CICM SAQ 2015.1 Q7
Question
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)
Answer
Answer and interpretation
a) Describe the available methods to estimate total energy expenditure in critically ill patients and outline their advantages and limitations
Empiric:
- 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.
b) Describe how you would prescribe her TPN
Standard TPN delivery 2 litre bags
If the total non-protein kCal required is 2000/day, ratio for CHO to fat is 70:30
Dextrose:
- 1400Kcal
- 824mls (412g dextrose at 50% solution at 3.4Kcal/gram and requiring 1400KCal)
Lipid:
- 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.
- Pass rate: 69%
- Highest mark: 7.8
Additional comments:
- Other valid methods for measurement of energy expenditure were given credit. Detail on nutritional requirements was lacking in some answers
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CICM
Chris is an Intensivist and ECMO specialist at The Alfred ICU, where he is Deputy Director (Education). He is a Clinical Adjunct Associate Professor at Monash University, the Lead for the Clinician Educator Incubator programme, and a CICM First Part Examiner.
He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives. He was one of the founders of the FOAM movement (Free Open-Access Medical education) has been recognised for his contributions to education with awards from ANZICS, ANZAHPE, and ACEM.
His one great achievement is being the father of three amazing children.
On Bluesky, he is @precordialthump.bsky.social and on the site that Elon has screwed up, he is @precordialthump.
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