Overview
This lecture provides a detailed, step-by-step guide on evaluating, initiating, and calculating requirements for Total Parenteral Nutrition (TPN) in clinical settings, emphasizing patient assessment, risks, formulas, and best practices.
Indications and Contraindications for Parenteral Nutrition
- PN is indicated when patients cannot absorb nutrients via the gut due to conditions like short gut syndrome or severe malabsorption.
- Do not start PN in patients who are expected to resume enteral intake within a few days.
- Avoid PN during the first week of critical illness unless patients are malnourished.
- PN is not an emergency intervention and should not be used to stabilize patients in the acute phase of infection or sepsis.
- Always prioritize enteral nutrition if possible, as gut use prevents bacterial translocation and maintains gut health.
Practical Considerations Before Initiating PN
- Central venous access is required for TPN due to the high osmolality; peripheral lines are only suitable for limited, lower-osmolality PPN.
- Never start TPN before placing and confirming central line access.
- Assess fluid needs, duration of PN, and risks versus benefits.
Calculating Parenteral Nutrition Requirements
Estimating Fluid Needs
- Use the Holliday-Segar Method: first 10 kg = 100 mL/kg, next 10 kg = 50 mL/kg, remainder = 20 mL/kg.
- Most adults tolerate 1.5-2.5 L/day of PN fluids.
Estimating Caloric/Metabolic Needs
- Predict needs using Harris-Benedict Equation, weight-based kcal/kg estimates, or metabolic cart studies.
- Normometabolic: 25-30 kcal/kg/day; hypometabolic: 20-25 kcal/kg/day; hypermetabolic: 30-35 kcal/kg/day.
- In obesity, use hypocaloric, high-protein feeding (e.g., 11-14 kcal/kg actual body weight for BMI 30-50).
Estimating Macronutrient Needs
- Protein: 0.8-1 g/kg for maintenance; up to 2-3 g/kg for severe burns or large wounds.
- Fats: Provide 30-40% of calories (IV 20% fat emulsion = 2 kcal/mL).
- Carbohydrates: Make up remaining calories; dextrose provides 3.4 kcal/g.
- GUR (Glucose Utilization Rate) should not exceed 4 mg/kg-min.
Estimating Micronutrient and Electrolyte Needs
- Monitor calcium/phosphate to avoid precipitation.
- Maximum daily: Na 130 mEq/L, K 80 mEq/L, Mg 12-16 mEq/L, Ca 10 mEq/L, Phos 25 mmol/L.
- Add daily water-soluble vitamins; avoid vitamin K in patients on warfarin.
- Adjust trace elements for liver/renal dysfunction.
Infusion and Tapering
- Initiate PN slowly (e.g., 25 mL/hr for 8 hrs, increase gradually).
- Taper PN to 50% for 2-4 hours before discontinuation.
- Transition to oral or tube feeding as soon as feasible, starting slow.
Special Considerations
- Replace electrolytes aggressively during initial PN days to prevent refeeding syndrome.
- Patients on propofol may need less IV fat.
- Adjust for co-existing conditions (e.g., increased protein for CRRT).
Key Terms & Definitions
- Parenteral Nutrition (PN) — IV delivery of nutrients when GI tract is unusable.
- Total Parenteral Nutrition (TPN) — Complete IV nutrition via central line.
- Peripheral Parenteral Nutrition (PPN) — Lower-concentration PN via peripheral line.
- Holliday-Segar Method — Formula for estimating daily fluid needs.
- GUR (Glucose Utilization Rate) — Rate at which body metabolizes IV glucose.
- Refeeding Syndrome — Dangerous electrolyte shifts upon reintroducing nutrition after starvation.
Action Items / Next Steps
- Review TPN calculations and practice example problems.
- Become familiar with institutional PN order sets and guidelines.
- Monitor labs closely after initiating PN, especially electrolytes and triglycerides.
- Read the 2016 ASPEN/SCCM guidelines for more details on nutrition support.