Description

During the first 7 days of the patient on mechanical ventilation (MV), measurements will be taken. For this, the patient will be in the supine position (DS) or the prone position (DP). The patient will be on a dose of analgesics, under sedation and muscle relaxation and will not present any respiratory stimulus.

Data from MV will also be recorded (prior to recruitment assessment): tidal volume (TV) in milliliters (ml), respiratory rate (RR) in a minute, fraction of inspired oxygen (FiO2), positive end-expiratory pressure (PEEP), in centimeters of water (cmH2O), plateau pressure (Pplat, measured after an end-inspiratory pause, cmH2O), mean airway pressure (PM, cmH2O), minute volume (calculated as the product of RR and TV) in milliliters per minute, ∆P (calculated as Pplat minus PEEP, cmH2O), static compliance (calculated as TV divided by ∆P, ml/cmH2O). Arterial blood gases will also be considered.

The proposed recruitment assessment will be the measurement of pulmonary hysteresis ratio. For measurements, a ventilator (Neumovent GraphNet Advance-TS, Córdoba Argentina) will be used. Additionally, a dedicated software connected to a computer will be used to perfThe evaluation of pressure in relation to volume will also be carried out, through the pressure/volume curve (PV curve), available in the respirator. A low-flow inflation and deflation PV curve from 0 up to 40 cm H2O and from 40 down to 0 cm H2O will be performed using the automatic tool on the ventilator (P/V tool; Neumovent GraphNet Advance-ts, Córdoba Argentina). The P-V curve can be visualized immediately on the screen of the mechanical ventilator and will be taken as a method to assess lung recruitment through the hysteresis-like behavior of the respiratory system. Inflation and deflation volume data will be corrected for changes in oxygen consumption.

Ratio hysteresis (RH) of the respiratory system will be measured by planimetry using SigmaPlot 12.0 software. There, through a cursor, for the measurement of hysteresis, the place where the volume was greater was drawn and the coincident pressure value was considered. Subsequently the ratio hysteresis will be calculated as the ratio between hysteresis and the product of the pressure span and the maximum volume reached (maximum hysteresis). Also, through the maximum distance between the inspiratory and expiratory limb of the curve weighted by the maximum volume, the normalized maximum distance (Vmax) is calculated. Recruitment potential will considered when there is a HR ≥ 28% and aVmax ≥ 41%.

Data Analysis. Categorical variables will be presented as number and percentage, while continuous variables will be presented as mean and standard deviation or median and interquartile range, as appropriate. The Chi2 test or Fisher's exact test will be used for qualitative variables and the Student test or the Mann-Whitney U test for quantitative variables. To evaluate correlation, the Pearson or Spearman test will be used, according to the distribution of the evaluated variables. A p ≤ 0.05 will be used.