The present study was conducted in the Anesthesiology and Reanimation Intensive Care Units of Pamukkale University Hospital with the approval of Pamukkale University Non-Interventional Clinical Research Ethics Committee dated 13.07.2021 and 13th board meeting numbered 60116787-020/83538. Between August 2021 and August 2022, 17 patients were included over 18 years of age who met the Berlin criteria and were diagnosed with ARDS sedated, intubated, and mechanically ventilated. Informed written consent was obtained from the relatives.

Patients included in the study were connected to a CARESCAPE R860 (GE Healthcare) mechanical ventilator. Rocuronium bromide was administered intravenously to eliminate the spontaneous respiratory effort and opioids for sedation. Tidal volume was adjusted as 6 ml/kg according to the estimated body weight; the respiratory rate was adjusted to ensure normocarbia in blood gas analysis; FiO2 was adjusted as PaO2 55 to 80 mmHg; end-inspiratory pause was adjusted to 20%, and inspiratory/expiratory ratio was adjusted as 1:2.

The ECOV-X (GE Healthcare) module was attached to the ventilator for gas measurements and allowed to warm up. A spirometer kit was inserted between the Y-piece in the ventilator circuit and the bacterial/viral filter with heat and moisture-retaining properties. An intratracheal pressure sensor was inserted to measure pressure levels independent of circuit and tube resistance and to evaluate them on the SpiroDynamics (GE Healthcare) application. EELV was measured with the ECOV-X (GE Healthcare) module using a modified NMBW technique with the change in FiO2. EELV was calculated using VO2 (oxygen consumption) and VCO2 (carbon dioxide production). After all the connections were completed, the VO2 and VCO2 values of the patients were measured. The PEEP titration procedure called Lung INview (GE Healthcare) was initiated in patients whose values stabilized within 30 min. Before the measurement, a recruitment maneuver was performed for 30-40 s at a PEEP level of 20 cmH2O. At four different PEEP levels of 15, 10, 5, and 0 cmH2O, a decreasing PEEP trial was performed, and the measurement results were recorded.

For the same PEEP levels, the shunt fraction decreases when a decreasing PEEP maneuver is used instead of an increasing PEEP maneuver. This suggests that the relationship between optimal PEEP and maximum compliance may be more accurate. Therefore, decreasing PEEP trials was preferred as the study protocol.

The measurement time at each PEEP level was chosen as 10 minutes. At the end of the measurement, static compliance was measured by applying an end-inspiratory pause. Tidal volume, peak, and driving pressure were recorded at each step. Respiratory system elastance was calculated using the equation by Henderson et al.(respiratory system elastance = driving pressure/tidal volume). The static strain was calculated using the equation using the tidal volume at the relevant PEEP value (static strain =VPEEP/FRC). The pressure-volume curve generated by the intratracheal pressure sensor at each PEEP level was evaluated using the SpiroDynamics application. A dynamic compliance curve was generated during the analysis, and volume changes in this curve were determined at each PEEP level. The difference in EELV at two different PEEP levels during a descending PEEP trial was calculated as ∆EELV, and the difference between ∆EELV and the volume derived from the pressure-volume curve was calculated as "volume gain" (gain = ∆EELV - volume derived from the curve). The estimated lung volume recovered was calculated using the formula ∆EELV - (∆PEEP x Compliance PEEPlow) and compared with the volume gain. The efficiency of the volume gains concept as an indicator of the volume gained by alveolar recruitment and its role in personalized PEEP titration was evaluated.