Acute respiratory failure (ALI and ARDS) is characterized by an increased inflammatory response and
hypercatabolism which can lead to significant nutritional deficits.
In response to the stress induced by infections, there is a strong protein catabolism at the expense of lean body mass and an increase in carbohydrate metabolism with consequent stress hyperglycaemia.
Respiratory failure can be worsened by preexisting sarcopenia, senile fragility, caloric-protein malnutrition, osteopenia and osteoporosis. Particularly at risk are patients with BMI <21Kg / m2 and Lean mass index <14-17 Kg / m2.
Nutritional support in patients in assisted ventilatory support has the primary objective of preventing cumulative caloric deficits, malnutrition, loss of lean mass and loss of strength of the respiratory muscles. ,br>
Furthermore, in critically ill patients, dysfunctions of the gastrointestinal tract occur quite frequently, characterized by alterations in absorption, transport and use of nutrients that contribute to malnutrition.
It is therefore important that nutritional support is optimized as far as protein-calorie requirement of an individual patient is concerned, and oriented towards a "personalized nutrition".
Evaluations to be carried out before starting Enteral Nutrition (EN) in the patient with respiratory failure:
• presence of conditions associated with hypermetabolism (i.e. sepsis or polytrauma)
• possible malnutrition
• degree of severity of the disease
• associated / preexisting clinical conditions
• gastro-intestinal function
• possible alcohol / drugs abuse
A comparison between IBW (Ideal Body Weight), ABW (Actual Body Weight) and BMI (Body Mass Index) is useful. It is therefore recommended to always measure patient's weight and height when entering the ICU.
Other signs of nutritional deficiency such as edema, cachexia, muscle wasting and swollen mucosa (glossitis, aftae, etc) should be assessed as well as a weight loss history, aiming to identify recent weight loss (> 5% in the last month) and BMI lower than 18.5 Kg / m2. Muscle strength can be measured through the hand grip tool.
Blood concentration of the various metabolic compounds reflect a balance between liver synthesis, circulation and degradation; these three elements often appear altered simultaneously in a critically ill patient, therefore they might be difficult to read and misleading.
Albumin is often related to an unfavorable prognosis in hospitalized patients, but it is a weak marker in the acute phase, having a long half-life (20 days); transferrin, prealbumin and fibronectin are very sensitive and change rapidly (7 days, 2 days and 4 hours respectively). Serum values, however also vary according to the acute phase, capillary permeability and inflammatory response.
A negative nitrogen balance (-5 to -30 g / day) reflects an important protein catabolism.
NITROGEN BALANCE calculation
= [protein intake (g / day) /6.25] - [Urinary urea N (g / day) + non urinary N (g / day)]
• Urinary urea (g / day) = urinary urea (g /l) /2.14 x diuresis (l) / day
• Non urinary N (g / day) = from 2 to 4 considering that the non urinary loss of nitrogen increases in some conditions such as severe burns, renal replacement treatment (RRT), abdominal drainage, etc.
During the acute phase of disease or in case of malnutrition, there are changes in the homeostasis of electrolytes, which can lead to hypernatremia, phosphate, magnesium and potassium deficiency, even if initially these elements show normal blood levels. Trace substances such as vitamins, hormones and enzymes reflect little of their intracellular concentration; determination of these parameters is used only for research purposes or to confirm diagnostic hypotheses.
NUTRITIONAL PARAMETERS AS RISK INDICES
It is recommended to assess nutritional risk of critically ill patients admitted to an ICU. Among the validated scores, we suggest the Nutritional Risk Screening (NRS 2002) and the NUTRIC Score. Compared to NRS 2002, the NUTRIC Score is considered more reliable in ICU setting. A high nutritional risk (NRS 2002 ≥5, NUTRIC Score ≥5 if IL-6 not available or> 5 if IL-6 available) selects those patients who will benefit most from an adequate nutritional intervention and early enteral nutrition (EN), as it has been demonstrated how an adequate nutrition can improve their outcome.
EN IN ICU
The goal of EN in the patient with respiratory failure is to preserve lean body mass, attenuate the metabolic response to stress and prevent cellular oxidative damage.
It has been demonstrated that NE promotes gut integrity and function of intestinal villi, supports the morphology and activity of GALT and MALT, promotes gut motility and reduces bacterial translocation with reducing risk of infectious complications.
