Pvi: 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)
- Iron → Hb synthesis
- Copper → Lipid metabolism, RBC maturation
- Iodine → thyroxine synthesis
- Manganese, fluoride, chromium, selenium
- H2O → replaces H2O loss