11.1 MNT in Critical Illness, Sepsis, Trauma, and Burns

Critical illness, major surgery, trauma, and burns push metabolism into a stress state very different from simple starvation. These questions test the timing of feeding, how to estimate needs, and why overfeeding is dangerous.

Sepsis, SIRS, and the Biphasic Metabolic Response

The metabolic response to critical injury, sepsis, and systemic inflammatory response syndrome (SIRS) was first systematically described by Sir David Cuthbertson. It is characterized by two distinct chronological phases: the Ebb Phase and the Flow Phase.

Trauma / Sepsis Event
       │
       ▼
[EBB PHASE: 24–48 Hours]
- Hypovolemic shock, blunted cardiac output
- Tissue hypoperfusion and cellular hypoxia
- Decreased BMR and body temperature
- Epinephrine, norepinephrine, cortisol surge
- PRIORITY: Hemodynamic resuscitation (fluids, pressors)
       │
       ▼ [Resuscitation Achieved: MAP >= 65 mmHg]
[FLOW PHASE: Days to Weeks]
- Hyperdynamic state (increased cardiac output)
- Elevated BMR and hyperthermia
- Accelerated skeletal muscle proteolysis (negative N balance)
- Profound peripheral insulin resistance (stress hyperglycemia)
- Hepatic acute-phase reactant synthesis (CRP, ferritin)
- PRIORITY: High-protein metabolic support, glucose control

Ebb Phase vs. Flow Phase Comparison

Energy Estimation in Critical Illness

• Indirect Calorimetry (IC): The undisputed gold standard for quantifying resting energy expenditure in mechanically ventilated patients. IC measures oxygen consumption ($\text{VO}_2$) and carbon dioxide elimination ($\text{VCO}_2$) to calculate REE via the Weir Equation: $\text{REE} = [3.941 \times \text{VO}_2(\text{L/min}) + 1.106 \times \text{VCO}_2(\text{L/min})] \times 1{,}440\text{ min/day}$
• Predictive Equations: Standard formulas (Mifflin-St. Jeor, Penn State, Harris-Benedict) frequently exhibit error rates of 30% to 50% in ICU populations due to fluid shifts, sedatives, paralytics, and fluctuating organ dysfunction.

Hypocaloric High-Protein Feeding (Permissive Underfeeding)

For critically ill patients with obesity (BMI $\ge 30\text{ kg/m}^2$), ASPEN/SCCM guidelines recommend intentional hypocaloric high-protein feeding (trophic or permissive underfeeding is also acceptable early in ARDS):

• Caloric Target:
  • BMI 30-50: $11\text{ to }14\text{ kcal/kg actual weight/day}$
  • BMI above 50: $22\text{ to }25\text{ kcal/kg IBW/day}$
• Protein Target:
  • BMI $30\text{ to }39.9\text{ kg/m}^2$: $\ge 2.0\text{ g/kg IBW/day}$
  • BMI $\ge 40\text{ kg/m}^2$: up to $2.5\text{ g/kg IBW/day}$

Rationale for Permissive Underfeeding

Full-calorie feeding in critically ill patients during the acute flow phase does not stop endogenous muscle proteolysis, because catabolism is driven by inflammatory cytokines and cortisol rather than substrate deficit. Overfeeding leads to life-threatening complications:

1. Hyperglycemia and Infection Risk: Excess exogenous glucose exacerbates stress-induced insulin resistance, impairing neutrophil phagocytosis.
2. Hypercapnia: Excess carbohydrate oxidation elevates carbon dioxide production ($\text{VCO}_2$), increasing the Respiratory Quotient ($RQ > 1.0$), which places excessive ventilatory load on compromised lungs and prevents successful weaning from mechanical ventilation.
3. Hepatic Steatosis: Excess glucose is shunted into de novo lipogenesis in the liver.
4. Azotemia and Fluid Retention: Fluid overload and accumulation of nitrogenous waste.

