4.2 Enteral & Parenteral Nutrition Support Basics

4.2 Enteral & Parenteral Nutrition Support Basics

When oral intake is inadequate, unsafe, or impossible, specialized nutrition support—comprising Enteral Nutrition (EN) and Parenteral Nutrition (PN)—must be initiated. The primary clinical mandate guiding support selection is: "If the gut works, use it!" Enteral nutrition preserves gut mucosal integrity, maintains gut-associated lymphoid tissue (GALT) immunity, prevents bacterial translocation, and carries lower infection rates and costs compared to parenteral administration.

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Access Selection & Route Determination

Selecting the appropriate feeding route depends on gastrointestinal function, anticipated duration of therapy, and aspiration risk.

Enteral Feeding Access Routes

• Short-Term Access (< 4–6 weeks):
  • Nasogastric (NG): Passed through the nose into the stomach. Requires intact gastric motility and normal gag reflex.
  • Nasoduodenal (ND) / Nasojejunal (NJ): Post-pyloric feeding passed beyond the ligament of Treitz. Indicated for severe gastroparesis, acute pancreatitis, high aspiration risk, or persistent gastric reflux.
• Long-Term Access (> 4–6 weeks):
  • Percutaneous Endoscopic Gastrostomy (PEG): Surgically or endoscopically placed directly into the stomach through the abdominal wall.
  • Percutaneous Endoscopic Jejunostomy (PEJ): Directly accesses the jejunum for long-term post-pyloric feeding.

Parenteral Feeding Access Routes

• Peripheral Parenteral Nutrition (PPN): Infused into small peripheral veins. Osmolality must remain ≤ 900 mOsm/L to prevent phlebitis and venous thrombosis. PPN is restricted to short-term support (< 10–14 days) in mild-to-moderately malnourished patients requiring low caloric density.
• Total Parenteral Nutrition (TPN): Infused into a large central vein (e.g., superior vena cava via subclavian or internal jugular catheter, or PICC line). Central placement accommodates hypertonic solutions (> 900 mOsm/L) meeting high energy and fluid needs over extended durations.

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Enteral Nutrition Formulations & Administration

Enteral formulas are classified based on nutrient complexity and density:

• Standard / Polymeric Formulas: Contain intact proteins (casein, soy), complex carbohydrates, and long-chain triglycerides. Caloric density ranges from 1.0 to 1.2 kcal/mL (~80–85% free water).
• Concentrated Formulas: Provide 1.5 to 2.0 kcal/mL (~69–77% free water). Formulated for patients requiring fluid restriction (e.g., heart failure, renal failure, acute respiratory distress syndrome).
• Elemental / Hydrolyzed Formulas: Contain pre-digested free amino acids, small peptides, and medium-chain triglycerides (MCTs). Designed for severe malabsorption, short bowel syndrome, or severe pancreatic insufficiency.
• Specialized / Disease-Specific: Tailored for renal (low electrolyte, high calorie), hepatic (high BCAA, low aromatic amino acids), or pulmonary disease (higher fat-to-carbohydrate ratio to reduce CO2 production).

Administration Methods

1. Continuous Infusion: Run via infusion pump over 24 hours. Preferred for critically ill patients and post-pyloric feeding.
2. Cyclic Infusion: Run over a set timeframe (e.g., 8–16 hours overnight). Promotes daytime mobility and oral intake.
3. Intermittent / Bolus Feeding: Infused in volumes of 250–400 mL over 15–30 minutes, 4 to 6 times daily via gravity or syringe. Strictly limited to gastric feeding (NG or PEG); bolus feeding into the small intestine causes severe diarrhea, cramping, and dumping syndrome.

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Enteral Feeding Rate & Free Water Calculations

NDTRs must master tube feeding calculations to verify caloric, protein, and fluid delivery.

Practical Calculation Example

Clinical Scenario: A patient requires 1,800 kcal/day and 75 g protein/day. The prescribed formula is a 1.5 kcal/mL formula providing 60 g protein/L and 77% free water (0.77 mL water per mL formula).

1. Calculate Total Formula Volume Needed: $\text{Total Volume} = \frac{\text{Target Calories}}{\text{Formula Density}} = \frac{1,800 \text{ kcal}}{1.5 \text{ kcal/mL}} = 1,200 \text{ mL/day}$

2. Calculate Hourly Infusion Rate (for 24-hour continuous delivery): $\text{Infusion Rate} = \frac{1,200 \text{ mL}}{24 \text{ hours}} = 50 \text{ mL/hour}$

3. Verify Protein Delivered: $\text{Protein Provided} = 1.2 \text{ L} \times 60 \text{ g/L} = 72 \text{ g protein/day} \quad (\text{meets target close to } 75 \text{ g})$

4. Calculate Free Water Provided by Formula: $\text{Free Water Volume} = 1,200 \text{ mL} \times 0.77 = 924 \text{ mL free water/day}$

5. Calculate Supplemental Water Flush Requirement: If total fluid goal is 1,800 mL/day: $\text{Additional Water Needed} = 1,800 \text{ mL} - 924 \text{ mL} = 876 \text{ mL/day}$ Divide into flushes: 219 mL water flush every 6 hours (or ~55 mL/hr flush).

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Parenteral Nutrition Substrates & Caloric Yields

Parenteral solutions deliver nutrients directly into the bloodstream using purified intravenous substrates. Substrate caloric values differ significantly from standard bomb-calorimeter dietary values due to hydration states and delivery forms.

Note: 10% IVFE yields 1.1 kcal/mL (rather than 1.0 kcal/mL) because added egg phospholipid emulsifiers and glycerol contribute extra calories.

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Complications & Refeeding Syndrome

Refeeding Syndrome Pathophysiology

Refeeding syndrome is a severe metabolic complication occurring when nutrition (especially carbohydrates) is reintroduced to severely malnourished or starved individuals (e.g., anorexia nervosa, chronic alcoholism, severe marasmus).

[Starvation State: Glucagon high, Catabolism, Electrolyte Depletion]
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        [Carbohydrate Reintroduction / Feeding]
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        [Insulin Secretion Spikes Rapidly]
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  [Anabolic Drive: Glucose, PO4, K, Mg Shift Intracellularly]
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[Hallmark Triad: Severe Hypophosphatemia, Hypokalemia, Hypomagnesemia]
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[Clinical Consequences: Cardiac Arrhythmia, Respiratory Failure, Seizures, Death]

Prevention & Protocol

To prevent refeeding syndrome:

• Screen at-risk patients prior to nutrition support initiation.
• Check and correct serum phosphorus, potassium, and magnesium baseline levels.
• Initiate feeding at low energy levels: 10–15 kcal/kg/day (or ~1,000 kcal/day max).
• Advance calories slowly over 4–7 days while monitoring serum electrolytes daily.