1. Parenteral Nutrition — Indications
1.1 When to Use Parenteral Nutrition
Parenteral nutrition (PN) should be considered when the enteral route is contraindicated, insufficient, or not feasible to meet the patient’s nutritional requirements. The enteral route is always preferred when functional; PN is not a substitute for EN in patients who can tolerate enteral feeding.
Absolute Indications for PN
| Indication | Clinical Scenario |
|---|
| Non-functional GI tract | Mechanical bowel obstruction; intestinal discontinuity; severe prolonged ileus unresponsive to prokinetics |
| Inability to access the GI tract | Failed enteral access (cannot place post-pyloric tube and gastric feeding contraindicated) |
| Short bowel syndrome | Insufficient absorptive surface (residual small bowel < 100-150 cm without colon in continuity) |
| High-output GI fistula | > 500 mL/day output proximal to any feasible enteral access site |
| Mesenteric ischemia | Active or recent bowel ischemia precluding enteral feeding |
| Severe GI hemorrhage | Ongoing hemorrhage precluding EN (transition to EN once controlled) |
Supplemental PN Indications
| Indication | Details |
|---|
| Insufficient EN delivery | EN consistently delivering < 60% of caloric/protein targets after optimization attempts (formula change, prokinetics, post-pyloric access) |
| EN intolerance | Persistent vomiting, abdominal distension, or high GRV despite maximal medical management |
| Transition period | Bridge therapy while awaiting enteral access (e.g., post-pyloric tube placement, PEG) |
1.2 Exclusive PN vs Supplemental PN
- Exclusive PN: All nutrition delivered intravenously; reserved for patients with complete EN contraindication
- Supplemental PN: PN added to partially tolerated EN to meet caloric/protein targets; the preferred approach when EN alone is insufficient (rationale: maintain gut integrity via trophic EN while supplementing with PN to meet overall targets)
2. Timing of PN Initiation — The Major Controversy
2.1 Overview of the Debate
The optimal timing for initiating PN when EN is not feasible or insufficient is one of the most debated topics in critical care nutrition. The two major international professional societies have historically differed on this question, reflecting divergent interpretations of the same evidence base.
2.2 Position Comparison
| Aspect | North American Societies (2016/2022) | European Nutrition Society (2019) |
|---|
| PN timing (EN contraindicated) | Withhold PN for the first 7 days in patients who were previously well-nourished | Initiate PN within 3-7 days if EN is not possible or insufficient |
| PN timing (EN contraindicated, malnourished) | Initiate PN as soon as possible in malnourished or high-risk patients | Initiate PN within 24-48 hours in patients unable to receive EN |
| Supplemental PN | Consider supplemental PN after day 7-10 if EN alone is insufficient (< 60% of target) | Consider supplemental PN after day 3-7 if EN alone is insufficient |
| Rationale | EPaNIC trial showed early PN (day 3) was harmful; concerns about overfeeding, hyperglycemia, infection risk | Early caloric deficit is harmful; need to meet energy targets; different interpretation of EPaNIC findings |
EPaNIC Trial (2011)
| Feature | Details |
|---|
| Design | Multicenter RCT; n = 4,640 |
| Comparison | Early PN (supplemental PN initiated within 48 hours of ICU admission when EN was insufficient) vs Late PN (PN withheld until day 8) |
| Primary outcome | No difference in mortality |
| Key findings | Early PN was associated with more infections (26.2% vs 22.8%, p = 0.008), longer ICU stay (median 4 vs 3 days, p = 0.02), more cholestasis, and prolonged mechanical ventilation |
| Criticisms | Early PN group may have been overfed; high dextrose loading; glucose control protocol may have influenced results; patients were predominantly cardiac surgical (low nutritional risk) |
