Nutrition in critical illness

Surgical approach (if relevant)

• Not an operation; however, ICU nutrition often follows a procedural pathway (feeding access and GI evaluation).
  • Assess GI tract usability: bowel sounds not required; focus on haemodynamic stability, abdominal exam, output, imaging if concerned.
  • Establish access: NG/OG tube first-line; consider post-pyloric tube if high aspiration risk or persistent intolerance; consider PEG/PEJ if prolonged feeding anticipated.
  • If enteral route fails/contraindicated: central venous access for PN (or peripheral PN short-term with osmolarity limits).

Anaesthetic management (if relevant)

• Not a single anaesthetic procedure; considerations apply to ICU patients requiring feeding access (e.g., endoscopic PEG, post-pyloric placement) or surgery while on nutrition support.
  • Type of anaesthesia: usually sedation/GA for PEG or endoscopy; GA for laparotomy; regional rarely primary.
  • Airway: aspiration risk common; ETT often preferred if GA/sedation in high-risk patients; SGA less suitable if full stomach/ileus.
  • Duration: PEG ~30–60 min; post-pyloric placement variable; major surgery variable.
  • Pain: PEG mild–moderate; laparotomy severe; plan multimodal analgesia, consider epidural for major abdominal surgery if appropriate.
  • Peri-procedural nutrition: stop feeds pre-GA per local policy; consider prokinetics; manage glycaemia; continue electrolytes/thiamine if refeeding risk.

Aims and principles

• Goals: reduce catabolism, preserve lean body mass, support immune function and wound healing, maintain gut integrity, avoid overfeeding and complications.
• Enteral nutrition (EN) preferred over parenteral nutrition (PN) when GI tract functional: lower infection risk, maintains mucosal barrier, cheaper.
• Avoid early full-calorie feeding in acute phase of critical illness; consider trophic/low-dose EN initially and advance as stable.

Metabolic response to critical illness (why nutrition is different in ICU)

• Stress response phases: early acute (ebb/flow overlap), then catabolic flow phase; later anabolic recovery if illness resolves.
  • Neuroendocrine: ↑ catecholamines, cortisol, glucagon; relative insulin resistance.
  • Inflammation: cytokines (IL-1, IL-6, TNF-α) drive proteolysis, lipolysis, acute phase response.
• Consequences: hyperglycaemia, increased hepatic gluconeogenesis, muscle protein breakdown (negative nitrogen balance), altered lipid handling, micronutrient depletion.
• Overfeeding risks: hypercapnia (↑ CO2 production), hepatic steatosis/cholestasis, hyperglycaemia, increased infection risk, refeeding complications.

Assessment and when to start

• Assess nutrition risk and baseline state: weight/BMI trends, recent intake, functional status, comorbidities, frailty, sarcopenia, alcohol use, malabsorption, prior bariatric surgery.
• Screening tools: MUST (ward), NRS-2002; ICU tools include NUTRIC score (incorporates illness severity).
• Start EN early when feasible (often within 24–48 h of ICU admission) once haemodynamically stabilising; use caution with escalating vasopressors and signs of gut ischaemia.
  • Contraindications to EN: bowel ischaemia, uncontrolled shock, high-output proximal fistula without distal access, mechanical obstruction, abdominal compartment syndrome (relative), uncontrolled GI bleeding (relative).

Energy targets (calories) and how to estimate

• Best method: indirect calorimetry (if available) to measure resting energy expenditure; avoids under/overfeeding.
• If not available: weight-based estimates (institution-dependent). Common pragmatic approach: 20–25 kcal/kg/day early; 25–30 kcal/kg/day later/recovery; adjust for obesity and burns/trauma.
  • Obesity: consider ideal/adjusted body weight; avoid overfeeding; higher protein proportion.
  • Burns/major trauma: higher requirements; specialist input; consider calorimetry.
• Account for non-nutrition calories: propofol (1.1 kcal/mL as lipid), citrate (CRRT), dextrose infusions.

Protein targets (often the key macronutrient in ICU)

• Aim to attenuate muscle loss: typical target 1.2–2.0 g/kg/day protein (use ideal/adjusted body weight in obesity).
  • Higher targets often used in burns/trauma; lower may be required temporarily in severe hepatic encephalopathy (individualise).
• Nitrogen balance: can be estimated using urinary urea nitrogen (UUN) but limited by practicality and non-urinary losses; trend rather than single value.

