Question 7

A 26-year-old female is admitted to the ICU post operatively with faecal peritonitis as a result of multiple bowel perforations secondary to Crohn’s disease. She has had the majority of her small bowel resected and is to be prescribed total parenteral nutrition (TPN).

a) Describe the available methods to estimate total energy expenditure in critically ill patients and outline their advantages and limitations. (70% marks)

The basal energy expenditure of this patient is determined to be 2000 kcal (8400 kJ) / day and she weighs 50 kg.

b) Describe how you would prescribe her TPN. (30% marks)

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College Answer



  • This may be based just upon weight or surface area – Most critically ill patients will have requirements of approx. 25 kCal/kg/day.
  • Advantages – quick, simple and cheap. Universally available
  • Disadvantages – may be inaccurate

Predictive equations:

  • Many versions such as Harris-Benedict, PennState, Faisy etc., based upon various direct measurements.
  • Advantages – quick, simple and cheap. Universally available
  • Disadvantages – Inaccuracy, usually underestimate requirements. Need for multiple correction factors.

Indirect Calorimetry:

  • Measures oxygen uptake and carbon dioxide production using the assumption that all of the oxygen uptake is used for oxidation of substrates.
  • Advantages: Most accurate method. Bedside monitor than can be integrated with ventilator.
  • Disadvantages: Expensive; requires technical expertise, limited availability. Inaccurate in the setting of high FiO2 or PEEP, leaks in circuit, recent ventilator changes, changes in oxygen concentration, hemodynamic instability, temperature changes or haemodialysis.

Fick method

  • Determines oxygen consumption from indwelling pulmonary artery catheter, then uses caloric value for oxygen to calculate energy expenditure.
  • Advantages: More accurate than predictive equations, cheaper and more available than indirect calorimtery.
  • Disadvantages: Highly invasive. Does not account for pulmonary oxygen consumption.


Standard TPN delivery 2 litre bags

If the total non-protein kCal required is 2000/day, ratio for CHO to fat is 70:30


  • 1400Kcal
  • 824mls (412g dextrose at 50% solution at 3.4Kcal/gram and requiring 1400KCal)


  • 600Kcal
  • Using 10% lipid (1.1kcal/ml), will need 545mls 10% lipid
  • Adjust if using propofol as sedation (approx. 1kcal/ml as fat)

Protein 1.5-2g/kg/day

  • 2 x 50 = 100 grams/day of amino acids
  • Using 10% solution amino acid solution (100g/L) 1 Litre of 10% amino acid solution

Electrolyte, vitamins and trace elements are added to the solution in a standard fashion, but may be individually tailored to the patient’s requirements.

Additional comments:
Other  valid  methods  for  measurement  of  energy  expenditure  were  given  credit.  Detail  on
nutritional requirements was lacking in some answers


a) Methods to estimate energy expenditure:

A Comparison of Methods
to Estimate Metabolic Energy Requirements
in Critical Illness
Method Physiology Advantages Limitations
Predictive  Equations
  • Calculation of metabolic requirements made on the basis of empirical experimental data
  • Typically, input information is gender, height, age and weight
  • Specific metabolic abnormalities (eg. burns or sepsis) can be factored in as multipliers
  • Range from complex equations to simple (25cal×kg per day) formulae
  • Cheap
  • Quick
  • Requires no expertise
  • Accurate for many circumstances, particularly straightfrward ICU patients
  • Predict requirements, i.e. useful goals of management
  • Tend to be inaccurate
  • The sicker the patient, the less accurate the predictions
Reverse Fick method
  • Determines oxygen consumption from pulmonary artery catheter:
  • Oxygen utilisation in metabolic processes is correlated to the metabolic rate.
  • Knowing the cardiac output, one can calculate the oxygen consumption of the organism from the arteriovenous oxygen content difference.
  • Accurate - more so than predictive equations
  • Reproduceable
  • Cheaper than the metabolic cart, and more widely available
  • Invasive
  • Does not incorprate the metabolic requirements of the lungs
  • Inaccurate in severe pulmonary pathology, eg. ARDS
Indirect calorimetry
  • Oxygen uptake and CO2 production are monitored by a specialized module attached to the ventilator
  • From the consumption of oxygen, one can estimate the metabolic rate (assuming all oxygen is used to oxidise substrate)
  • The most accurate method of determining energy use
  • Module can integrate with the ventilator

Indications may include:

  • Extremes of obesity
  • Extremes of core body temperature (eg. in hypothermia)
  • Extremes of age
  • Very expensive
  • It makes the assumpation that all oxygen use is for oxidation of substrate
  • It is a complex procedure and it requires special equipment
  • It is a measure of metabolic fuel consumption, not demand.
  • It is not associated with any clinical benefit.
  • Inaccurate at high PEEP
  • Inaccurate with high FiO2
  • Invalid in the presence of circuit leak
  • Difficult to interpret if the ventilator settings keep changing rapidly


How do you prescribe TPN?

  • 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

This discussion of TPN is prescription is brief and offers little in addition to the college answer, but this is perhaps for the beast, as the authors' tendency to hold forth extensively on TPN is clearly demonstrated eslewhere. The 2014 ASPEN Clinical Guidelines on "Parenteral Nutrition Ordering, Order Review, Compounding, Labeling, and Dispensing" are probably the most important official resource for this answer.


Fink's Textbook of Critical Care: Chapter 94: Critical Care Nutrition by JUAN B. OCHOA, DAREN K. HEYLAND, STEPHEN A. McCLAVE.

Boullata, Joseph I., et al. "ASPEN Clinical Guidelines Parenteral Nutrition Ordering, Order Review, Compounding, Labeling, and Dispensing." Journal of Parenteral and Enteral Nutrition (2014): 0148607114521833

Singer, Pierre, et al. "ESPEN guidelines on parenteral nutrition: intensive care."Clinical Nutrition 28.4 (2009): 387-400.

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.

Casaer, Michael P., et al. "Early versus late parenteral nutrition in critically ill adults." N Engl J Med 365.6 (2011): 506-517.

Marik, Paul E., and Michael Hooper. "Parenteral versus enteral nutrition in the critically ill patient: a re-analysis of a flawed meta-analysis." Intensive care medicine 39.5 (2013): 979-980.

Doig, Gordon Stuart. "Parenteral versus enteral nutrition in the critically ill patient: additional sensitivity analysis supports benefit of early parenteral compared to delayed enteral nutrition." Intensive care medicine 39.5 (2013): 981-982.

Doig, Gordon S., et al. "Early parenteral nutrition in critically ill patients with short-term relative contraindications to early enteral nutrition: a randomized controlled trial." JAmA 309.20 (2013): 2130-2138.

Fremont, Richard D., and Todd W. Rice. "How soon should we start interventional feeding in the ICU?." Current opinion in gastroenterology 30.2 (2014): 178.

Stawny, M., et al. "Pharmaceutical point of view on parenteral nutrition." The Scientific World Journal 2013 (2013).

Waitzberg, Dan L., Raquel Susana Torrinhas, and Thiago Manzoni Jacintho. "New parenteral lipid emulsions for clinical use." Journal of Parenteral and Enteral Nutrition 30.4 (2006): 351-367.