What is nutritional support in critical care?

Nutritional Support in Critical Care

Nutritional support in critical care is the provision of enteral or parenteral nutrition to critically ill patients to prevent malnutrition, preserve lean body mass, and support recovery, with enteral nutrition preferred as the first-line route initiated within 24-48 hours of ICU admission when hemodynamically feasible. 1

Core Principles and Metabolic Context

Critical illness triggers profound metabolic derangements characterized by:

• Catabolism and hypermetabolism with accelerated breakdown of lean muscle tissue to provide substrates for the acute stress response 1
• Insulin resistance and altered substrate utilization that evolves through distinct phases of illness 1
• Endogenous glucose production that is elevated during the acute phase, reducing the body's ability to utilize exogenous nutrition 1

The metabolic response progresses through three phases: the "ebb" phase (acute early, ICU days 1-2) with hemodynamic instability and lower energy expenditure; the "flow" phase (acute late, ICU days 3-7) with intense catabolism; and the recovery phase (after day 7) when anabolic processes can resume 1. Nutritional strategies should align with these metabolic phases, as aggressive feeding during acute phases may cause harm rather than benefit. 1

Route of Nutrition Delivery

Enteral Nutrition (Preferred)

Enteral nutrition is the preferred method of nutritional support for all critically ill patients who can tolerate it. 1

The superiority of enteral nutrition is based on:

• Maintenance of gut mucosal integrity and reduced intestinal permeability 1
• Lower rates of infectious complications compared to parenteral nutrition (22.8% vs 26.2%, p=0.008) 1
• Reduced stress ulcers and protection of gastrointestinal function 1
• Lower costs and fewer catheter-related complications 1
• Shorter duration of mechanical ventilation and reduced ICU length of stay 1, 2

Gastric feeding is as safe as post-pyloric feeding for most critically ill patients, with post-pyloric routes reserved only for those at high aspiration risk or requiring frequent procedural fasting 2. Routine measurement of gastric residual volumes is not recommended 2.

Parenteral Nutrition (When Enteral Fails)

Parenteral nutrition should be used when:

• Enteral nutrition is contraindicated (mesenteric ischemia, mechanical bowel obstruction) 1
• Enteral nutrition cannot be tolerated for a prolonged period 1

Critical timing consideration: Withholding parenteral nutrition for up to one week is recommended in critically ill patients who cannot achieve adequate enteral intake, provided micronutrients are supplied. 2 This recommendation is based on the landmark EPaNIC trial showing that late initiation of PN (day 8) versus early initiation (within 48 hours) resulted in faster ICU discharge (HR 1.06, p=0.04), fewer infections, reduced cholestasis, and shorter mechanical ventilation duration 1.

Timing of Nutrition Initiation

Enteral nutrition should be initiated within 24-48 hours of ICU admission for all eligible patients. 1, 2

Early enteral nutrition (within 24-48 hours) is associated with:

• Shorter duration of mechanical ventilation 2
• Lower rates of nosocomial infection 2
• Reduced mortality 2

Special Populations - Hemodynamically Unstable Patients

Once hemodynamically stabilized, early enteral nutrition is recommended even for patients receiving vasoactive medications, extracorporeal life support (ECMO/ECLS), or post-cardiac surgery. 2 The key is stabilization, not complete resolution of hemodynamic support. Delayed enteral nutrition is warranted only in patients with severe, uncontrolled shock 3.

Energy and Protein Targets

Energy Prescription

Target less than 100% of measured or estimated energy expenditure during the acute early and acute late phases (ICU days 1-7) due to endogenous glucose production. 1

Recent high-quality evidence challenges traditional aggressive feeding:

• The TARGET trial (3,957 patients) showed that augmented energy delivery (~30 kcal/kg/day, 50% higher than routine care) did not improve 90-day mortality or any secondary outcomes 1
• Hypocaloric and trophic feeding strategies showed no benefit over standard care when provided early in critical illness 1
• Overfeeding during acute phases may cause harm, particularly when substantial energy is provided during periods of decreased energy expenditure and enhanced endogenous production 1

Indirect calorimetry is the gold standard for measuring energy expenditure, but predictive equations remain widely used despite significant inaccuracies, especially at extremes of weight, in severely ill patients, and in older or malnourished populations 1. When indirect calorimetry is unavailable, VCO2-based estimation from the ventilator (REE = VCO2 × 8.19) or VO2 from pulmonary artery catheters may provide better accuracy than traditional predictive equations 1.