In patients with respiratory failure, adequate energy supply, associated with proteins and cations (K+, Mg+) improves the contractility, strength and resistance of respiratory muscles (essential for an early weaning from the ventilator),
In patients with pulmonary deficit, a clinical nutrition program must take into consideration risks associated with high fluids volume, possible electrolyte imbalances, and metabolic stress (increase in CO2).
In a stable patient, early EN (within the first 24-48 hours of admission in ICU) is preferable, especially in patients with high nutritional risk (NRS 2002 ≥5, NUTRIC Score ≥5 if IL-6 not available or> 5 if IL-6 available).
NUTRITIONAL NEED ASSESSMENT
There are several methods of assessing patients’ energy needs, but none of them has been validated in ICU. Ideally, indirect calorimetry should be used, but calorimeters are not always available, therefore this technique is not part of the current clinical practice.
Usually, the basal metabolic rate is calculated with the formula of Harris-Benedict:
Man: BEE (kcal / day) = 66.45 + [13.75 x weight (kg)] + [5.00 x height (cm)] - [6.76 x age (years)]
Woman: BEE (kcal / day) = 655.10 + [6.56 x weight (kg)] + [1.85 x height (cm)] - [4.65 x age (years)]
[BEE = Basal Energy Expenditure]
For mechanically ventilated patients, the following Faisy equation is considered valid:
EE (kcal / day) = 8 x weight (kg) + 14 x height (cm) + 32 x MV + 94 x BT - 4834 [MV = minute ventilation (L / min); BT = body temperature (° C)]
If a calorimeter is not available, according to the ESPEN 2019 guidelines, the use of the measurement of VO2 (oxygen consumption) or VCO2 (production of carbon dioxide) from the ventilator data can be used. These values allow an accurate assessment of the energy expenditure compared to the predictive equation.
In clinical practice, the following simple equations, which have proven to be sufficiently reliable, can be used to determine total energy needs:
if BMI <30 kg/m2 → 20-25 kcal / kg ABW / day
if BMI between 30 and 50 kg/m2 → 11 - 14 kcal / kg ABW / day
if BMI> 50 kg/m2 → 22 - 25 kcal / kg IBW / day
ABW: Actual Body Weight
IBW: Ideal Body Weight
In critically ill patients, if feasible, EN should start early during the acute phase (1 – 5 days from entering ICU), within 48 hours of admission, with continuous enteral infusion, with the aim to gradually provide 80% of the caloric prescription within the 3-4th day.
Particular attention should be paid not to exceed with feeding. In fact, overfeeding is related to delayed weaning from mechanical ventilation (due to hypercapnia), can lead to hypernatriemia, electrolyte alterations, hyperglycaemia (and therefore increased risk of infection), immunosuppression and liver steatosis.
Underfeeding is dangerous as well. It is associated with failure of weaning from mechanical ventilation, due to reduction of muscle strength and respiratory drive.
An adequate protein intake in a critically ill patient is extremely important to maintain lean body mass. Scientific guidelines suggest the following protein intake in critically ill patients:
if BMI <30 kg/m2 → 1.2 - 2 g/kg ABW/day
if BMI between 30 and 39 kg/m2 → up to 2 g/kg IBW/day
if BMI> 39 kg/m2 → up to 2.5 g/ kg IBW day.
Particular attention must be given to the choice of the type of protein source, taking into account the incidence of gastrointestinal intolerance in critically ill patients: high quality protein are most suitable. Predigested protein source can ease digestion and absorption.
In addition, it should be remembered that to facilitate respiratory functions, ventilated patients are kept in a prone position: this position, nevertheless, reduces gastrointestinal tolerability, increases the risk of gastroesophageal regurgitation and therefore may increase the risk of intolerance of the enteral formula.
For all the above mentioned reasons, the choice of the type of protein should prefer whey protein over caseinates: whey proteins, do not precipitate in the acid pH environment of the stomach, promote faster gastric emptying and reduce the risk of reflux.
In addition, whey proteins are rich in cysteine, which is the limiting amino acid for the glutathione peroxidase redox activity: a higher cysteine content increases the free radicals clearance activity and helps reducing the oxidative stress.
Patients with respiratory failure have difficulty eliminating CO2 from the lungs, which results in dyspnoea, hypercapnia, hypoxia and respiratory acidosis. This exacerbates muscle loss through oxidative stress and inflammatory response. To mitigate these problems, the goal is to reduce the metabolic production of CO2 and the respiratory quotient.
Several studies have shown that high fat formulas reduce CO2 production and therefore the respiratory quotient. In addition, a high intake of lipids can be the most efficient means of increasing energy intake without excessive fluid load.