Pvii: Describe the pharmacology of enteral & parenteral nutrition

Definitions

  • Enteral nutrition = when nutrients are given through the GI tract as a mixture of protein, carbs, fats, vitamins & minerals to the stomach/SB via a tube
  • Parenteral nutrition = when nutrients are suppled by an IV catheter, bypassing the stomach & bowel

Metabolic Changes of the Critically Ill

  • Often underfed
  • ↑protein catabolism
  • ↑BMR 40%
  • Insulin resistance
  • Malnutrition → ↑morbidity & mortality

 Aim 25kcal/kg/day + 1.5g protein/kg/day

  • All methods of enteral nutrition have risks

Enteral

Pros

  • Lower cost
  • Maintains GI structure/function
  • ↓risk of stress ulcers
  • Enhances gut immune function
  • Less hyperglycaemia
  • Less risk infection

Cons

  • High risk undernutrition
  • Aspiration risk
  • Can exacerbate subclinical gut ischaemia
  • Viscus perforation
  • Successful enteral feeding = ≥40mL/hr with 4h aspirates <250mL

ENTERAL FEEDING FORMULATIONS

  • Standard polymeric
  • Small peptide
  • Disease specific

Standard formulas: 1kcal/mL, osmolality 300 – 500mOsm/kg

    • Protein 10 – 15%
    • Carbohydrate 50 – 60%
    • Fat 30 – 35%
    • Vitamins
    • Minerals/trace elements
  • Iso-osmolar optimises absorption (high osmolarity can cause diarrhoea & delay gastric emptying)
    • 70 – 80% H2O

Parenteral Feeding Formations

Pros

  • Start early
  • No delay caloric intake
  • No reliance on gut function
  • Safe for mechanical ventilation

Cons

  • Catheter sepsis/Occlusion/insertion risks
  • ↑glucose
  • ↑lipids
  • Abnormal LFTs
  • ↓eGFR

Composition

  • Water 30mL/kg/day
  • Energy → glucose/lipids → 125kJ/kg/day
    • Glucose required by brain, RBC
    • Lipids from soybean emulsion provide E for cell wall integrity & PGI synthesis
  • Nitrogen → amino acids
    • 5g/kg/day for protein synthesis
  • Electrolytes
    • Na+ (1 – 2mol/kg/day) → nerve conduction & ECF tonicity
    • K+ (0.7 – 1mol/kg/day) → membrane potential & ICF tonicity
    • Ca2+ (0.1mol/kg/day) → bone metabolism, muscle contraction
    • Phosphate (0.7) → bone metabolism, phosphorylation of energy bonds
    • Mg2+ (0.1) → bone anabolism, enzyme cofactor
  • Vitamins → catalyse/substrate of enzyme systems
  • Trace elements
    • Zinc → constituent of enzymes (CA)
    • IronHb synthesis
    • CopperLipid metabolism, RBC maturation
    • Iodine → thyroxine synthesis
    • Manganese, fluoride, chromium, selenium
  • H2O → replaces H2O loss