What are the phases of the metabolic response to injury?

Metabolic Response to Injury Phases

The metabolic response to injury occurs in three distinct phases: the acute early phase (ebb phase), the acute late phase (flow phase), and the recovery phase (anabolic phase), each characterized by specific metabolic changes that significantly impact patient morbidity and mortality.

Ebb Phase (Acute Early Phase, ICU Day 1-2)

• Duration: Minutes to hours after injury
• Characteristics:
  • Decreased metabolic rate
  • Reduced oxygen consumption and temperature
  • Hemodynamic instability
  • Hormonal changes including insulin resistance
  • Endogenous glucose production
  • Lower energy expenditure compared to pre-injury state 1
• Metabolic Purpose: Prioritization of energy substrates to vital tissues as a survival mechanism 1
• Clinical Implications:
  • Aggressive nutrition in this phase may be harmful as endogenous production is already enhanced 1, 2
  • Focus should be on hemodynamic stabilization rather than nutritional optimization

Flow Phase (Acute Late Phase, ICU Day 3-7)

• Duration: Days to weeks
• Characteristics:
  • Elevated metabolic rate (hypermetabolism)
  • Increased protein catabolism with breakdown of skeletal muscle
  • Nitrogen loss in urine peaking during days 3-8 following injury
  • Tendency for water and sodium retention
  • Continued insulin resistance
  • Altered pathways of energy production 1, 3
• Metabolic Purpose: Breakdown of tissue to provide substrates for "fight or flight" response and reduce risk of bleeding and infection 1
• Clinical Implications:
  • Increased nutritional requirements but limited ability to utilize delivered nutrients
  • Higher protein requirements (1.2-2 g/kg/day) to minimize muscle loss 2
  • Weight generally increases due to fluid retention 1

Recovery Phase (Anabolic Phase, After ICU Day 7)

• Duration: Weeks to months
• Characteristics:
  • Anabolic metabolism predominates
  • Resynthesis of lost tissue
  • Excretion of excess water and sodium
  • Improved ability to process delivered nutrients 1
• Metabolic Purpose: Restoration of lean body mass and functional recovery
• Clinical Implications:
  • Increased energy and protein requirements to support anabolism
  • Weight typically decreases during post-acute phase and then increases during convalescence due to increase in fat-free mass 1
  • Energy intake of 1.4-1.5 times measured resting energy expenditure may be optimal 1

Nutritional Considerations Across Phases

• Acute Early Phase:

  • Withholding aggressive nutrition may be beneficial 1
  • Target less than 100% of energy expenditure due to endogenous production 2

• Acute Late Phase:

  • Increased nutritional requirements but persistent catabolism
  • Protein catabolism may not be fully counteracted by nutritional support 1

• Recovery Phase:

  • Nutrition becomes more physiologically available and important 1
  • Combined with physical exercise to rebuild peripheral protein mass 1

Common Pitfalls and Caveats

• There is no known clinical marker to identify when a patient shifts from one phase to another 1
• Traditional "stress factors" applied to predictive equations (e.g., +10% for each 1°C elevation of temperature) are speculative with little evidence for utility 1
• One-size-fits-all and set-and-forget approaches to nutrition do not adequately address the complex metabolic changes in critical illness 1
• Overfeeding in early phases can be harmful, while underfeeding in recovery phase may delay rehabilitation 2
• The need for nutritional therapy increases with duration of acute illness; if not systematically provided, life-threatening malnutrition will develop 1

Understanding these metabolic phases is crucial for optimizing nutritional support and improving outcomes in injured patients. Nutritional therapy should be dynamically adjusted according to the metabolic response and its individual phase 4.