Total parenteral nutrition

Total parenteral nutrition

Total parenteral nutrition (TPN) is the intravenous delivery of the complete nutritional requirement — water, carbohydrate, amino acids, lipid, electrolytes, vitamins and trace elements — when the gastrointestinal tract cannot be used at all. Partial or supplemental PN supplements inadequate enteral intake.

The governing principle is "if the gut works, use it." Pathy's records a downward trend in the use of parenteral nutrition in critical care over recent years, chiefly because of studies showing higher morbidity with parenteral compared with enteral nutrition.

In acute pancreatitis this hierarchy is explicit: early oral feeding is preferred; enteral tube feeding (nasogastric or nasojejunal) within 72 hours if oral feeding is not tolerated; and parenteral nutrition is the last option if caloric goals cannot be met enterally — because PN carries a significantly higher risk of infection including bacteraemia and infected necrosis.

Non-functioning or inaccessible gut: intestinal obstruction, high-output enterocutaneous fistula, prolonged paralytic ileus, severe short bowel syndrome, mesenteric ischaemia with extensive resection.

Severe malabsorption or intestinal failure where enteral feeding cannot meet requirements; severe inflammatory bowel disease unresponsive to enteral support; radiation enteritis.

Peri-operative: severely malnourished patients unable to be fed enterally for more than 7 days; post-operative complications preventing enteral access.

In advanced disease: Pathy's notes that in motor neuron disease, parenteral nutrition can be useful in advanced disease when gastrostomy is not feasible.

General rule for older adults: consider PN when the gut cannot be used for more than 5–7 days in a well-nourished patient, or more than 3 days in an already malnourished patient.

Access: central venous access is required for full-strength TPN because of high osmolarity — a tunnelled catheter, PICC line or implanted port. Peripheral parenteral nutrition is possible only with low-osmolarity solutions and for short periods. Use a dedicated lumen with strict aseptic technique.

Energy: approximately 25–30 kcal/kg/day, reduced to 20–25 kcal/kg/day in the acutely ill and the elderly to avoid overfeeding.

Protein: approximately 1.0–1.5 g/kg/day of amino acids in older adults; Essentials notes protein requirement estimates range from 0.5 to more than 1.0 g/kg/day, and that higher protein intake is independently associated with a dose-responsive lower risk of incident frailty in older women.

Carbohydrate: dextrose supplying about 50–60% of non-protein calories; do not exceed roughly 4–5 mg/kg/min to avoid hyperglycaemia and hepatic steatosis.

Lipid: 20–30% of total energy, providing essential fatty acids; modern olive-oil and fish-oil-containing emulsions are preferred to pure soybean-oil emulsions.

Micronutrients: daily electrolytes, trace elements (zinc, selenium, copper, chromium, manganese) and vitamins including thiamine must be added — deficiency states develop rapidly on PN.

Daily initially: capillary glucose, fluid balance, weight, and serum potassium, phosphate and magnesium for the first 3–5 days.

Twice weekly then weekly: urea, creatinine, sodium, calcium, liver function tests, triglycerides, full blood count and CRP; periodic trace element and vitamin levels in long-term PN.

Clinical monitoring: catheter site inspection, temperature charting, and daily reassessment of whether the gut can be used — PN should be stopped as soon as enteral feeding is feasible.

Catheter-related mechanical: pneumothorax, haemothorax, arterial puncture, air embolism, catheter malposition, thrombosis and occlusion.

Catheter-related bloodstream infection — the most feared complication; higher infection risk than enteral feeding is the principal reason PN use has declined.

Metabolic: hyperglycaemia (and rebound hypoglycaemia on abrupt cessation), hypertriglyceridaemia, electrolyte disturbances, azotaemia, hypercapnia from overfeeding, and metabolic bone disease in long-term PN.

Hepatobiliary: steatosis, cholestasis, gallbladder sludge and gallstones — the last especially relevant in older adults, in whom Pathy's already notes increased biliary cholesterol and reduced gallbladder tone predispose to stone formation.

Gut-related: intestinal mucosal atrophy and bacterial translocation from disuse of the gut.

Refeeding syndrome — the most important early hazard in the malnourished older patient. Reintroduction of carbohydrate triggers an insulin surge with intracellular shift of phosphate, potassium and magnesium, plus thiamine depletion, causing arrhythmia, cardiac failure, respiratory failure, rhabdomyolysis, seizures, delirium and death.

Identify risk: low BMI, significant unintentional weight loss, minimal intake for several days, low baseline potassium, phosphate or magnesium, alcohol misuse, and loss of subcutaneous fat or muscle. Electrolytes may be normal despite total-body deficiency.

Prevention protocol: give thiamine before any dextrose-containing solution; start feeding at 100–150 g of dextrose or 10–20 kcal/kg in the first 24 hours, advancing by about 33% of goal every 1–2 days; correct severe electrolyte deficits before or alongside feeding; and monitor vital signs 4-hourly for the first 24 hours with daily weights and fluid balance. Evidence and Ethical Considerations in the Elderly

Pathy's is deliberately cautious about outcomes: the results of percutaneous parenteral feeding in subacute patients were not optimal, and undernourished or high-risk surgical patients did not have post-operative complications reduced to the level of well-nourished patients by either enteral or parenteral support — suggesting factors beyond pure starvation are operating.

Enteral tube feeding likewise did not affect 24-month survival in nursing home residents over 65 with severe cognitive impairment, and aspiration pneumonia occurred at a similar rate with or without enteral feeding — the same reasoning applies with even greater force to PN in advanced dementia.

Ethical framing: artificial nutrition is a medical treatment, not basic care. Decisions require goals-of-care discussion, capacity assessment, and consideration of advance directives; in India this is framed by the Mental Healthcare Act 2017 and Supreme Court jurisprudence on advance medical directives.