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Parenteral nutrition (PN) is the intravenous delivery of all essential macronutrients and micronutrients — glucose, amino acids, lipid emulsions, electrolytes, vitamins, and trace elements — directly into the bloodstream, bypassing the gastrointestinal tract entirely. It is indicated when the GI tract is non-functional, inaccessible, or inadequate to meet nutritional requirements, and when this situation is expected to persist for more than 5–7 days (in hospital) or the patient is already nutritionally depleted. Total Parenteral Nutrition (TPN) provides complete nutrition intravenously; Supplemental PN provides additional calories when enteral nutrition is insufficient. PN is delivered via a central venous catheter (CVC) for concentrated (>850 mOsm/L) formulations, or via peripheral intravenous access for peripheral PN (PPN, osmolarity ≤850 mOsm/L) for short-term use. The macronutrient composition of PN is calculated based on the patient's clinical condition and weight: energy 25–30 kcal/kg/day (reduced to 20 kcal/kg/day in the first 48 hours of critical illness to avoid overfeeding), protein 1.2–2.0 g/kg/day (up to 2.5 g/kg/day in severe catabolism), lipid emulsion 0.8–1.5 g/kg/day (maximum infusion rate 0.15 g/kg/hour), and glucose with a maximum oxidation rate of 4–5 mg/kg/min (approximately 6–7 g/kg/day) above which excess glucose is converted to fat and CO2. Refeeding syndrome — characterised by dangerous hypophosphataemia, hypokalaemia, and hypomagnesaemia when nutrition is initiated in severely malnourished patients — is the most dangerous metabolic complication of PN and must be actively prevented.
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Formula
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Energy: 25–30 kcal/kg/day; Protein: 1.2–2.0 g/kg/day; Lipid: 0.8–1.5 g/kg/day (max 0.15 g/kg/hr); Glucose: max 4–5 mg/kg/min; Glucose (g/kg/day) = mg/kg/min × 1.44Leggenda delle variabili
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| Simbolo | Nome | Unità | Descrizione |
|---|---|---|---|
| PN | Parenteral Nutrition | kcal/day, g/day | Intravenous delivery of macronutrients and micronutrients bypassing the GI tract |
| GIR | Glucose Infusion Rate | mg/kg/min | Rate of glucose delivery; maximum oxidation rate = 4–5 mg/kg/min |
| CLABSI | Central Line-Associated Bloodstream Infection | per 1000 catheter-days | Most common serious complication of long-term PN; prevented by strict aseptic technique |
| PNALD | Parenteral Nutrition-Associated Liver Disease | — | Spectrum of hepatic complications from long-term PN including steatosis, cholestasis, and cirrhosis |
Come Parenteral Nutrition Calculator
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- 1Confirm PN indication: GI tract non-functional or inaccessible (short bowel syndrome, intestinal obstruction, high-output fistula, paralytic ileus >5–7 days, severe GI bleeding, post-major GI surgery).
- 2Calculate body weight (use actual weight if BMI <30; use adjusted body weight if obese).
- 3Set energy target: 25–30 kcal/kg/day; reduce to 20 kcal/kg/day in ICU first 48 hours; include calories from all sources (glucose, lipid, protein — but protein calories are conventionally excluded from non-protein energy calculations).
- 4Calculate protein requirement: 1.2–1.5 g/kg/day (stable hospitalised); 1.5–2.0 g/kg/day (critically ill); up to 2.5 g/kg/day (burns, major trauma).
- 5Calculate lipid: 0.8–1.5 g/kg/day as lipid emulsion (soybean, olive oil, or mixed lipid); infuse over ≥16 hours; maximum rate 0.15 g/kg/hour to prevent impaired lipoprotein lipase clearance.
- 6Calculate glucose: total energy − lipid energy − protein energy = remaining glucose energy; ensure glucose rate does not exceed 4–5 mg/kg/min (maximum oxidation rate).
- 7Screen for refeeding syndrome risk and manage: check phosphate, magnesium, potassium before starting; supplement thiamine 200–300 mg/day before and during first 48 hours; start at 10 kcal/kg/day and increase over 5–7 days in high-risk patients.
