Total Parenteral Nutrition (TPN)

OVERVIEW

Total parental nutrition is the complete provision of nutrition intravenously, bypassing the GI tract.

INDICATIONS = unable to establish enteral feeding

General

• EN contraindicated (have low threshold to use perioperatively depending on nutritional state)
• EN fails to meet nutritional requirements

Specific

• prolonged bowel obstruction and ileus
• short bowel syndrome with severe malabsorption
• severe dysmotility
• high output intestinal fistulae
• anastomotic break down
• intolerance to EN

GENERAL

• ensure adequate central venous access (subclavian lines have lowest infection rates – aseptic insertion, 2% chlorhexidine in alcohol, permeable polyurethane dressings, antimicrobial catheters)
• dedicated lumen for TPN (daily infusion set changes)
• if requires TPN for > 2 months -> consider tunneled line
• assess why patient cannot be enterally fed -> this is known to be safer
• 12 hourly reassessment of whether patient can be enterally fed
• requires close liaising with dietician and pharmacy

ASSESSMENT OF NUTRITIONAL STATUS (CALORIC REQUIREMENTS)

• calculate Resting Energy Expenditure (using the Harris-Benedict equation)

REE (males) = 66.5 + (13.7 x body weight in kg) + (5.0 x height in cm) (6.8 x age in years)
REE (females) = 66.5 + (9.6 x body weight in kg) + (1.7 x height in cm) (4.7 x age in years)

• use ideal body weight
• resting energy expenditure in calories
• multiply this by a stress factor to allow for effects of disease (no exercise = 1.2, very heavy exercise BD = 1.9)

• more accurate to measure REE by indirect calorimetry
• most hospitalized patients require 25-30 kcal/kg/day
• mechanically ventilated are on the lower aspect of range
• burns and trauma patient may require 45 kcal/kg/day
• high protein hypocaloric nutrition preferred in obese patents (esp BMI >40)

NUTRITIONAL REQUIREMENTS

Protein

• utilisation of exogenous protein = 1.5g/kg/day
• other techniques:

-> non-protein calorie to nitrogen ratio (100-200kcal/g of nitrogen often used)
-> nitrogen balance = (protein intake (g) / 6.25) – (urinary nitrogen (g) + 4) -> 4 = empirical factor added to account for non-urinary nitrogen loss (faeces and sweat)

• crystalline solutions of L-amino acids (varying essential amino acids)

Carbohydrates

• daily requirement of glucose = 4-5g/kg/day in severely catabolic patients

Lipids

• requirements 1g/kg/day
• 1/3 of energy given as non-protein
• advantages: prevents fatty acid deficiency and provides more concentrated calories
• disadvantages: can cause pancreatitis, immunosuppression, overfeeding -> increase in CO2 production
• check lipids daily and check clearance of lipids

Minerals and Micronutrients

• Na+, K+, Ca2+, Mg2+, phosphate -> guided by serum plasma levels

• inorganic trace elements
• organic vitamins (thiamine, folic acid, fat soluble vitamins D, E, K, A, water soluble vitamins B and C)

H2O

• 25-40mL/kg/day (+ losses)

MONITOR FOR COMPLICATIONS

• catheter related sepsis
• catheter occlusion
• hyperglycaemia
• hypercholesterolaemia
• refeeding syndrome (Po43-, K+, Mg2+)
• abnormal LFTs
• trace elemental deficiency: > 2-4 weeks of poor nutrition
• copper: anaemia, neutropenia
• iodine: hypothyroidism
• chromium: glucose intolerance
• zinc: mental apathy, diarrhoea, rash
• selenium: cardiomyopathy

-> treatment: supplement, follow levels (serum, tissue, white cells)

PRESCRIPTION

• total energy: 25 kcal/kg/day
• protein: 1.5g/kg/day
• carbohydrate: 4g/kg/day
• lipids: 1g/kg/day
• H2O: 30mL/kg/day + other losses
• electrolytes
• organic vitamins
• inorganic trace elements

AN APPROACH

• exhaust all means of feeding enterally
• dietician involvement
• calculate requirements (caloric and nutritional)
• start slowly
• monitor for refeeding and complications
• frequent reassessment about whether patient can be fed enterally

VIDEO

References and Links

Journal articles

• Ziegler TR. Parenteral nutrition in the critically ill patient. N Engl J Med. 2009 Sep 10;361(11):1088-97. PMC3064749.

FOAM and web resources

• ICN — EXPLAINED: Parenteral Nutrition by Roger Harris (2014)