| Citation | Casaer MP, Mesotten D, Hermans G, et al. N Engl J Med. 2011;365(6):506-517 |
| DOI | 10.1056/NEJMoa1102662 |
Heidegger et al. (SPN Trial, 2013)
| Feature | Details |
|---|
| Design | Multicenter RCT; n = 305 |
| Comparison | Supplemental PN starting day 4 (when EN < 60% of target as measured by IC) vs EN alone |
| Primary outcome | Reduced nosocomial infections in the supplemental PN group (27% vs 38%, p = 0.0338) |
| Key findings | Supplemental PN improved caloric delivery and reduced infections; no difference in mortality or ICU LOS |
| Note | Used IC to guide targets (avoided overfeeding); smaller study |
| Citation | Heidegger CP, Berger MM, Graf S, et al. Lancet. 2013;381(9864):385-393 |
| DOI | 10.1016/S0140-6736(12)61351-8 |
CALORIES Trial (2014)
| Feature | Details |
|---|
| Design | Multicenter RCT; n = 2,400 |
| Comparison | EN vs PN initiated within 36 hours of ICU admission |
| Primary outcome | No difference in 30-day mortality (33.1% vs 34.2%) |
| Key findings | PN and EN had similar mortality and infectious complications when caloric delivery was equivalent; PN had more hyperglycemia; EN had more vomiting and hypoglycemia |
| Citation | Harvey SE, Parrott F, Harrison DA, et al. N Engl J Med. 2014;371(18):1673-1684 |
| DOI | 10.1056/NEJMoa1409860 |
NUTRIREA-2 Trial (2018)
| Feature | Details |
|---|
| Design | Multicenter RCT; n = 2,410 |
| Comparison | Early EN vs early PN in ventilated adults with shock |
| Primary outcome | No difference in 28-day mortality |
| Key findings | EN was associated with more GI complications (vomiting, bowel ischemia, GRV > 500, pseudo-obstruction); PN was not inferior; EN-related bowel ischemia occurred in 2% of EN group |
| Implication | In patients with shock, early EN has significant GI risks; early PN may be a reasonable alternative |
| Citation | Reignier J, Boisrame-Helms J, Brisard L, et al. Lancet. 2018;391(10116):133-143 |
| DOI | 10.1016/S0140-6736(17)32146-3 |
2.4 Practical Synthesis — When to Start PN
| Clinical Scenario | Recommended PN Timing |
|---|
| EN feasible and tolerated | PN not needed |
| EN partially tolerated (< 60% target) — low nutritional risk (mNUTRIC 0-4) | Supplemental PN after day 7-10 |
| EN partially tolerated (< 60% target) — high nutritional risk (mNUTRIC >= 5) | Supplemental PN after day 3-5 (individualized) |
| EN completely contraindicated — low nutritional risk | Exclusive PN by day 5-7 (withhold first 3-5 days; provide IV dextrose for glucose) |
| EN completely contraindicated — high nutritional risk or malnourished | Exclusive PN within 24-48 hours |
| Shock requiring high-dose vasopressors | PN preferred over EN if nutrition cannot be deferred (NUTRIREA-2) |
3. PN Composition
3.1 Macronutrient Components
Dextrose (Carbohydrate)
| Parameter | Recommendation |
|---|
| Caloric value | 3.4 kcal/g dextrose (monohydrate form used in IV solutions) |
| Typical contribution | 50-70% of non-protein calories |
| Concentration range | 10-70% dextrose solutions (final concentration after compounding) |
| Maximum glucose infusion rate (GIR) | <= 5 mg/kg/min (exceeding this threshold promotes hyperglycemia, hepatic steatosis, and excess CO2 production) |
| Monitoring | Blood glucose every 4-6 hours; insulin infusion per protocol if glucose > 180 mg/dL |
GIR calculation:
GIR (mg/kg/min) = [Dextrose (g/day) x 1,000] / [Weight (kg) x 1,440]
Example: Patient weighing 70 kg receiving 250 g dextrose/day:
GIR = (250 x 1,000) / (70 x 1,440) = 250,000 / 100,800 = 2.48 mg/kg/min (acceptable)
Amino Acids (Protein)
| Parameter | Recommendation |
|---|
| Caloric value | 4 kcal/g amino acid |
| Typical contribution | Protein targets mirror EN (1.2-2.0 g/kg/day; higher in specific populations) |
| Concentration range | 4.25-15% amino acid solutions |
| Note | Amino acid calories are traditionally included in total caloric delivery but are primarily used for protein synthesis, not energy; some clinicians target non-protein calories separately |
