Introduction and Indications

Total parenteral nutrition (TPN) aims to provide adequate calories, amino acids, and nutrients to prevent negative energy and nitrogen balance in the acute phase of illness.
The ultimate goal is to promote positive nutritional balance to support growth approximating the intrauterine rate until adequate enteral intake is established.
Early initiation of parenteral nutrition within four hours of birth is highly beneficial and improves short-term and medium-term developmental outcomes.
It is indicated in neonates with a birth weight less than 1500 g or a gestational age less than 31 weeks.
It is also required when significant enteral feeding is not anticipated for more than three to five days.
Surgical conditions such as necrotizing enterocolitis, gastroschisis, omphalocele, tracheoesophageal fistula, and short bowel syndrome are absolute indications for parenteral nutrition.

Route of Administration

Peripheral veins can be used for short durations until central access is obtained.
Peripheral administration is limited to solutions with an osmolarity below 900 mOsm/L and a dextrose concentration strictly less than 12.5%.
Central venous catheters, such as an umbilical venous catheter or a peripherally inserted central catheter (PICC), are required for infusing hypertonic solutions.
Surgical insertion of a central catheter is considered only if long-term parenteral nutrition is anticipated and percutaneous insertion is not feasible.

A daily energy intake of 110 to 120 kcal/kg is needed to meet the metabolic demands of a healthy preterm neonate and support adequate growth.
Energy should be balanced among carbohydrates (60-75%), fats (25-40%), and proteins (10-15%).

Carbohydrates are the primary energy substrate and are delivered as dextrose.
The initial glucose infusion rate (GIR) should be 4 to 6 mg/kg/minute.
The rate is advanced in a gradual, stepwise fashion by 2 mg/kg/minute per day to a maximum of 12 to 14 mg/kg/minute.
Insulin infusions should not be routinely used to increase the glucose infusion rate to promote growth.
Insulin is indicated only if the infant develops persistent hyperglycemia associated with glycosuria despite reducing the glucose intake.

Preterm infants catabolize body protein rapidly if amino acids are not provided in the first postnatal days.
Amino acid infusions must be initiated immediately after birth at a dose of 1.5 to 3.5 g/kg/day.
Early provision of protein normalizes insulin secretion in extremely low birth weight neonates and promotes lean body mass accretion.
The dose is gradually increased to a maximum of 3.5 to 4.0 g/kg/day.

Intravenous lipid emulsions prevent essential fatty acid deficiency, which can develop within 72 hours if exogenous fat is withheld.
Lipids should be started on the first day at a dose of 1.0 to 1.5 g/kg/day.
The dose is advanced by 0.5 to 1.0 g/kg/day to a maintenance maximum of 3.0 to 3.5 g/kg/day.
Lipid syringes and tubing must be covered with aluminum foil to shield them from ambient light and prevent the formation of toxic lipid hydroperoxides.

Sodium and potassium are not added to the parenteral solution during the first two to three days of life.
They are introduced only after serum potassium falls to normal levels and a steady urine output is established.
Calcium and phosphorus must be supplemented early to prevent the metabolic bone disease of prematurity.
An optimal calcium-to-phosphorus ratio of 1.3:1 to 1.7:1 by weight helps prevent precipitation in the intravenous line.
Magnesium is supplemented from the start, though postnatal levels must be monitored if the mother received magnesium sulfate.

Multivitamin injections are added directly to the lipid emulsion to minimize photodegradation and adherence to the tubing.
Zinc is universally recommended from day one due to high gastrointestinal and urinary losses.
Other trace elements like copper, selenium, manganese, and iodine are typically provided after two weeks if the neonate remains completely dependent on parenteral nutrition.

Meticulous monitoring is mandatory to prevent nutrient-related complications.
Blood glucose should be checked two to three times a day while increasing the glucose infusion rate, and then daily once stable.
Serum electrolytes, blood urea nitrogen, and blood gases should be checked twice a week initially, followed by weekly assessments.
Liver function tests, triglycerides, calcium, and phosphorus require weekly monitoring.
Weight must be monitored daily, while head circumference and length should be measured weekly.

Sepsis risk is exceedingly high due to central catheters, multiple intravenous fluids, and skin punctures.
Metabolic complications include hypoglycemia, hyperglycemia, azotemia, hypertriglyceridemia, and metabolic acidosis.
Neonatal refeeding syndrome can occur due to inappropriate rates of minerals and amino acids.
This syndrome presents with low serum phosphate, potassium, and magnesium, alongside high serum calcium and sodium.
Parenteral nutrition-associated liver disease (PNALD) is a severe complication of prolonged use, presenting with progressive cholestasis and direct hyperbilirubinemia.
Minimizing the duration of fasting and initiating minimal enteral feeding helps stimulate bile secretion and protects the liver.

Parenteral nutrition is gradually weaned as the neonate successfully tolerates escalating enteral feeds.
It can be completely discontinued once enteral feeds reach 140 to 150 ml/kg/day in extremely preterm neonates, or 120 to 140 ml/kg/day in neonates born at or after 28 weeks of gestation.