Hypercaloric and hyperproteic complete nutrition formulas are widely used in patients with ARF, particularly in the presence of volume overload. Water overload, lung edema and renal failure are frequent in patients with ARF, and are associated to a poorer prognosis. Therefore, in patients with ARF that require water restriction measures, high calorie content nutrition formulas are recommended (1.5–2kcal/ml), with a protein supply of 1.2–2.0g/kg/day.1 In recent years a number of studies have supported the use of trophic and non-complete enteral nutrition in patients with acute respiratory distress syndrome (ARDS) and in those in which prolonged MV is expected. No significant differences in terms of mortality, the duration of MV or infectious complications have been documented.2–4 Nevertheless, methodological limitations mean that these findings must be interpreted with caution.

Patients requiring MV during at least 96h are at nutritional risk. In individuals subjected to prolonged MV (over 8 days), it is advisable to secure at least 80% of the calorie target in a gradual manner in the first 8 days of MV, whether using enteral nutrition (EN) or total parenteral nutrition (TPN) or complementary feeding. This strategy has been associated to increased survival after 6 months and to improved physical recovery three months after discharge from the Intensive Care Unit (ICU).5 At present, it seems adequate for critical patients in the first week of admission to receive a calorie supply of 20–25kcal/kg/day, with a high protein supply (>1.5g/kg/day).6

Indirect calorimetry is the method of choice for calculating the calorie requirements, though its use is limited, particularly when high oxygen concentrations are used7.

Different studies have shown the benefit of starting early nutritional support in critical patients, though it must be remembered that an excessive energy supply causes lipogenesis, with an increased production of CO2, and this may contribute to weaning failure. The composition in terms of macronutrients possibly plays no relevant role in the production of CO2 in patients, provided the nutritional requirements are adjusted and overfeeding is avoided.8 On the other hand, diets rich in fats and poor in carbohydrates have not shown significant efficacy in critical patients with ARF.9

A mixture of long-chain fatty acids and medium-chain fatty acids in 1/1 or 1/2 proportion is able to reduce the amount of linoleic acid and reduce the effects upon pulmonary vasoconstriction. Linoleic acid can contribute to increase lung shunting and to unfavorable immune changes.9 However, no improved clinical outcomes have been demonstrated with such mixtures.

To date, the published data on the clinical efficacy of diets with combinations of fats rich in eicosapentanoic acid, docosahexaenoic acid, γ-linolenic acid and antioxidants in the nutrition of patients with ARDS have been contradictory. The heterogeneity of the studies, the lack of information referred to clinical management, the use of formulas with mixtures of pharmaconutrients, and the disparity of results imply that the routine use of such enteral formulas cannot be recommended.10 Lipid emulsions enriched with ω-3 fatty acids have not demonstrated a decrease in mortality, and the data referred to other outcomes such as nosocomial infection and mean duration of stay are contradictory.11,12

Hypophosphoremia is a frequent laboratory test finding in patients with ARF. It is associated to respiratory failure and can at least partially influence weakness of the respiratory muscles.13 The rapid correction of respiratory acidosis after MV can also contribute to hypophosphoremia by increasing glycolysis and facilitating phosphorus penetration into the intracellular compartment. It is particularly important to determine its levels in malnourished patients or individuals at risk of suffering refeeding syndrome.11

Although a common practice in the ICU, few studies have evaluated the safety of enteral nutrition in patients in prone decubitus. In a prospective, controlled before-after study of 38 patients ventilated in prone decubitus and receiving enteral nutrition, the authors recorded no increased incidence of digestive or respiratory complications versus supine decubitus, and recommended raising the patient headrest to 25° during MV in prone decubitus.14 The authors of another prospective study in patients ventilated in prone decubitus concluded that enteral nutrition is feasible and safe, and is not associated to an increased risk of gastrointestinal complications or bronchoaspiration pneumonia – provided tolerance is closely monitored.15 Both studies administered enteral nutrition via a nasogastric tube.

In patients requiring noninvasive MV, oral intake is typically inadequate and difficult to apply. Few studies have evaluated the administration of enteral nutrition in these patients, though some publications have reported an increase in airway complications in patients administered this type of nutrition and subjected to noninvasive MV – though with no impact upon mortality.16 Another prospective study did not find fasting or underfeeding to worsen the prognosis.17 Given the risk of bronchoaspiration, it seems reasonable to administer enteral nutrition at low doses, with close monitoring of signs of intolerance, and complementing with parenteral nutrition if the patient does not tolerate complete enteral dosing.

With regard to the use of high-flow oxygen therapy devices, these allow satisfactory reintroduction of oral feeding or the use of oral nutritional supplements, with no contraindications apart from those secondary to the patient disease process.18