Characteristic Features of Artificial Nutrition in Acute Respiratory Failure

Similar to other critically ill patients, some days of starving or hypoalimentation lead to caloric deficit; this is even more relevant in ARDS. Nutrition support is then necessary to prevent cumulative caloric deficits, malnutrition, loss of lean body mass, and deterioration of respiratory muscle strength [13, 24-26].

Several aspects of energy consumption physiology and of substrate supply, disposal and transformation, oxidation, or storage, generally not considered in the absence of respiratory failure, may have a relevant impact and have to be carefully taken into account when feeding critically ill patients with severe respiratory failure.

The human body extracts the energy that is needed to comply to processes requiring mechanical (muscular contraction), chemical (biosynthesis), and osmotic (active transport) work by oxidizing a mixture of CHO, lipids, and protein to produce carbon dioxide (CO2), water, and nitrogen. Part of the energy packed in substrates is in fact utilized to generate adenosine triphosphate (ATP), while the remainder is converted to heat used to maintain body temperature. The energy ingested in excess to the expenditure is inevitably stored [27, 28]. As only small amounts of energy can be derived by phosphorylation at substrate level (glycolysis), energy is mainly released from substrate oxidation, so that VO₂ and REE are closely correlated.