Comparison of respiratory mechanics according to tidal volume obtained from predicted, current estimated and no body weight prediction method in patients under pressure controlled mechanical ventilation.

All the calculation will be done for every participant. Each phase correspond to one of the three interventions. Each phase will be performed once over a single day. PHASE 1: calculation of predicted body weight through the formulas:50+0,91(height in centimeters)-152,4 for men and 45,5 +0,91 (height in centimeters)-152,4 for women. The result will be multiplied by 6 ml (this is the ARDS recommendation of protective ventilation) and the final result will be the tidal volume. We'll adjust the pressure limit that offers this tidal volume and keep the ventilation for 2 hours. After, the respiratory mechanics will be calculated according to the found tidal volume. PHASE 2: calculation of the current estimated body weight through the formula: [0,98 X calf circumference(cm) + 1,16 X knee height(cm)]+[1,73 X arm circumference (cm)]+0,37 X subscapularis skinfold(mm)]-81,69 for men and [1,27 X calf ircumference(cm)]+[0,87 X knee height(cm)]+0,98 X arm circumference (cm)]+0,4 X subscapularis skinfold(cm)]-62 for women. The result will be multiplied by 6 ml and the final result will be the tidal volume. We'll adjust the pressure limit that offers this tidal volume and keep the ventilation for 2 hours. After, the respiratory mechanics will be calculated according to the found tidal volume. PHASE 3: for no body weight prediction method we'll use the tidal volume observed at the mechanical ventilator initially. After, the respiratory mechanics will be calculated according to this tidal volume. Then the results of respiratory mechanics corresponding to each phase will be compared and defined which prediction body weight method is more appropriate, closer to normal values os respiratory mechanics.