What is the metabolic response to injury and how is it managed?

The Metabolic Response to Injury: Afferent, Neuromuscular, and Efferent Components

The metabolic response to injury is a complex, coordinated physiological process involving afferent signals, central processing, and efferent responses that mobilize substrates to support healing and immune function while significantly impacting morbidity and mortality outcomes.

Phases of Metabolic Response to Injury

The metabolic response to injury follows a distinct pattern with specific phases:

1. Ebb Phase (Afferent Component)

• Timing: Minutes to hours post-injury 1
• Characteristics:
  • Decreased energy expenditure compared to pre-injury state 2
  • Haemodynamic instability with prioritization of blood flow to vital organs 2
  • Hormonal changes including insulin resistance 2
  • Afferent signals from injury site via:
    • Neural pathways (pain signals, inflammatory mediators)
    • Humoral pathways (cytokines, stress hormones) 2

2. Flow Phase (Neuromuscular Component)

• Timing: Days to weeks post-injury 1
• Characteristics:
  • Hypermetabolism with increased energy expenditure (20-50% above baseline) 1
  • Systemic Inflammatory Response Syndrome (SIRS) 2
  • Catabolism of glycogen, fat, and protein 2
  • Release of glucose, free fatty acids, and amino acids into circulation 2
  • Diversion of substrates from peripheral protein maintenance to healing and immune response 2
  • Central processing in hypothalamus and brainstem coordinates metabolic changes

3. Recovery/Anabolic Phase (Efferent Component)

• Timing: After 7+ days, depending on injury severity 2
• Characteristics:
  • Resynthesis of lost tissue 2
  • Improved metabolic processing of delivered nutrients 2
  • Efferent responses include:
    • Hormonal changes (insulin sensitivity improves)
    • Decreased catabolism
    • Rebuilding of peripheral protein mass 2

Metabolic Changes During Injury Response

Carbohydrate Metabolism

• Insulin resistance develops despite hyperinsulinemia 2
• Increased hepatic glucose production via glycogenolysis and gluconeogenesis 2
• Decreased peripheral glucose utilization 3
• Hyperglycemia is common and associated with worse outcomes 1

Protein Metabolism

• Accelerated skeletal muscle breakdown 2
• Amino acid release for:
  • Gluconeogenesis (alanine, glutamine) 2
  • Acute phase protein synthesis 2
  • Wound healing 2
• Negative nitrogen balance persists despite nutritional support 2
• Muscle loss is a significant burden for functional recovery 2

Lipid Metabolism

• Enhanced lipolysis and lipid oxidation 2
• Mobilization of free fatty acids as energy substrates 2
• Decreased glucose oxidation as a protein-sparing mechanism 2

Management of the Metabolic Response to Injury

Nutritional Support

• Early Phase (ICU day 1-2):

  • Initiate enteral nutrition within 24-72 hours when possible 1
  • Avoid overfeeding as the body may not process nutrients effectively 2
  • Target 70-80% of estimated requirements 2

• Late Acute Phase (ICU day 3-7):

  • Gradually increase nutritional support 2
  • Monitor for tolerance and metabolic complications 1

• Recovery Phase (after ICU day 7):

  • Provide full nutritional support 2
  • Protein requirements: 1.5-2.0 g/kg/day for general trauma 1
  • Energy requirements: 1.4-1.5 times measured resting energy expenditure 1

Glucose Control

• Maintain blood glucose within target range using insulin therapy 3
• Avoid excessive carbohydrate administration to prevent hyperglycemia 1
• Monitor for insulin resistance which is characteristic of the stress response 2

Adjunctive Therapies

• Physical activity/rehabilitation to minimize muscle loss 2
• Consider addressing non-nutritional factors that may impair recovery:
  • Diminished cardio-respiratory function
  • Anemia
  • Acute and chronic intoxications
  • Anti-inflammatory medications 2

Monitoring and Assessment

• Indirect calorimetry (gold standard) for measuring energy expenditure 1
• Nitrogen balance to assess protein catabolism 1
• Regular monitoring of electrolytes and glucose 1
• Assessment of functional recovery as the primary outcome measure 2

The metabolic response to injury represents a critical survival mechanism that diverts resources toward healing and immune function at the expense of peripheral tissues. Understanding this complex process with its afferent signals, central processing, and efferent responses is essential for optimizing patient care and improving outcomes following injury.