Nutrition and TPN
Remember the basics
It's easy to forget, amongst all the antibiotics, pressors and sedatives that we also need to be thinking about the basic building blocks of nutrition for our patients.
Ignoring the oxymoron that is NHS 'food', we need to know what the nutritional requirements are for an unwell human, so that we can supply them in some form to our intubated patient who cannot eat or drink for themselves.
Your patient not only needs to supply their basal metabolic rate, but they also need to contend with the increased energy demand and catabolism of being critically unwell.
How can nutritional requirement be assessed?
NICE recommend nutritional screening on hospital admission, as well as regular reassessment using a validated scoring tool such as:
- Malnutrition Universal Screening Tool (MUST) - BMI, weight loss and acute disease effect are used to calculate risk of malnutrition
- Nutrition Risk in the Critically Ill (NUTRIC) - designed for critically ill patients
Bear in mind, however, neither of these are actually properly validated in critically ill patients.
A simpler approach is just to realise that patients with:
- Pre-ICU weight loss
- Poor muscle mass
- ICU admission of more than 48 hours
Are all high risk for malnutrition.
How can you measure someone's current nutritional status?
Start with a history:
- Comorbidities
- Current illness
- Drug history
- Dietary history
Then some basic body measurements:
- Appearance, cachexia
- Height and weight
- Change in weight over time
- Muscle mass
- Body fat percentage
Then look at some bloods and fluids:
- Electrolytes
- Plasma proteins - albumin is a bad marker of nutritional status because of its very long half life
- 24 hour urine urea and creatinine - gives an idea of muscle mass and nitrogen metabolism
What happens to energy requirement in critical illness?
Unsurprisingly, it goes up.
In an ideal situation you're going to figure out the patient's energy expenditure using indirect calorimetry by measuring their oxygen consumption (VO2) and CO2 production (VCO2).
In a pinch, even just using one of these parameters is still better than using generic feeding equations.
Can you name any feeding equations?
- Harris-Benedict
- Schofield
These approximate energy expenditure using sex, height, weight, age and activity level, but can vary enormously in what answer they churn out.