7.3 Nutritional Support in the Critically Ill

Physiological Importance of Nutrition in Critical Illness

Critical illness triggers a systemic inflammatory response characterized by severe hypermetabolism and catabolism. Hormonal surges (cortisol, catecholamines) and inflammatory cytokines drive the breakdown of skeletal muscle protein and fat stores to provide substrates for gluconeogenesis.

Respiratory Consequences of Malnutrition

Malnutrition directly impairs the respiratory system, presenting significant challenges for the mechanically ventilated patient:

1. Diaphragmatic Atrophy: Severe catabolism combined with ventilator-induced diaphragmatic inactivity (disuse atrophy) leads to rapid loss of diaphragmatic muscle mass. Studies demonstrate a significant decrease in diaphragmatic thickness within 48 hours of mechanical ventilation in malnourished patients.
2. Impaired Ventilatory Drive: Protein-calorie malnutrition blunts the hypoxic and hypercapnic ventilatory drives.
3. Loss of Surfactant: Malnutrition impairs the synthesis of pulmonary surfactant, leading to micro-atelectasis and worsened $V/Q$ mismatch.
4. Impaired Immunity: Malnutrition depresses cell-mediated immunity and secretory IgA production, increasing susceptibility to nosocomial infections like Ventilator-Associated Pneumonia (VAP).

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Enteral Nutrition (EN): Protocols and Advantages

Current guidelines recommend initiating Enteral Nutrition (EN) within 24 to 48 hours of ICU admission in hemodynamically stable patients who cannot maintain oral intake.

Route of Administration

• Gastric Feeding (NG/OG): The standard route. Easy to place and allows natural digestive processes.
• Post-Pyloric Feeding (Nasojejunal - NJ): The feeding tube is advanced past the pylorus into the duodenum or jejunum. This is indicated for patients with severe gastroparesis, high gastric residual volumes ($> 500$ mL), or a history of recurrent micro-aspiration.

Physiological Advantages over Parenteral Nutrition

The gut is a major immunological organ. Bypassing it carries significant risks:

• Maintenance of Mucosal Integrity: Enterocytes (lining the intestinal mucosa) derive their nutrients directly from the gut lumen. Without enteral nutrition, the intestinal villi atrophy, and the "tight junctions" between enterocytes break down.
• Prevention of Bacterial Translocation: Breakdown of the mucosal barrier allows enteric bacteria and endotoxins to cross into the portal circulation and lymphatic system, triggering a systemic inflammatory response and multi-organ failure.
• Immunological Support: EN stimulates the Gut-Associated Lymphoid Tissue (GALT), which produces secretory IgA, the primary defense against bacterial adherence in both the gut and respiratory tract.

Aspiration Risk Mitigation

The primary risk of EN is pulmonary aspiration of gastric contents. The RT and nursing staff must implement strict protocols:

• Maintain head of the bed (HOB) elevation at 30 to 45 degrees.
• Turn off feeding infusions during repositioning, chest physiotherapy, or transport.
• Monitor for signs of intolerance (abdominal distension, vomiting).

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Parenteral Nutrition (PN): Indications and Risks

Parenteral Nutrition (PN) bypasses the gastrointestinal tract, delivering nutrients directly into the venous system.

Clinical Indications

PN is indicated only when the gastrointestinal tract is non-functional or inaccessible. Examples include:

• Complete mechanical bowel obstruction or severe paralytic ileus.
• Severe bowel ischemia or high-output enterocutaneous fistulas.
• Hemodynamic instability requiring high-dose vasopressor support (due to the risk of non-occlusive mesenteric ischemia).

Complications

• Central Line-Associated Bloodstream Infections (CLABSI): High-glucose TPN solutions provide an ideal medium for bacterial growth.
• Severe Hyperglycemia: Impairs neutrophil function, worsening infection rates.
• Hepatic Steatosis: Excess glucose delivery can lead to fatty liver and cholestasis.

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Metabolic Complications and Ventilatory Weaning

Refeeding Syndrome

Refeeding syndrome is a potentially fatal shift in fluids and electrolytes that occurs when nutrition is reintroduced to a chronically malnourished patient (e.g., severe anorexia, chronic alcoholism, or prolonged starvation).

• Pathophysiology: During starvation, the body relies on fat and ketone metabolism. Upon reintroducing carbohydrates, insulin secretion surges. Insulin stimulates the sodium-potassium-ATPase pump, driving glucose, potassium, magnesium, and phosphorus out of the extracellular space and into the cells.
• Hypophosphatemia: The hallmark of refeeding. Phosphorus is essential for ATP production. Severe hypophosphatemia ($< 1.5$ mg/dL) deprives the diaphragm of energy, causing sudden diaphragmatic failure and weaning failure.
• Hypokalemia and Hypomagnesemia: Can trigger fatal arrhythmias (e.g., Torsades de Pointes).
• Prevention: Start feeding slowly (e.g., 25% of goal calories) and aggressively replete electrolytes before starting nutrition.

Overfeeding and Carbon Dioxide Production

Overfeeding, particularly with carbohydrates, increases the body's carbon dioxide production, which can cause respiratory acidosis and weaning failure.

• Respiratory Quotient (RQ): The ratio of $CO_2$ produced to $O_2$ consumed ($RQ = \dot{V}CO_2 / \dot{V}O_2$).
  • Carbohydrates: $RQ = 1.0$ (highest $CO_2$ production).
  • Proteins: $RQ = 0.8$.
  • Lipids: $RQ = 0.7$ (lowest $CO_2$ production).
  • Lipogenesis (Overfeeding): $RQ = 1.0$ to $1.3$ (excess carbohydrates are converted to fat, releasing massive amounts of $CO_2$).
• Weaning Trap: In patients with severe COPD or limited ventilatory reserve, the excess $CO_2$ load generated by overfeeding forces them to increase their minute ventilation. If they cannot meet this demand, they develop respiratory acidosis and fail weaning.
• Action: If a patient fails weaning with an unexplained high minute ventilation and $PaCO_2$ retention, perform indirect calorimetry to measure RQ. If the RQ is $> 1.0$, reduce total calories and increase the lipid-to-carbohydrate ratio.