Lipid Emulsions (Fat)
| Parameter | Recommendation |
|---|
| Caloric value | 10 kcal/g fat (as administered in emulsion form: 20% emulsion = 2.0 kcal/mL) |
| Typical contribution | 25-40% of non-protein calories |
| Maximum dose | <= 1.0-1.5 g/kg/day (to reduce hypertriglyceridemia and immune suppression risk) |
| Infusion rate | Infuse over >= 12 hours (continuous) or may be part of a 3-in-1 admixture |
3.2 Lipid Emulsion Types
| Lipid Source | Trade Examples | Composition | Advantages | Disadvantages |
|---|
| Soybean oil (SO) | Intralipid | 100% soybean oil; rich in omega-6 PUFA (linoleic acid) | Widely available; inexpensive | Pro-inflammatory (high omega-6:omega-3 ratio); immunosuppressive at high doses; hepatic steatosis risk |
| Olive oil-based | ClinOleic | 80% olive oil, 20% soybean oil; rich in MUFA (oleic acid) | Less pro-inflammatory than SO; less immunosuppressive | Limited availability in some regions |
| SMOF lipid | SMOFlipid | 30% soybean oil, 30% MCT, 25% olive oil, 15% fish oil | Balanced fatty acid profile; omega-3s (EPA/DHA); anti-inflammatory; alpha-tocopherol (vitamin E) enriched; reduced hepatotoxicity | More expensive; limited head-to-head outcome data vs soybean oil |
| MCT/LCT | Lipofundin MCT/LCT | 50% MCT, 50% soybean oil | Faster oxidation of MCTs; less hepatic fat deposition | Still contains soybean oil omega-6 |
| Fish oil (pure) | Omegaven | 100% fish oil; rich in omega-3 (EPA, DHA) | Potent anti-inflammatory; parenteral nutrition-associated liver disease (PNALD) rescue | Approved mainly in pediatrics (FDA); used off-label in adults; expensive; sole-source use controversial |
Recommendation: The European nutrition society recommends olive oil-based or SMOF lipid emulsions as preferred over pure soybean oil emulsions in ICU patients. The North American societies do not make a strong recommendation for one lipid type over another but note that limiting soybean oil exposure may be beneficial.
3.3 Fluid Volume Considerations
| PN Component | Typical Volume Contribution |
|---|
| Amino acids + dextrose (2-in-1) | 1,000-2,000 mL/day |
| Lipid emulsion (if separate) | 250-500 mL/day |
| Total PN volume (3-in-1) | 1,200-2,500 mL/day |
| Concentrated PN (fluid restriction) | As low as 800-1,000 mL/day (higher concentrations of dextrose/amino acids; reduced free water) |
Fluid-restricted patients (heart failure, ARDS, renal failure): Use concentrated PN formulations (high-concentration dextrose and amino acids; calorie-dense lipid emulsions at 20% or 30%). Coordinate with pharmacy to minimize total daily volume while meeting macronutrient targets.
3.4 Electrolytes and Additives in PN
| Component | Standard Daily Addition | Monitoring |
|---|
| Sodium | 1-2 mEq/kg | Basic metabolic panel daily |
| Potassium | 1-2 mEq/kg | Basic metabolic panel daily; EKG as indicated |
| Phosphorus | 20-40 mmol | Daily during initiation; critical for refeeding prevention |
| Magnesium | 8-24 mEq | Daily during initiation |
| Calcium (as gluconate) | 10-15 mEq | Ionized calcium daily; watch for calcium-phosphate precipitation |
| Acetate / Chloride | Adjusted for acid-base status | Basic metabolic panel; adjust ratio for metabolic acidosis (more acetate) or alkalosis (more chloride) |
| Zinc | 3-5 mg | Weekly serum zinc if supplementing |
| Copper | 0.3-0.5 mg | Omit or reduce in cholestasis (biliary excretion) |
| Chromium | 10-15 mcg | Omit in renal failure (renally excreted) |
| Manganese | 55-150 mcg | Omit or reduce in cholestasis; neurotoxicity risk with accumulation |
| Selenium | 60-100 mcg | Weekly serum selenium if supplementing |
| MVI (multivitamin injection) | 1 vial daily | Standard addition |
| Trace element injection | 1 vial daily | Standard addition; adjust individual elements as above |
4. PN Monitoring
4.1 Monitoring Schedule
| Parameter | Frequency During Initiation (Days 1-7) | Frequency After Stabilization |
