A Novel Noninvasive Tool to Assess Recruitability During Assisted Ventilation: The Sigh to Tidal Respiratory System Compliance Ratio

Background: Lung recruitability refers to the capacity of lung parenchyma to regain aeration and functional units in response to increased airway pressure. Assessing lung recruitability is clinically relevant because it may guide ventilator settings, particularly the selection of positive end-expiratory pressure (PEEP). No validated method exists to assess recruitability in pressure support ventilation (PSV) because the active respiratory drive of the patient interferes with the methods available for controlled mechanical ventilation. The application of a mandatory sigh (a breath 2-3 times greater in volume than tidal breath) during PSV is proven to be safe and physiologically beneficial. Research Question: Because a mandatory sigh transiently generates airway pressures higher than those of a conventional tidal breath, is it possible to detect lung recruitment potential during PSV by comparing the respiratory system compliance (Crs) of a sigh breath (Crs sigh) with that of a tidal breath (Crs tidal)? Study Design and Methods: This bicentric prospective study comprised 2 steps lasting 15 minutes each: clinical PEEP and clinical PEEP plus 3 cm H2O. Patients were in PSV mode with a periodic sigh set by the attending clinician. The level of pressure support was set by the attending clinician. The sigh was set as a sustained inflation of 30 cm H2O lasting 3 s/min. For every patient, we calculated Crs sigh, Crs tidal, and—exclusively at clinical PEEP—sigh to tidal compliance ratio (S/T; or Crs sigh to Crs tidal ratio). Results: One hundred ten patients were enrolled. Crs tidal was feasible. S/T of > 1 (36% of patients) was associated significantly with a higher Crs tidal and better oxygenation when PEEP was increased by 3 cm H2O as compared with baseline (median, 53 mL/cm H2O [interquartile range (IQR), 44-63 mL/cm H2O] vs 48 mL/cm H2O [IQR, 41-59 mL/cm H2O]; P < .001; and median PaO2 to FIO2 ratio of 308 [IQR, 191-352] vs 269 [IQR, 164-299]; P < .001; respectively). Conversely, S/T of ≤ 1 was associated with a significant reduction in both Crs tidal and PaO2 to FIO2 ratio when PEEP was increased. Interpretation: Our results show that normalizing Crs sigh over Crs tidal—leading to the S/T index—provides useful bedside information about the potential of lung recruitment in patients undergoing PSV. Clinical Trial Registration: ClinicalTrials.gov; No.: NCT07172061; URL: www.clinicaltrials.gov