Esempi risolti
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All electrolytes, vitamins, and trace elements must be added; central line required; glucose monitoring 4-hourly initially
Standard PN prescription for a post-operative patient with expected prolonged ileus. The glucose rate is well within the maximum of 4–5 mg/kg/min.
NICE recommends no more than 10 kcal/kg/day for the first 2 days in extreme malnutrition risk; monitor ECG for QTc prolongation
In severe malnutrition, rapid refeeding causes life-threatening electrolyte shifts. Starting at 10 kcal/kg/day and advancing slowly prevents refeeding syndrome.
ESPEN ICU guidelines recommend against early full caloric targets in the first 48 hours to avoid overfeeding-related complications
In early acute illness, full caloric targets worsen outcomes through overfeeding. A permissive underfeeding approach targets 70–80% of calculated needs in the acute phase.
Long-term PN carries risks: PNALD (PN-associated liver disease), line infections (0.5–1 per catheter-year), thrombosis, metabolic bone disease
Home PN is a life-sustaining treatment for patients with severe intestinal failure. Cycled overnight delivery allows mobility during the day and reduces liver fat deposition.
Applicazioni pratiche
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Post-operative nutritional support for patients with prolonged ileus, anastomotic dehiscence, or high-output enteral fistulae., where accurate parenteral nutrition analysis through the Parenteral Nutrition supports evidence-based decision-making and quantitative rigor in professional workflows
Long-term home PN for patients with intestinal failure (short bowel syndrome, chronic intestinal pseudo-obstruction, radiation enteritis)., where accurate parenteral nutrition analysis through the Parenteral Nutrition supports evidence-based decision-making and quantitative rigor in professional workflows
Haematology: supporting bone marrow transplant patients through severe GI mucositis when enteral feeding is not feasible., where accurate parenteral nutrition analysis through the Parenteral Nutrition supports evidence-based decision-making and quantitative rigor in professional workflows
ICU nutrition: supplemental PN when enteral nutrition targets cannot be achieved in high-nutritional-risk ICU patients., where accurate parenteral nutrition analysis through the Parenteral Nutrition supports evidence-based decision-making and quantitative rigor in professional workflows
Gastroenterology: peri-operative nutritional optimisation for malnourished IBD patients before major bowel surgery to reduce anastomotic complications.
Casi speciali
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PN in Renal Failure
Patients with acute kidney injury or chronic kidney disease on dialysis require modified PN formulations. Protein targets differ: in non-dialysed CKD, protein may be restricted to 0.8–1.0 g/kg/day (depending on eGFR); in patients on dialysis or CRRT, losses via filtration increase protein needs to 1.5–2.5 g/kg/day. Electrolyte additions require adjustment: reduce or eliminate phosphate and potassium, as these accumulate in renal failure. Standard multivitamins must be modified to avoid accumulation of fat-soluble vitamins (A, E, K).
PN in Liver Failure
Liver failure affects multiple aspects of PN prescription: protein should not be restricted (contrary to older practice — protein restriction does not prevent hepatic encephalopathy and worsens malnutrition and outcomes); use branched-chain amino acid (BCAA) enriched formulations in refractory encephalopathy; reduce glucose to avoid worsening hyperglycaemia (insulin resistance is common); use olive oil or fish oil lipid emulsions instead of soybean oil to reduce PNALD. Supplement thiamine and zinc, which are commonly deficient in liver disease.
PN and Immunosuppressed Patients
Haematology patients (post-BMT, haematological malignancy with mucositis) frequently require PN as first-line nutritional support when the oral and GI mucosa is severely damaged. Glutamine supplementation in PN (0.3–0.5 g/kg/day) has historically been recommended in critically ill and cancer patients, though more recent RCT evidence (METAPLUS, SIGNET) shows no survival benefit and possible harm in some ICU populations. Current ESPEN guidelines do not recommend routine glutamine supplementation in PN.