|---|
| Blood glucose | Every 4-6 hours | Every 6-8 hours (or per insulin protocol) |
| Basic metabolic panel (Na, K, Cl, CO2, BUN, Cr, glucose) | Daily | Daily to every other day |
| Phosphorus | Daily (critical for refeeding) | 2-3 times per week |
| Magnesium | Daily | 2-3 times per week |
| Ionized calcium | Daily | 2-3 times per week |
| Triglycerides | Baseline, then 24-48 hours after initiation, then 1-2 times per week | Weekly |
| Liver function tests (AST, ALT, ALP, total/direct bilirubin) | Baseline, then 2-3 times per week | Weekly |
| Prealbumin | Not useful during acute inflammation | May track weekly during recovery (when CRP is declining) |
| C-reactive protein | 2-3 times per week | Weekly (to contextualize prealbumin trends) |
| Nitrogen balance (24-hour UUN) | Weekly (if feasible) | Weekly |
| Weight | Daily (trend; interpret cautiously with fluid shifts) | Daily |
| Fluid balance | Daily | Daily |
5. PN Complications
Hyperglycemia
| Aspect | Details |
|---|
| Incidence | Very common (> 50% of PN patients) |
| Mechanism | Dextrose infusion; insulin resistance of critical illness; hepatic glucose production |
| Target range | 140-180 mg/dL (see glycemic control section) |
| Management | Insulin infusion protocol; reduce dextrose concentration if persistent hyperglycemia despite insulin; ensure GIR <= 5 mg/kg/min |
| Impact | Hyperglycemia > 180 mg/dL during PN is associated with increased infections, mortality, and length of stay |
Hypertriglyceridemia
| Aspect | Details |
|---|
| Threshold | Hold or reduce lipids if triglycerides > 400 mg/dL; some guidelines use > 250 mg/dL as a threshold for dose reduction |
| Risk factors | Propofol (contains 10% soybean oil emulsion — must account for additional lipid and caloric load: propofol provides 1.1 kcal/mL), sepsis, renal failure, pancreatitis, high-dose lipid infusion |
| Management | Reduce lipid dose to 0.5-0.7 g/kg/day; hold lipids if TG > 400; recheck in 24-48 hours; consider lipid-free PN temporarily; account for propofol lipid load |
| Propofol caloric contribution | Propofol lipid calories = propofol rate (mL/hr) x 1.1 kcal/mL x 24 hours (subtract from PN lipid dose) |
Hepatic Complications (PN-Associated Liver Disease — PNALD)
| Complication | Timeframe | Mechanism | Prevention/Management |
|---|
| Hepatic steatosis | Days to weeks | Excessive dextrose (GIR > 5 mg/kg/min); overfeeding; soybean oil lipids | Avoid overfeeding; limit GIR; use mixed lipid emulsions; cycle PN (10-12 hour infusion with 12-14 hour rest) |
| Cholestasis | Weeks to months | Bile stasis from lack of enteral stimulation; manganese/copper accumulation; soybean oil lipids | Initiate even trophic EN as soon as possible; reduce or omit manganese and copper; consider SMOF or fish oil lipids; cycle PN |
| Elevated LFTs (transaminases, ALP, bilirubin) | Variable | Multifactorial | Evaluate for other causes (medications, biliary obstruction, sepsis); optimize PN composition; transition to EN as soon as possible |
Electrolyte Abnormalities
- Hypophosphatemia: Most dangerous in the context of refeeding syndrome (see Section 7)
- Hypokalemia and hypomagnesemia: Common with insulin-mediated intracellular shifts upon carbohydrate reintroduction
- Hypernatremia/hyponatremia: Related to free water content of PN and concurrent IV fluids
5.2 Infectious Complications
| Complication | Risk Factor | Prevention |
|---|
| Central line-associated bloodstream infection (CLABSI) | PN is an independent risk factor for CLABSI due to the nutrient-rich solution providing an ideal growth medium | Dedicated PN lumen (do not use PN lumen for other infusions or blood draws); strict aseptic technique; chlorhexidine dressing changes per CLABSI bundle; minimize line manipulation; consider antimicrobial-impregnated catheters |
| Fungemia (Candida) | PN, broad-spectrum antibiotics, central line | Blood cultures if fever develops; empiric antifungal consideration in high-risk patients |
