Predictive equations to estimate nutritional requirements

Because the metabolic cart for indirect calorimetry is frequently a cumbersome pain in the arse to deploy, lazy people have developed arithmetical shortcuts which - to some extent - predict the energy requirements of critically ill patients without having to resort to precise measurements of their gas exchange volumes. In the past papers,  Question 28 from the second paper of 2007 and Question 7 from the first paper of 2015 required some knowledge of these equations' strengths and weaknesses, but so far the college has not asked anything too indepth.

The Harris-Benedict Equation

This formula was arrived at after numerous empirical experiments, from data collected by the Nutrition Laboratory of the Carnegie Institution, under the direction of Francis G. Benedict.

They published in 1918, and their equation (with some revisions) to this day remains the most commonly used method of estimating the basal metabolic rate.

This equation can also be adjusted to accomodate different, non-basal rates of metabolism - by simply multiplying it by a factor which estimates the increased energy demand.

For instance, if one is engaged in heavy exercise, on can multiply one's BMR value by 1.75-2.00.

Similarly, multipliers are available to reflect the fact that critically ill patients frequently have very abnormal energy requirements, owing to their state of critical illness (or in some cases the perverse treatments they are receiving).

Even after all these years, critics reluctantly agree that "All of the variables used in the equations have sound physiologic basis for use in predicting BEE"(basal energy expenditure).

The Ireton-Jones Formula

This is a more recent formula than the Harris-Benedict, and it was devised specifically to estimate the deranged patterns of energy expenditure in ventilated burns patients.

This equation adjusts for the changes in metabolic demand due to mechanical ventilation, and makes special provisions for the hypercatabolic state of trauma and burns.

The Fusco formula

The simplicity of this equation almost allows one to forgive that it mixes Imperial and scientific measurements. I suppose we are to be thankful that one's height is not measured in handspans instead.

This equation was developed with a view to estimate the energy requirements of obese patients, and to prevent their overfeeding in intensive care.

The Frankenfield formula

This equation was developed with sepsis and trauma in mind.

The investigators had found that minute volume correlated with basal metabolic rate, and thus have factored it into their equation. Haemoglobin also features, and its difficult to explain why. The possibilities for multipliers are endless; the investigators found a coefficient which one can use to multiply their dobutamine dose, thereby factoring it into the BMR calculations. Ultimately, the authors are forced to admit that their regression equations "probably apply only to severe trauma and sepsis", and that "other studies should be conducted to predict energy expenditure in other patient types".

A comparison of predictive formulae

A comparison of these predicitive equations with indirect calorimetry and the Fick method was performed. None except the Ireton-Jones and the Frankenfield equation were particularly good. In the end, analysis revealed that even those two best performers had a pretty poor correlation coefficient, suggesting that for any individual patient they would produce inaccurate results.

In short, predictive equations - though useful as vague estimates - can never be as good as direct measurements.