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Severe Thermal Burns

Major thermal burn trauma represents the most extreme hypermetabolic and hypercatabolic challenge encountered in clinical medicine. In burns exceeding $40\%$ Total Body Surface Area (TBSA), metabolic rates can surge up to $100\%\text{ to }200\%$ above normal basal expenditure ($2\times\text{ BMR}$).

Pathophysiology of Burn Hypermetabolism

• Massive Neuroendocrine Discharge: Massive release of catecholamines (epinephrine, norepinephrine), cortisol, and glucagon drives sustained glycogenolysis, lipolysis, and muscle proteolysis.
• Evaporative Water and Heat Loss: Destruction of the cutaneous stratum corneum barrier causes massive transdermal fluid evaporation. Because evaporating 1 mL of water consumes approximately 0.58 kcal of latent heat, burn patients suffer continuous heat loss, driving shivering and non-shivering thermogenesis.
• Massive Exudative Protein Loss: Open burn wounds continuously weep albumin, immunoglobulins, and trace elements, resulting in daily urinary and wound nitrogen losses exceeding 20–30 g/day.

Assessment of Burn Surface Area: The Wallace Rule of Nines

For adult burn assessment, the body surface area is divided into anatomical multiples of nine:

Energy Estimation: The Curreri Formula

The Curreri Formula is the classic mathematical equation used in NDLE examinations to calculate daily caloric requirements in severe thermal burns:

$\text{Curreri Energy (kcal/day)} = (24\text{ kcal} \times W) + (40\text{ kcal} \times \% \text{TBSA})$

Where:

• $W$ = Pre-injury actual body weight in kilograms (kg)
• $\% \text{TBSA}$ = Percentage of total body surface area burned, expressed as a whole number (e.g., 40 for 40% TBSA)

Clinical Limitation & The Burn Plateau: Because hypermetabolism plateaus when burns reach 50% to 60% TBSA, calculations typically cap the $\% \text{TBSA}$ at 50% to avoid gross overfeeding. Where available, metabolic cart indirect calorimetry remains the gold standard.

Protein Requirements in Major Burns

• Prescription: $1.5\text{ to }2.5\text{ g/kg/day}$ (constituting 20% to 25% of total caloric intake).
• Calorie-to-Nitrogen Ratio: The Non-Protein Calorie to Nitrogen ratio ($\text{NPC:N}$) should be narrowed to $80:1\text{ to }100:1$ to optimize nitrogen retention, promote granulation tissue formation, and support graft take.

Micronutrient Pharmacology in Burn Management

Trace element and vitamin losses through burn exudates are immense. Standard multivitamin dosages are grossly inadequate:

1. Vitamin C (Ascorbic Acid):
   • Dose: 500 mg twice daily (BID) (or 1,000 mg/day IV/oral).
   • Physiological Role: Essential coenzyme for prolyl and lysyl hydroxylase, which hydroxylate proline and lysine residues in nascent collagen fibers. Hydroxyproline stabilizes the collagen triple helix, while hydroxylysine enables cross-linking, providing tensile strength to healing skin grafts and granulating wound beds.
2. Vitamin A:
   • Dose: 10,000 IU/day for 7 to 10 days.
   • Physiological Role: Enhances epithelial cell differentiation, stimulates collagen synthesis, and reverses the suppressive effects of systemic corticosteroids on wound healing.
3. Zinc Sulfate:
   • Dose: 220 mg/day (providing approximately 50 mg of elemental zinc).
   • Physiological Role: Essential catalytic cofactor for DNA polymerase, RNA polymerase, and matrix metalloproteinases. Accelerates re-epithelialization and immune cell proliferation.
   • Clinical Caution: High-dose zinc induces intestinal mucosal metallothionein, which binds copper with high affinity, preventing copper absorption and precipitating severe copper-deficiency microcytic anemia and neutropenia. Limit zinc supplementation to 14–21 days unless active deficiency is confirmed.
4. Copper and Selenium: Exudative losses through open burn blisters deplete circulating copper and selenium. Supplementation (copper 4 mg/day, selenium 300–500 $\mu\text{g}$/day) is essential to preserve antioxidant enzyme activity (superoxide dismutase and glutathione peroxidase) and protect against nosocomial infection.