5.3 Mechanical Complications
- Complications related to central venous access (pneumothorax, arterial puncture, thrombosis) — these are catheter-related rather than PN-specific
- Catheter occlusion from PN precipitates (calcium-phosphate, lipid residue) — flush with normal saline; use ethanol lock for lipid occlusion if needed
6. Transition from PN to EN
6.1 Principles
- Always transition to EN as soon as the GI tract is functional — even partial EN (trophic) with PN supplementation is preferable to exclusive PN
- Transition should be gradual to avoid rebound hypoglycemia from abrupt PN discontinuation
- Overlap EN and PN during the transition period
6.2 Transition Protocol
| Step | Action |
|---|
| 1 | Initiate EN at trophic rate (10-20 mL/hr) while continuing full PN |
| 2 | Advance EN by 10-25 mL/hr every 4-8 hours as tolerated |
| 3 | When EN delivers >50-60% of caloric target, begin reducing PN proportionally |
| 4 | Reduce PN in a stepwise fashion (reduce by 25-50% of original dose every 12-24 hours) |
| 5 | When EN reaches >75-80% of target, discontinue PN |
| 6 | Before final PN discontinuation: taper dextrose infusion rate or provide D10W at 50-75 mL/hr for 1-2 hours to prevent rebound hypoglycemia |
| 7 | Monitor blood glucose every 1-2 hours for the first 6-12 hours after PN discontinuation |
6.3 Monitoring During Transition
- Blood glucose every 2-4 hours during the taper
- Electrolytes (K, Mg, PO4) daily
- Caloric/protein delivery tracking (sum of EN + PN) to avoid underfeeding gaps or overfeeding overlap
7. Refeeding Syndrome
7.1 Pathophysiology
Refeeding syndrome (RFS) occurs when nutrition (particularly carbohydrate) is reintroduced after a period of starvation or severe malnutrition. During starvation, the body shifts from carbohydrate to fat as its primary energy source, depleting intracellular stores of phosphate, potassium, and magnesium. Upon carbohydrate reintroduction:
- Insulin secretion increases rapidly in response to glucose
- Insulin drives intracellular uptake of phosphate, potassium, magnesium, and water
- Severe hypophosphatemia (the hallmark) leads to ATP depletion, impaired cellular function, and potentially fatal cardiac, respiratory, and neurologic complications
- Hypokalemia causes cardiac arrhythmias
- Hypomagnesemia exacerbates arrhythmia risk and contributes to refractory hypokalemia and hypocalcemia
- Fluid retention (sodium and water reabsorption driven by insulin) can precipitate acute heart failure and pulmonary edema
- Thiamine depletion — increased carbohydrate metabolism rapidly consumes thiamine (vitamin B1), which is an essential cofactor for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase; deficiency can cause Wernicke encephalopathy and lactic acidosis
7.2 Risk Factors for Refeeding Syndrome
High risk (one or more of the following):
| Criterion | Threshold |
|---|
| BMI | < 16 kg/m2 |
| Unintentional weight loss | > 15% in the preceding 3-6 months |
| Minimal or no nutritional intake | > 10 days |
| Low baseline electrolytes before feeding | Low phosphate, potassium, or magnesium |
Moderate risk (two or more of the following):
| Criterion | Threshold |
|---|
| BMI | < 18.5 kg/m2 |
| Unintentional weight loss | > 10% in the preceding 3-6 months |
| Minimal or no nutritional intake | > 5 days |
| History of alcohol misuse | Or drugs including insulin, chemotherapy, antacids, diuretics |
Additional ICU-specific risk factors:
- Chronic alcoholism
- Anorexia nervosa / eating disorders
- Oncology patients (especially head and neck cancer, esophageal cancer)
- Elderly patients with poor oral intake before admission
- Chronic malabsorptive conditions
- Prolonged ICU stay with inadequate nutrition support
- Post-bariatric surgery patients
7.3 Refeeding Syndrome Prevention Protocol
| Day | Caloric Prescription | Monitoring | Supplementation |
|---|
