Question 9

With respect to the use of Parenteral Nutrition (PN) in critically ill patients, briefly discuss using the following headings in your answer:

a) Supplemental PN – rationale, timing, along with your evidence-based approach to supplemental PN. (60% marks)

b) Principles of prescription of Total Parenteral Nutrition (TPN) for a critically ill ICU patient. (40% marks)

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College answer

This question was poorly answered with a common lack of details in specific areas, which were a lack of evidence, or evidence that was incorrectly cited / interpreted. There commonly no clear approach provided about the candidates own practice. Furthermore, in part b) of the question, there was minimal details about required vascular access and ongoing monitoring.


If one were still surprised by such things, one might retort that vascular access and monitoring are not a part of the TPN prescription, as (for example) they are usually not charted along with the TPN constituents on the fluid/medication chart. The questions reads like a repeat of Question 7 from the first paper of 2015, except this time the candidates were also expected to answer something that wasn't asked. An answer which ticks all the boxes for the examiners would resemble the following:

a) Supplemental PN

Rationale for supplemental PN

  • Malnutrition is a significant contributor to ICU morbidity and mortality
  • Many patients admitted to the ICU may already be malnourished at the time of admission
  • Enteral nutrition does not meet 100% requirements in most patients (Cahill et al, 2010)
  • With supplemental PN, 100% of nutritional requirements could be fully met earlier in the ICU stay

Timing of supplemental PN

  • 2022 ASPEN guidelines recommend supplemental PN after 7-10 days, "if unable to
    meet 60% of energy and protein requirements by the enteral route alone"
  • 2017 ESPEN guidelines recommend supplemental PN after 7 days "if the energy and nutrient requirements cannot be met by oral and enteral intake alone (< 50% of caloric requirement)"
  • ESPEN do not recommend contemplating supplemental PN "if the expected time period of PN is < 4 days"
  • Clinical trials (eg. Gao et al, 2022) defined early supplemental PN as starting day 3 following surgery

Evidence to support this practice

  • Systematic review and meta-analysis by Alsharif et al (2020) turned up only five studies, of which the largest was Heidegger et l (2013; n=320).
  • Supplemental PN did not affect morbidity and mortality
  • More recently,  Gao et al, 2022, n=230 in China - early supplemental PN in major abdominal surgery patients was associated with a reduced risk of infection, but other outcomes were unchanged.

Own practice

  • The CICM exam candidate is suggested to adopt a non-controversial position here which obeys the ASPEN and ESPEN kabbalah while demonstrating some consultant-level individuality, eg. "in my practice, I will add PN to EN on day 7 of inadequate nutritional intake, or earlier in patients with pre-existing severe malnutrition, or those in whom EN is clearly and predictably unlikely to be established within this timeframe, such as those with severe ileus or GI tract injury"

b) TPN prescription:

  • Usually the bag is about 2L
  • Carbohydrate: fat ratio: 70:30.
  • Protein is also required: 1.5-2g/kg/day
    • Fat is supplied as 10% lipid emulsion, at  1.1 kcal/ml
    • Carbohydrate is supplied as 50% dextrose, at 3.4 kcal/gram, or 1.7 kcal/ml
    • Protein is supplied as 10% amino acid solution, as 100g/L
  • Normal requirements are 25 kcal/kg/day
  • Thus, 17.5 kcal/kg/day is supplied by carbohydrate, and 7.5 kcal/kg/day is supplied by fat
  • Thus, a normal ICU patient getting 2000 kcal/day requires the following dose of TPN:
    • 1400 kcal/day of 50% dextrose (which makes about 824 ml)
    • 600 kcal/day of 10% lipid emulsion (which makes about 545ml)
    • 100 g/day of protein, which makes about 1000ml of 10% amino acid solution
  • Vascular access must be central venous
  • Monitoring:
    • Trace elements and fat-soluble vitamin levels at baseline
    • BSL every 6 hours on day 1, and then daily thereafter
    • CMP/EUC/LFTs watching for refeeding syndrome and hepatosteatosis (frequency depends on risk)
    • Serum cholesterol and triglycerides weekly at first, and then three-monthly
    • Regular fluid balance and weight measurements


The monitoring guidance comes from BAPEN (Parenteral Nutrition Monitoring) which is in turn based mainly on ESPEN and NICE guidelines, but it may be a bit dated (2016)

Hartl, Wolfgang H., et al. "Complications and monitoring–guidelines on parenteral nutrition, Chapter 11." GMS German Medical Science 7 (2009).

Russell, Mary K., and Paul E. Wischmeyer. "Supplemental parenteral nutrition: review of the literature and current nutrition guidelines." Nutrition in Clinical Practice 33.3 (2018): 359-369.

Berger, Mette M., et al. "Clinical nutrition issues in 2022: What is missing to trust supplemental parenteral nutrition (SPN) in ICU patients?." Critical Care 26.1 (2022): 1-5.

Cahill, Naomi E., et al. "Nutrition therapy in the critical care setting: what is “best achievable” practice? An international multicenter observational study." Critical care medicine 38.2 (2010): 395-401.

Gao, Xuejin, et al. "Effect of early vs late supplemental parenteral nutrition in patients undergoing abdominal surgery: a randomized clinical trial." JAMA surgery 157.5 (2022): 384-393.

Alsharif, Dalal J., et al. "Effect of supplemental parenteral nutrition versus enteral nutrition alone on clinical outcomes in critically ill adult patients: a systematic review and meta-analysis of randomized controlled trials." Nutrients 12.10 (2020): 2968.

Heidegger, Claudia Paula, et al. "Optimisation of energy provision with supplemental parenteral nutrition in critically ill patients: a randomised controlled clinical trial." The Lancet 381.9864 (2013): 385-393.

Compher, Charlene, et al. "Guidelines for the provision of nutrition support therapy in the adult critically ill patient: The American Society for Parenteral and Enteral Nutrition." Journal of Parenteral and Enteral Nutrition 46.1 (2022): 12-41.

Martindale, Robert G., et al. "Guidelines for the provision and assessment of nutrition support therapy in the adult critically ill patient: Society of Critical Care Medicine and American Society for Parenteral and Enteral Nutrition: Executive Summary*." Critical care medicine 37.5 (2009): 1757-1761.

Weimann, Arved, et al. "ESPEN guideline: clinical nutrition in surgery." Clinical nutrition 36.3 (2017): 623-650.