When is parenteral nutrition recommended and what is its typical composition?

Parenteral Nutrition: Indications and Composition

When Parenteral Nutrition is Recommended

Parenteral nutrition (PN) should be initiated when patients cannot meet their nutritional requirements via oral or enteral routes for 7-10 days and have a non-functioning, inaccessible, or unsafe gastrointestinal tract. 1

Absolute Indications for PN

PN is mandatory in the following clinical scenarios:

• Intestinal obstruction not amenable to feeding tube placement beyond the obstruction 1
• Prolonged gastrointestinal failure where PN becomes life-saving 1
• Short bowel syndrome with severe malabsorption or high fluid/electrolyte losses unmanageable enterally 1
• Severe dysmotility making enteral feeding impossible 1
• High-output enterocutaneous fistulae or surgical anastomotic breakdown 1
• Intestinal ischemia or severe shock with impaired splanchnic perfusion 1
• Patient intolerance of enteral nutrition when nutrition cannot be maintained orally 1

Perioperative Indications

• Severely malnourished surgical patients (>10-15% weight loss) who cannot be adequately fed orally or enterally should receive 7-10 days of preoperative PN 1
• Postoperative PN is indicated only when patients cannot meet caloric requirements within 7-10 days via oral or enteral routes 1, 2
• Well-nourished patients should NOT receive routine perioperative PN as it offers no benefit and increases morbidity 1

Special Populations

• Cancer patients: PN is indicated only when enteral nutrition is not feasible and the patient has acceptable performance status; routine PN in well-nourished oncology patients is NOT recommended 1
• Home PN: Reserved for patients with irreversible intestinal failure who cannot meet nutritional requirements enterally but can receive therapy outside acute care settings 1

Critical Principle: Enteral Nutrition First

When the gut is functional, enteral nutrition or a combination of enteral plus supplementary parenteral nutrition is ALWAYS the first choice. 1 Supplemental PN should be considered only when >60% of energy needs cannot be met enterally 1, 2

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Typical Composition of Parenteral Nutrition

Energy Requirements

• Total calories: 25-30 kcal/kg ideal body weight per day 1, 2
• Under severe stress, requirements may approach 30 kcal/kg ideal body weight 1

Macronutrient Distribution

Protein (Amino Acids)

• 1.5 g/kg ideal body weight per day in stressed/illness conditions 1, 2
• 0.8-1.0 g/kg per day for unstressed patients 1
• Represents approximately 20% of total energy requirements 1

Carbohydrates (Glucose)

• 50-60% of total energy (or 60-70% of non-protein calories) 1, 2
• Avoid exceeding 7 mg/kg per minute to prevent hepatic complications 1
• The glucose:fat ratio has shifted from 50:50 toward 60:40 or 70:30 due to concerns about hyperlipidemia and fatty liver 1

Lipids (Fat Emulsions)

• 30-40% of total energy (not exceeding 1 g/kg per day for long-term PN) 1, 2
• Essential fatty acid requirement: 7-10 g daily (14-20 g LCT fat from soya oil) 1
• MCT/LCT and fish oil emulsions are safe and effective alternatives 1

Optimal Protein:Fat:Glucose Ratio

The recommended caloric ratio is approximately 20:30:50% (protein:fat:glucose) 1

Micronutrients

Vitamins and Trace Elements

• Full range supplemented daily when total or near-total PN is required 1, 2
• Well-nourished patients recovering oral/enteral nutrition by postoperative day 5 require minimal IV supplementation 1
• Increased amounts may be needed with high digestive losses (e.g., zinc, magnesium) 2

Electrolytes

• Composition should reflect fluid losses 1
• Regular monitoring required, especially with high-output stomas or fistulae 2

Administration Principles

• 24-hour continuous infusion achieves optimal nitrogen sparing when all components are administered simultaneously 1
• Cyclic administration is recommended for home PN 1
• Central venous access is required due to hypertonicity; tunneled catheters or implanted ports are preferred for long-term use 1, 3
• No weaning necessary when transitioning from PN to enteral nutrition 1, 2

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Critical Pitfalls to Avoid

• Overfeeding: Avoid exceeding 1 g/kg/day lipids or 7 mg/kg/min glucose, which can cause chronic cholestasis and liver fibrosis 1, 2
• Delaying PN inappropriately: In severely malnourished patients (>20% weight loss), start at 25% of calculated energy requirements to prevent refeeding syndrome 4
• Using PN when enteral is possible: Enteral nutrition has stimulatory effects on GI structure/function and lower costs 1
• Inadequate monitoring: Monitor blood glucose daily, electrolytes regularly, and assess for catheter-related complications 2, 3