| Pre-feeding | — | Check baseline PO4, K, Mg, Ca, glucose, thiamine level (if available) | Thiamine 200-300 mg IV daily x 3 days BEFORE starting nutrition or concurrently with first feed; correct electrolyte deficiencies |
| Day 1 | Start at 10 kcal/kg/day (or as low as 5 kcal/kg/day in extreme cases, e.g., BMI < 14) | PO4, K, Mg every 6-12 hours; glucose every 4-6 hours; daily weights; strict I/O | Phosphate 0.3-0.6 mmol/kg/day IV (adjust for levels); K 1-2 mEq/kg/day; Mg 0.3-0.4 mEq/kg/day; MVI daily |
| Day 2-3 | If electrolytes stable: advance to 15 kcal/kg/day | Continue electrolyte monitoring every 8-12 hours | Continue thiamine, electrolyte supplementation |
| Day 4-5 | If electrolytes stable: advance to 20 kcal/kg/day | Electrolytes every 12-24 hours | Continue thiamine through day 5-7 minimum |
| Day 6-7 | Advance to 25 kcal/kg/day (or full target) | Transition to daily electrolytes | Adjust supplementation based on levels |
| Day 7+ | Full target calories | Standard monitoring | Standard supplementation |
7.4 Electrolyte Replacement in Refeeding
| Electrolyte | Target Level | Replacement Strategy |
|---|
| Phosphate | > 2.5 mg/dL (0.81 mmol/L) | Mild (2.0-2.5): PO4 15-30 mmol IV over 4-6 hours. Moderate (1.0-1.9): PO4 30-45 mmol IV over 4-6 hours. Severe (< 1.0): PO4 45-60 mmol IV over 8-12 hours; recheck in 2-4 hours |
| Potassium | > 4.0 mEq/L | KCl 20-40 mEq IV over 1-2 hours (max 20 mEq/hr via central line; 10 mEq/hr peripheral); recheck 2-4 hours after replacement |
| Magnesium | > 2.0 mg/dL (0.82 mmol/L) | MgSO4 2-4 g IV over 2-4 hours; recheck 4-6 hours after replacement |
| Thiamine | — (empiric dosing) | Thiamine 200-300 mg IV daily for minimum 3-5 days; transition to 100 mg oral/IV daily |
7.5 Clinical Manifestations of Refeeding Syndrome
| System | Manifestations |
|---|
| Cardiac | Arrhythmias (VT, VF, torsades de pointes), heart failure, hypotension, cardiac arrest |
| Respiratory | Respiratory muscle weakness, failure to wean from ventilator, respiratory failure |
| Neurologic | Wernicke encephalopathy (confusion, ophthalmoplegia, ataxia), delirium, seizures, coma |
| Hematologic | Hemolytic anemia, thrombocytopenia, leukocyte dysfunction |
| Metabolic | Lactic acidosis, hyperglycemia, metabolic alkalosis |
| Musculoskeletal | Rhabdomyolysis, weakness |
| Fluid | Peripheral edema, pulmonary edema |
8. Micronutrient Supplementation in Critical Illness
8.1 Thiamine (Vitamin B1)
| Parameter | Details |
|---|
| Role | Essential cofactor for pyruvate dehydrogenase (aerobic glucose metabolism), alpha-ketoglutarate dehydrogenase (Krebs cycle), and transketolase (pentose phosphate pathway) |
| Deficiency prevalence in ICU | 20-70% of critically ill patients (depending on population and assay used) |
| High-risk populations | Chronic alcoholism; malnutrition; refeeding syndrome; chronic diuretic use; hyperemesis; bariatric surgery; prolonged TPN without adequate supplementation |
| Clinical manifestations of deficiency | Wernicke encephalopathy (confusion, oculomotor dysfunction, ataxia — classic triad present in only ~16% of cases); beriberi (wet: high-output heart failure; dry: peripheral neuropathy); lactic acidosis (impaired aerobic metabolism) |
| Dosing — refeeding prevention | 200-300 mg IV daily for 3-5 days, then 100 mg IV/PO daily |
| Dosing — suspected Wernicke | 500 mg IV three times daily for 3-5 days, then 250 mg IV daily for 3-5 days, then 100 mg PO daily |
| Dosing — sepsis (empiric) | 200 mg IV every 12 hours for 7 days (based on sepsis vitamin cocktail studies) |
| Administration | IV push or short infusion (30 minutes); stable in PN admixtures |
| Safety | Very low toxicity; anaphylaxis is extremely rare with IV administration |
8.2 Vitamin C (Ascorbic Acid)
Vitamin C is a potent antioxidant and essential cofactor for catecholamine and cortisol synthesis. Its role in sepsis and critical illness has been extensively studied.