Enteral nutrition (EN): practical ICU approach

• Route: NG/OG first-line; post-pyloric (nasojejunal) if high aspiration risk, gastroparesis, recurrent high gastric residuals with intolerance, pancreatitis (select cases).
• Feeding strategy: continuous infusion often better tolerated in ICU; start low (trophic) and advance every 4–12 h as tolerated to target.
• Gastric residual volumes (GRV): thresholds vary; interpret with clinical context (vomiting, distension, aspiration). Avoid reflexively stopping feeds for modest GRVs without symptoms.
• Prokinetics for intolerance: metoclopramide or erythromycin; consider post-pyloric feeding if persistent.
• Aspiration prevention: head-up 30–45°, oral care, appropriate tube position, consider post-pyloric route in high risk, minimise deep sedation when possible.
• Formula choice: standard polymeric feeds usually adequate; consider energy-dense if fluid restricted; consider renal/hepatic-specific formulas selectively (evidence mixed).

Parenteral nutrition (PN): indications and key points

• Indications: non-functional/inaccessible GI tract or EN contraindicated; failure to meet needs with EN over time in high-risk patients (local policy).
  • Examples: short bowel, high-output fistula without distal feeding option, prolonged ileus/obstruction, severe malabsorption, mesenteric ischaemia, severe GI bleeding where EN not possible.
• Access: central venous catheter preferred for full PN (osmolarity); peripheral PN limited and short-term.
• Composition: glucose + amino acids + lipids + electrolytes + vitamins/trace elements; tailor to organ dysfunction and labs.
• Complications: catheter-related bloodstream infection, thrombosis, hyperglycaemia, hypertriglyceridaemia, liver dysfunction (cholestasis/steatosis), electrolyte disturbances, refeeding syndrome.
• Monitoring: daily U&E, phosphate, Mg, glucose initially; triglycerides; LFTs; fluid balance; line care and asepsis.

Micronutrients and pharmaconutrition

• Thiamine: give before and during carbohydrate reintroduction in malnourished/alcohol-dependent patients to prevent Wernicke’s and contribute to refeeding prevention.
• Trace elements/vitamins: ensure daily provision in PN; in EN, standard feeds usually cover once at goal rate; consider extra in burns, CRRT, high-output losses.
• Glutamine: routine supplementation not recommended in general ICU (signal of harm in some populations); follow local/national guidance.
• Omega-3/immune-modulating formulas: evidence mixed; may be considered in selected surgical/ARDS contexts per local protocol but not universal.

Glycaemic control and nutrition

• Critical illness causes insulin resistance; both EN and PN can precipitate hyperglycaemia.
• Target glucose: avoid tight control; aim for moderate range per ICU policy (commonly ~6–10 mmol/L) balancing hypoglycaemia risk.
• Practical: insulin infusion protocols; consider reducing carbohydrate load if persistent hyperglycaemia; review sepsis/steroids/vasopressors.

Refeeding syndrome (high-yield viva topic)

• Definition: potentially fatal shifts in fluids/electrolytes after reintroduction of nutrition in malnourished patients; driven by insulin-mediated cellular uptake.
• Who is at risk: little/no intake for >5–10 days, significant weight loss, low BMI, alcohol misuse, anorexia, oncology, elderly/frail, post-bariatric, chronic malabsorption, prolonged diuretics/antacids.
• Biochemistry: hypophosphataemia (hallmark), hypokalaemia, hypomagnesaemia; thiamine deficiency; sodium/water retention → oedema, heart failure.
• Clinical: arrhythmias, cardiac failure, respiratory failure (diaphragm weakness), rhabdomyolysis, seizures, delirium, haemolysis.
• Prevention/management: give thiamine (and multivitamins) before feeding; start low calories (e.g., 10 kcal/kg/day or less if very high risk) and increase slowly; aggressive electrolyte replacement; close monitoring (phosphate/Mg/K daily or more).

Special situations

• Renal failure/CRRT: protein needs often higher with CRRT due to amino acid losses; monitor electrolytes closely; fluid restriction may require energy-dense feeds.
• Liver failure: avoid protein starvation; manage encephalopathy with standard measures (lactulose/rifaximin) rather than routine protein restriction; consider branched-chain AA in selected cases.
• Pancreatitis: EN preferred (gastric or jejunal depending on tolerance); PN if EN not possible; avoid prolonged starvation.
• ARDS/ventilation: avoid overfeeding (↑ CO2 production); permissive underfeeding early may be acceptable; ensure adequate protein.
• Post-op/major surgery: early EN where possible; consider enhanced recovery principles; treat ileus, minimise opioids, mobilise early.