PN and Glycaemic Control
Hyperglycaemia is the most common metabolic complication of PN, affecting up to 50% of patients. Target blood glucose is 6–10 mmol/L (108–180 mg/dL) in most patients; tight control (4.5–6.0 mmol/L) increases hypoglycaemia risk without improving outcomes. Insulin can be added directly to the PN bag (for stable patients on fixed PN prescriptions) or infused separately via sliding scale. Glucose monitoring every 4–6 hours is essential during the first 48–72 hours of PN and whenever the PN formulation changes.
Parenteral Nutrition Macronutrient Targets
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| Nutrient | Standard Hospital Target | Critical Care ICU (Acute Phase) | Long-Term Home PN |
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| Energy (kcal/kg/day) | 25–30 | 20–25 (first 48h) | 25–35 |
| Protein (g/kg/day) | 1.2–1.5 | 1.3–2.0 | 1.0–1.5 |
| Lipid (g/kg/day) | 0.8–1.5 (max 0.15 g/kg/hr) | 0.7–1.2 | 0.8–1.5 |
| Glucose (mg/kg/min) | ≤4–5 (max rate) | ≤4 (avoid overfeeding) | ≤4–5 |
| Glucose (g/kg/day) | ≤7.2 | ≤5.8 | ≤7.2 |
Domande frequenti
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When should parenteral nutrition be started?
PN should be started when: (1) the GI tract is non-functional or inaccessible for ≥5–7 days in a previously well-nourished patient; (2) the GI tract is inadequate to meet nutritional requirements for ≥3 days in a malnourished patient; or (3) short-term complete bowel rest is required (e.g., some intestinal fistulae, severe GI bleeding, ileus). Enteral nutrition (tube feeding) is strongly preferred over PN whenever the GI tract is functional, as it maintains gut mucosal integrity, reduces bacterial translocation, and is safer and less expensive.
What is PN-associated liver disease (PNALD)?
PN-associated liver disease (PNALD, also called IFALD — intestinal failure-associated liver disease) is a spectrum of hepatic complications from long-term PN including steatosis (fat accumulation), cholestasis (bile flow impairment), fibrosis, and cirrhosis. It occurs in up to 15–40% of long-term PN patients. Risk factors include: duration of PN, lack of enteral stimulation (gut rest), excessive glucose delivery, soybean oil-based lipid emulsions, recurrent line infections, and short bowel syndrome. Modern fish oil-containing or olive oil-based lipid emulsions reduce PNALD risk. Cycled PN (overnight rather than continuous) also reduces cholestasis.
What are the main complications of parenteral nutrition?
PN complications include: central line-associated bloodstream infections (CLABSI — most common and serious), venous thrombosis, pneumothorax or haemothorax during line insertion, metabolic complications (hyperglycaemia, hypertriglyceridaemia, electrolyte imbalances, refeeding syndrome), and long-term complications (PNALD, metabolic bone disease, trace element deficiency or toxicity). Strict aseptic technique during line care is the most important factor in reducing CLABSI risk — care bundles reduce CLABSI rates by up to 70%.
What is the maximum glucose infusion rate in PN?
The maximum glucose oxidation rate in humans is approximately 4–5 mg/kg/min (approximately 6–7 g/kg/day for a 70 kg adult). Above this rate, excess glucose cannot be oxidised and is instead converted to fat (lipogenesis), with consequent CO2 overproduction, hypertriglyceridaemia, hepatic steatosis, and worsening of respiratory failure in patients with limited ventilatory reserve. In PN prescription, the glucose rate should always be calculated and confirmed to be within this maximum.
What electrolytes and micronutrients are added to PN?
Standard PN formulations include: sodium (60–150 mmol/day), potassium (40–120 mmol/day), phosphate (20–40 mmol/day), magnesium (8–20 mmol/day), calcium (5–10 mmol/day), and chloride (as counterion). A complete multivitamin preparation and trace element solution (zinc, selenium, copper, manganese, chromium, molybdenum, iodine) are added daily. Requirements change significantly in disease states — renal failure reduces phosphate and potassium needs; critical illness increases phosphate and zinc demands; short bowel syndrome increases zinc losses.
Can PN and enteral nutrition be given simultaneously?