| Parameter | Details |
|---|
| Role | Antioxidant; cofactor for catecholamine synthesis (dopamine beta-hydroxylase); collagen synthesis; immune function; enhances iron absorption |
| Deficiency in ICU | Common (up to 40-70% of critically ill patients have low vitamin C levels) |
| Standard supplementation | 100-200 mg/day (as part of standard MVI in PN) |
Key Trials in Sepsis
| Trial | Design | Intervention | Key Findings |
|---|
| CITRIS-ALI (2019) | Multicenter RCT; n = 167; ARDS + sepsis | Vitamin C 50 mg/kg IV every 6 hours x 96 hours vs placebo | No difference in primary outcome (modified SOFA or inflammatory biomarkers at 96 hours); secondary analysis showed reduced 28-day mortality (29.8% vs 46.3%, p = 0.03) — but this was a secondary outcome and should be interpreted cautiously |
| VITAMINS (2020) | Multicenter RCT; n = 216; septic shock | Vitamin C 1.5 g IV q6h + hydrocortisone 50 mg IV q6h + thiamine 200 mg IV q12h vs hydrocortisone alone | No difference in time alive and vasopressor-free to day 7; no mortality difference |
| LOVIT (2022) | Multicenter RCT; n = 872; sepsis | Vitamin C 50 mg/kg IV every 6 hours x 96 hours vs placebo | Potential harm: higher composite of death or persistent organ dysfunction at day 28 in vitamin C group (44.5% vs 38.5%, RR 1.21, 95% CI 1.04-1.40) |
Current recommendation: High-dose vitamin C (>= 6 g/day) is NOT routinely recommended in sepsis based on current evidence. Standard supplementation (100-200 mg/day) remains part of routine micronutrient support in PN. Further trials are needed.
| Citation (LOVIT) | Lamontagne F, Masse MH, Menard J, et al. N Engl J Med. 2022;386(25):2387-2398 |
|---|
| DOI | 10.1056/NEJMoa2200644 |
8.3 Vitamin D
| Parameter | Details |
|---|
| Deficiency prevalence | 40-70% of ICU patients have 25(OH)D levels < 20 ng/mL (deficiency); up to 80-90% may have suboptimal levels |
| Role | Calcium homeostasis; immune modulation (innate and adaptive immunity); skeletal muscle function; anti-inflammatory effects |
| Association with outcomes | Low vitamin D is associated with increased mortality, longer ICU stay, and greater infection risk — but association does not prove causation |
| Key trial: VIOLET (2019) | RCT; n = 1,078; critically ill adults with 25(OH)D < 20 ng/mL; single dose of 540,000 IU vitamin D3 vs placebo. No difference in 90-day mortality (23.5% vs 20.6%), hospital LOS, or ventilator-free days |
| Key trial: VITdAL-ICU (2014) | RCT; n = 475; critically ill adults with 25(OH)D <= 20 ng/mL; loading dose 540,000 IU oral vitamin D3 vs placebo. No difference in 28-day mortality overall; possible benefit in severe deficiency subgroup (25(OH)D <= 12 ng/mL): lower hospital mortality (28.6% vs 46.1%, p = 0.04) |
| Recommendation | Check 25(OH)D level in all ICU patients. Supplement if deficient (< 20 ng/mL). Standard approach: cholecalciferol 50,000 IU weekly or 10,000 IU daily for loading, then 1,000-2,000 IU daily. High single-dose bolus (> 300,000 IU) has not shown benefit and may have risks. |
8.4 Selenium
| Parameter | Details |
|---|
| Role | Component of glutathione peroxidase (antioxidant defense); selenoproteins (thyroid hormone metabolism, immune function) |
| Deficiency in ICU | Common; depletion accelerated by systemic inflammation and oxidative stress |
| Evidence | Mixed results in clinical trials. The REDOXS trial (see Part 4) found that high-dose selenium (as part of an antioxidant cocktail with glutamine) was associated with a trend toward increased mortality. Smaller earlier trials suggested potential benefit in sepsis. |
| Recommendation | Standard trace element supplementation in PN (60-100 mcg/day selenium). High-dose supplementation (>= 500 mcg/day) is NOT recommended based on current evidence. |