Protein Prescription

Protein intake should be advanced toward 1.2 g/kg/day after hemodynamic stabilization, though optimal timing remains unclear 3. For pediatric patients, targets of 3.5-4 g/kg/day are recommended during transition and early enteral phases 4.

Feeding Advancement and Protocols

Enteral feeds should be advanced stepwise using a standardized feeding protocol until prescribed goals are met. 2

Feeding protocols provide:

• Improved time to enteral nutrition initiation 2
• Increased overall nutrient intake 2
• Reduced adverse events such as feeding intolerance in high-risk populations 2

Achieving at least two-thirds of prescribed nutrient goals during the first week is associated with better clinical outcomes. 2

Immunonutrition and Specialized Formulas

Immune-enhancing formulas (containing arginine, nucleotides, and fish oil) may reduce length of stay and infection rates in septic patients, but effects on mortality remain unproven. 1 One trial showed reduction in ICU stay and infections without mortality benefit 1, while another reported mortality reduction from 32% to 19% and infection reduction from 20% to 7% 1. However, immunonutrition should not be routinely provided to all critically ill patients 3.

Nutritional Assessment and Monitoring

Initial Assessment

All critically ill patients should have nutritional status assessed within 24-48 hours of ICU admission using anthropometric measurements expressed as z-scores. 2

Required measurements include:

• Weight and height/length with BMI-for-age z-scores (or weight-for-length z-scores for children <2 years) 2
• Head circumference in children younger than 36 months 2
• Mid-upper-arm circumference (MUAC) as part of the initial anthropometric panel 2

All anthropometric data must be expressed as z-scores rather than percentiles for greater precision. 2

Comprehensive Assessment for At-Risk Patients

Patients identified as at-risk require full nutritional assessment within 48 hours, including:

• Medical and dietary history focusing on recent weight loss percentage and duration/severity of reduced intake 2
• Physical examination for muscle wasting, subcutaneous fat loss, and edema 2
• Evaluation of underlying disease severity 2

Ongoing Monitoring

Nutritional status should be re-evaluated at least weekly throughout the ICU stay. 2 Muscle wasting occurs rapidly in critically ill patients and requires continuous assessment 1, 2.

Clinical Impact of Malnutrition

Malnutrition affects 15-25% of critically ill children at PICU admission and up to 40% of hospitalized children with chronic conditions 2. Malnourished critically ill patients experience:

• Increased morbidity including infections, weakness, and prolonged mechanical ventilation 1
• Longer hospital stays 1, 2
• Increased mortality 1, 2
• Compromised wound healing and immune dysfunction 1

Critical Pitfalls to Avoid

• Do not initiate early parenteral nutrition (within 48 hours) unless enteral nutrition is absolutely contraindicated; wait until day 8 if enteral nutrition is inadequate 1
• Do not aggressively feed during the acute phase (ICU days 1-7) targeting 100% of energy expenditure, as this may cause harm without benefit 1
• Do not rely solely on predictive equations for energy estimation in patients at extremes of weight, severely ill, or malnourished; use indirect calorimetry when available 1
• Do not postpone enteral nutrition in hemodynamically stable patients receiving vasoactive support or ECMO 2
• Do not use routine prokinetic agents due to insufficient evidence of benefit 2
• Do not use adult screening tools (NRS-2002, MUST, MNA) in pediatric patients 2
• Do not rely on BMI alone to identify at-risk patients; use comprehensive anthropometric assessment 2