Yes — supplemental PN can be given alongside enteral nutrition when enteral intake is insufficient but some GI function is present. This 'combined' approach avoids the disadvantages of sole reliance on PN (gut disuse, cost) while ensuring adequate macronutrient delivery. ESPEN recommends considering supplemental PN if full target enteral intake cannot be achieved by day 3–4 of ICU admission in patients who are at high nutritional risk (NRS-2002 ≥5 or severely malnourished).
What is the difference between TPN (total parenteral nutrition) and PPN (peripheral parenteral nutrition)?
Total Parenteral Nutrition (TPN) is delivered via a central venous catheter (PICC, subclavian, or internal jugular line) and can provide full nutritional requirements including concentrated glucose solutions (10–50%). It is used for complete GI failure or when full nutritional replacement is needed for extended periods. Peripheral Parenteral Nutrition (PPN) uses a peripheral venous cannula and is limited by osmolarity (≤850 mOsm/L) to prevent peripheral vein thrombophlebitis, allowing only dilute solutions with lower glucose concentration and typically supplementary rather than complete nutritional delivery. PPN is suitable for short-term use (up to 7–10 days).
What is refeeding syndrome and how is it detected?
Refeeding syndrome occurs within 72 hours of starting nutritional support in severely malnourished patients. Starvation causes cellular depletion of phosphate, potassium, and magnesium. When carbohydrates are introduced, insulin secretion stimulates cellular uptake of these electrolytes, causing dangerous extracellular falls. Clinical manifestations include: cardiac arrhythmias and cardiac failure (hypophosphataemia — most dangerous), respiratory failure, neuromuscular weakness, peripheral oedema, and Wernicke's encephalopathy (thiamine deficiency). Serum phosphate is the primary monitoring marker — a fall to <0.5 mmol/L requires immediate supplementation and possible feeding pause.
Errori comuni da evitare
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- !Starting PN when enteral nutrition would be feasible — EN is safer, cheaper, and preserves gut mucosal integrity; PN should not be defaulted to simply because it seems easier.
- !Not calculating the maximum glucose infusion rate — exceeding 4–5 mg/kg/min causes CO2 overproduction, hepatic steatosis, and worsening hyperglycaemia.
- !Failing to prescribe thiamine before starting PN in malnourished or high-risk patients — thiamine deficiency causes life-threatening Wernicke's encephalopathy.
- !Using a PN line for blood sampling, medication infusion, or central venous pressure monitoring — the PN line should be dedicated solely to PN to minimise CLABSI risk.
- !Not monitoring for refeeding syndrome with daily electrolytes (phosphate, potassium, magnesium) in the first 72–96 hours of PN initiation in high-risk patients.
- !Continuing PN beyond its indication without reassessing when enteral feeding could be restarted — prolonged unnecessary PN increases PNALD, infection, and cost.
Consiglio Pro
For a quick bedside PN energy estimate: 25 kcal/kg × body weight in kg gives the approximate total daily caloric target. Allocate approximately 40% to lipid, 30% to glucose (non-protein calories), and the remaining 30% as protein (at 4 kcal/g). Always verify the calculated glucose rate does not exceed 4–5 mg/kg/min before signing the PN prescription.
Lo sapevi?
The first successful long-term intravenous feeding of a human patient was reported by Dr Stanley Dudrick and colleagues at the University of Pennsylvania in 1968. Their landmark patient was a premature infant with intestinal atresia who survived for 45 days on complete intravenous nutrition alone and grew normally. Dudrick and Jonathan Rhoads then demonstrated the technique in dogs and adult patients, establishing total parenteral nutrition as a clinical reality and founding the entire field of clinical nutrition support.
Regional Guides
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🇬🇧 UK▾
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Riferimenti
- ›ESPEN Guideline on Parenteral Nutrition (Clin Nutr 2020)
- ›ESPEN Guideline on Critical Care Nutrition (Clin Nutr 2019)
- ›NICE CG32 — Nutrition Support for Adults (2006 updated 2017)
- ›Skipper A — Refeeding Syndrome or Refeeding Hypophosphataemia (Nutr Clin Pract 2012)
- ›Dudrick SJ et al — First TPN Paper (Surgery 1968)
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