| Monitoring | Serum selenium levels can be checked but are not routinely monitored |
8.5 Zinc
| Parameter | Details |
|---|
| Role | Immune function (T-cell and NK-cell activity); wound healing; protein synthesis; > 300 enzyme cofactor |
| Deficiency in ICU | Very common; redistributed from plasma to liver during acute phase response (serum zinc is a negative acute-phase reactant, similar to albumin) |
| Populations at highest risk | Burns (massive zinc losses in wound exudate), chronic alcoholism, malabsorption, high-output GI fistulae, chronic diarrhea, CRRT |
| Supplementation | Standard: 3-5 mg/day in PN; additional: 10-30 mg/day in patients with large wound/GI losses or burns |
| Burns-specific dosing | Zinc supplementation 25-40 mg/day (often combined with copper and selenium) for patients with major burns |
| Monitoring | Serum zinc (interpret cautiously — levels drop with inflammation regardless of stores) |
8.6 Copper
| Parameter | Details |
|---|
| Role | Ceruloplasmin synthesis (iron metabolism); cytochrome c oxidase (mitochondrial function); lysyl oxidase (collagen/elastin cross-linking); superoxide dismutase (antioxidant) |
| Deficiency signs | Microcytic anemia (refractory to iron), neutropenia, osteoporosis, neurologic dysfunction |
| PN supplementation | 0.3-0.5 mg/day standard; reduce or omit in cholestasis (copper is excreted in bile; accumulation risk with cholestasis) |
| Monitoring | Serum copper and ceruloplasmin; interpret in context of inflammation (copper is a positive acute-phase reactant — levels may be falsely elevated during inflammation) |
8.7 Summary of Micronutrient Recommendations
| Micronutrient | Standard PN Dose | High-Risk Indications for Extra Supplementation | Do NOT Give High Dose |
|---|
| Thiamine (B1) | 6 mg/day (in MVI) | Refeeding risk: 200-300 mg IV/day; Wernicke: 500 mg IV TID | — |
| Vitamin C | 200 mg/day (in MVI) | Standard supplementation only | >= 6 g/day in sepsis (LOVIT: potential harm) |
| Vitamin D | Check level; supplement if deficient | 50,000 IU weekly or 10,000 IU/day loading if < 20 ng/mL | Single dose > 300,000 IU (no benefit) |
| Selenium | 60-100 mcg/day | Standard dose only | >= 500 mcg/day (REDOXS: trend toward harm) |
| Zinc | 3-5 mg/day | Burns: 25-40 mg/day; high GI losses: 10-30 mg/day | — |
| Copper | 0.3-0.5 mg/day | Wound healing | Reduce/omit in cholestasis |
9. Glycemic Control During Nutrition Support
9.1 Target Blood Glucose Range
| Parameter | Recommendation |
|---|
| Target range | 140-180 mg/dL (7.8-10.0 mmol/L) |
| Avoid | Tight glycemic control (80-110 mg/dL) — the NICE-SUGAR trial demonstrated that tight control increased mortality (27.5% vs 24.9%, p = 0.02) and severe hypoglycemia (6.8% vs 0.5%) compared to conventional control (target < 180 mg/dL) |
| Insulin protocol | Continuous IV insulin infusion preferred in the ICU; subcutaneous basal-bolus for stable patients transitioning to ward |
| Monitoring | Blood glucose every 4-6 hours during PN/EN; every 1-2 hours during insulin infusion titration |
9.2 Nutrition-Specific Glycemic Management
| Scenario | Approach |
|---|
| Persistent hyperglycemia (> 180 mg/dL) on EN | Start insulin infusion; do NOT hold EN for hyperglycemia alone; consider diabetes-specific formula |
| Persistent hyperglycemia on PN | Start insulin infusion; reduce dextrose concentration; ensure GIR <= 5 mg/kg/min; add insulin to PN bag (regular insulin, typically starting at 0.1 units per gram of dextrose in the PN) |
| Hypoglycemia (< 70 mg/dL) after EN hold | Start D10W at 50-75 mL/hr; recheck glucose in 15-30 minutes; restart EN when possible |
| Hypoglycemia after PN interruption | Never abruptly stop PN; taper dextrose; run D10W at 50-75 mL/hr if PN interrupted unexpectedly |
References