How much protein and energy are needed to equilibrate nitrogen and energy balances in ventilated critically ill children

How to monitor and how to decrease protein hypercatabolism in critically ill children? The authors reply

To the Editor: It is known that the metabolic response to surgery, injury, and inflammation is characterized by protein catabolism (1). Most of the studies that reported protein intake have been using nitrogen balance as a gold standard of protein metabolism when a negative balance correlates with catabolism (2). Biolo (3) observed that the amino acid efflux from skeletal muscle provided precursors to “de novo” protein synthesis and energy fuel to the liver and to the rapidly dividing cells of the intestinal mucosa and the immune system. Gielen et al (4) hypothesized that higher amino acid concentration profiles could be a sign of hypercatabolism, but no other measurements of protein balance or other nutritional comparison were considered, bar the assessment of body mass index. Despite certain reservations regarding the accuracy of nitrogen balance, it may provide a better estimation of protein loss (2, 4). The measurement of amino acids profile without other nutritional variables might not be the best way to predict and calculate protein catabolism; furthermore, it behaves differently in postsurgical cardiac patients (1). Hsu et al (2) used a more complete nutritional evaluation by performing a study in adults with very similar design, but comparing urinary nitrogen excretion, nitrogen balance, serum albumin/pre albumin concentrations, and clinical outcome among patients receiving moderate and conventional insulin therapy in a medical ICU. They evidenced a lesser negative nitrogen balance and similar benefits, mainly in the first 72 hours of critical illness, in a group with moderate insulin therapy with a lower prevalence of hypoglycemia comparing to Gielen et al (4). It is well documented that patients who undergo cardiopulmonary bypass similar to other critically ill patients have hypercatabolism as a result of complex neuroendocrine responses (1, 3). The hormonal and metabolic responses to cardiopulmonary bypass are greater in neonates compared with older children and adults due to a variety of metabolic and physical changes after birth. With the marked variation in all patients and clinical conditions evaluated by Gielen et al (4) (different surgeries, underlying diseases, and cyanotic and acyanotic cardiac defects), it is rather difficult to compare protein metabolism using a single biomarker (amino acid profile). Higher levels of protein intake are required to reach a lower negative or positive nitrogen balance during the most catabolic phase of illness including the first 72 hours of a postoperative period of time after a cardiac surgery (1). In a systematic review, Bechard et al (5) found that at least 1.5 g/kg/d of protein intake was necessary to achieve positive protein balance in critically ill children receiving mechanical ventilation. It was observed that there is a strong association between malnutrition and poor wound healing, impaired immunity, increased risk of postoperative pneumonia, loss of skeletal muscle, and in patients after cardiac surgery additional loss of myocardial muscle, thus a complete and sequential nutritional evaluation should be performed (1). Should amino acid profiles be used as a biomarker of response to injury or to predict outcome using any kind of treatment in critically ill patients? Despite this question, it is essential to ensure an adequate supply of amino acids and maintain a proper nitrogen balance. The authors have disclosed that they do not have any potential conflicts of interest.

Caroline Pritsch

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How to monitor and how to decrease protein hypercatabolism in critically ill children