The goal of this physiological study is to determine whether ventilator-delivered sigh breaths during pressure support ventilation (PSV) provide a reliable bedside index of lung recruitability and can guide PEEP optimization.The main questions it aims to answer are:

• Does respiratory system compliance measured at the end of a sigh (Crs_sigh) differ from compliance obtained with an inspiratory hold during an assisted breath (Crs_assisted), and can the Crs_sigh/Crs_assisted ratio indicate recruitability?
• Does adjusting PEEP based on the sigh-derived recruitability index improve respiratory mechanics and gas exchange compared with usual clinical PEEP settings?

Background and RationaleInvasive mechanical ventilation is often necessary in critically ill adults but may contribute to ventilator-induced lung injury (VILI) if ventilator settings are not individualized. During assisted ventilation (pressure support ventilation, PSV), ongoing patient effort complicates the assessment of PEEP response, PEEP titration, and lung recruitability.Ventilator-delivered sigh breaths (i.e., brief sustained inflations) can improve gas exchange and promote alveolar recruitment, and may permit artifact-free assessment of respiratory mechanics in assisted modes. This study evaluates whether Crs measured at the end of a sigh provides reliable, clinically useful information on recruitability and whether the ratio of Crs during sigh to Crs during an assisted breath (Crs_sigh/Crs_assisted) can guide positive end-expiratory pressure (PEEP) optimization.Primary ObjectiveTo assess lung recruitability during PSV by normalizing Crs measured at the end of a sigh to Crs obtained with an inspiratory hold during an assisted breath.Secondary Objectives

• To quantify the proportion of patients in whom Pplat during assisted breathing is not reliable (non-readable or unstable ≥3 s).
• To determine whether adjusting PEEP according to the sigh-derived recruitability index improves Crs.

Inclusion criteria:Adults (≥18 years) receiving PSV with the ventilator's sigh function active.Exclusion criteria:

• Clinical contraindication to increasing PEEP
• Hemodynamic instability defined as SOFA cardiovascular score ≥3

Protocol OverviewEach participant undergoes two sequential, non-randomized steps:

• Step 1 (Clinical PEEP): Measurements are performed after ≥15 minutes at the treating team's current PEEP.
• Step 2 (Clinical PEEP +3 cmH₂O): PEEP is increased by 3 cmH₂O; measurements are repeated after ≥15 minutes of stabilization.

In both steps, a sigh is programmed as a sustained inflation at 30 cmH₂O for 3 seconds (pressure-controlled), per routine practice and prior literature.Sigh Setting

• Sigh frequency: 1 sustained inflation every minute.
• Sigh target: 30 cmH₂O for 3 seconds.
• If end-inspiratory flow did not reach 0 L/min, the inspiratory time of the sigh was extended to ensure an end-inspiratory alveolar pressure of 30 cmH₂O.
• The sigh pressure was identical in both PEEP steps.

Data CollectionBaseline demographics (age, sex, BMI), comorbidities, and hemodynamics are recorded. At the end of each 15-minute step (baseline and PEEP+3), we perform an end-inspiratory hold and an end-expiratory hold on a tidal assisted breath and measured:

• Plateau pressure (Pplat): Pplat is considered reliable only if the pressure trace is visually stable (flat) during the inspiratory hold.
• Static driving pressure: DP_st = Pplat - PEEP.
• Dynamic driving pressure: DP_dyn = Pressure Support + (ΔP_occ × 0.75), where ΔP_occ is the difference between total PEEP and the low airway pressure during the expiratory-hold maneuver.
• Tidal volume (Vt).
• Respiratory system compliance on tidal assisted breath: Crs_tidal = Vt / DP_st.
• P0.1.
• Occlusion pressure (P_occ).
• Pressure Muscle Index: PMI = P_peak - Pplat.
• Ventilatory Ratio (VR).
• Arterial blood gas (ABG). Compliance During Sigh and S/T IndexCrs during the mandatory sigh (Crs_sigh) is computed at each step using the sigh volume and pressure recorded once the inspiratory flow reaches 0 L/min and airway pressure (Paw) is visually stable.The S/T index is defined as the ratio Crs_sigh / Crs_tidal.Using baseline Crs_tidal, we estimate the sigh-induced volume (mL):V_sigh,expected = (Psigh_peak - PEEP) × Crs_tidal,baseline.We also derive a compliance valid for the pressure range above Pplat up to the sigh pressure ("over-plateau" compliance):Crs_overplat = (Vt_sigh - Vt_tidal) / (Psigh - Pplat).

Inclusion Criteria:

• Adults (≥18 years) receiving PSV with the ventilator's sigh function active.

Exclusion Criteria:

• Clinical contraindication to increasing PEEP
• Hemodynamic instability defined as SOFA cardiovascular score ≥3

Data may be shared after reasonable request to the Principal investigator.