin.

No stool for 3 or more days without obstruction is lower GI paralysis (ESICM working group 2012); start a bowel regimen early with opioids.

Parenteral nutrition and glucose control

ASPEN/SCCM 2016 withholds exclusive PN for 7 days in low-risk patients, starts it as soon as possible in high-risk or severely malnourished ones, and considers supplemental PN after 7–10 days if EN meets under 60% of goals. ASPEN 2022 recommends against supplemental PN before day 7 (strong; the trials excluded malnourished patients) and accepts PN or EN when calories match. ESPEN 2023 starts PN within 3–7 days when EN is contraindicated, allows early progressive PN in severe malnutrition, and weighs supplemental PN case by case in week 1 after every EN strategy has been tried.

Dextrose (3.4 kcal/g) should not exceed 5 mg/kg/min and lipid 1.5 g/kg/day including propofol (ESPEN 2023); amino acids give 4 kcal/g. ASPEN 2022 accepts soybean or mixed-oil emulsions; ESPEN prefers mixed emulsions and allows fish oil 0.1–0.2 g/kg/day. Vitamins and trace elements go in daily because PN contains none (ESPEN 2023); CRRT for more than 2 weeks can cause copper deficiency.

• Dedicated lumen of a central line; peripheral PN only at 900 mOsm/L or less (ASPEN).
• Filter all PN, lipid or not, through a 1.2-micron filter (ASPEN 2021 retired the 0.22-micron filter).
• Hang lipid alone up to 12 h and mixtures containing lipid up to 24 h (lipid labels); change lipid tubing within 24 h (CDC 2011).
• High-alert drug (ISMP): independent double-check.
• Glucose at least every 4 h at first (ESPEN 2023); triglycerides and liver tests at baseline, then weekly.

SCCM 2024 starts insulin for persistent glucose ≥180 mg/dL, advises against titrating to 80–139 rather than 140–200, prefers an IV infusion in the acute phase, and checks at least hourly while glucose is unstable; ESPEN 2023 gives insulin above 180 mg/dL. NICE-SUGAR (6,104 patients) found more 90-day deaths with 81–108 mg/dL (27.5% vs 24.9%). In shock, use arterial or venous samples on a blood gas or lab analyzer. When feeds stop with insulin running, cut or stop the insulin per protocol or give dextrose.

Refeeding syndrome

Starvation empties cells of phosphate, potassium, magnesium and thiamine while serum levels can stay normal. Carbohydrate from any source, IV dextrose included, raises insulin, which pulls phosphate into glycolysis and potassium in through Na/K-ATPase, so levels can fall within hours (ASPEN 2020). Low phosphate means less ATP and 2,3-DPG: diaphragm weakness and failure to wean, poor contractility, arrhythmias, hemolysis and seizures. Thiamine is a cofactor of carbohydrate metabolism; deficiency causes Wernicke encephalopathy, lactic acidosis and wet beriberi. ASPEN 2020 doubts volume overload is part of the syndrome, though edema can follow.

ESPEN 2023 calls phosphate under 0.65 mmol/L (about 2.0 mg/dL), or a fall of more than 0.16 mmol/L (about 0.5 mg/dL), refeeding hypophosphatemia: measure electrolytes 2–3 times daily and restrict energy for 48 h. The rule comes from Doig 2015 (339 patients, 13 ICUs): 20 kcal/h for at least 2 days, then 40 and 60 kcal/h and 80% of goal before full feeding. Days alive after ICU discharge did not differ significantly, but 60-day survival was 91% vs 78%.

ASPEN 2020 also lists higher-risk conditions: alcohol or drug use disorder, eating disorders, cancer, malabsorption, bariatric surgery, dysphagia, protracted vomiting, food insecurity, and prolonged fasting or major surgery without nutrition. NICE is more cautious: at most 10 kcal/kg/day, and 5 kcal/kg/day with BMI under 14 or negligible intake for over 15 days.

• Before feeding: K, Mg and phos; hold calories while any is severely low (ASPEN 2020).
• Thiamine 100 mg before feeding or dextrose-containing fluids, then 100 mg/day for 5–7 days or longer in severe starvation or alcohol use disorder; suspected Wernicke needs treatment doses (Sedation sheet).
• Multivitamin daily for at least 10 days, in the PN bag or enterally.
• 10–20 kcal/kg or 100–150 g dextrose for 24 h, counting IV dextrose and drug diluents; advance by a third of goal every 1–2 days.
• Labs every 12 h for 3 days; vital signs every 4 h for 24 h; telemetry if unstable; daily weight and I&O.
• Levels hard to correct or falling fast: cut calories by 50%, then re-advance; dose phosphate by weight and level (Electrolytes sheet).

The evidence and what is unsettled

What changed practice: less feeding in week 1, no routine residuals, a week's wait for supplemental PN, and dose mattering more than route. Still open: protein dose and timing after week 1, whether calorimetry-guided feeding helps, how to feed the malnourished patients most trials excluded, the pressor dose too high for trophic feeds, and whether nutrition plus exercise restores function after the ICU.

Exam pearls

• Start EN within 24–48 h once resuscitated; bowel sounds and flatus are not required.
• Hold EN for MAP <50 or escalating pressors; low-dose feeds once shock is controlled, watching for bowel ischemia.
• Week 1: give less (12–25 kcal/kg/day, ASPEN 2022); protein 1.2–2.0 g/kg/day.
• No supplemental PN before day 7 unless the patient is severely malnourished.
• Only a radiograph confirms a blindly placed tube; auscultation is unreliable; check the mark every 4 h.
• Head of bed 30–45°, continuous feeds, a prokinetic, then post-pyloric feeding for intolerance or high aspiration risk.
• Do not hold feeds for a residual under 500 mL or for diarrhea alone; look for drugs and C. difficile.
• Refeeding: thiamine first, start at 10–20 kcal/kg, and check phos, K and Mg every 12 h for 3 days.
• Propofol supplies 1.1 kcal/mL as fat; count it and check triglycerides.
• Insulin above 180 mg/dL; tight 81–108 control raised deaths in NICE-SUGAR.