Assessment of V/Q mismatch during pressure support ventilation with electrical impedance tomography: a prospective physiological study

Introduction: Spontaneous breathing may have both protective and negative effects in patients with ARDS, according to the severity of lung injury. Scarce evidence is available for physicians to safely guide the transition from controlled to assisted ventilation of ARDS patients. We aimed at describing variations of V/Q matching, measured with electrical impedance tomography (EIT), in patients recovering from ARDS, ventilated with different levels of pressure support. Methods: We performed a single-centre prospective observational study (Clinicaltrial.gov: NCT05781802), including adult mechanically ventilated patients admitted to the ICU with a diagnosis of ARDS according to the Berlin definition. The period of interest for the study was the transitioning phase from controlled to pressure support ventilation (PSV), and two observations were conducted. Data collection occurred at high and low pressure support, with each patient serving as his own control. The two conditions were defined according to a P0.1 threshold of 2 cmH2O (i.e. P0.1 < 2 cmH2O was considered "High PS" and P0.1 > 2 cmH2O was considered "Low PS"). The primary outcome was V/Q matching at the two different conditions. Results: We included a total of 15 patients receiving pressure support ventilation, after a median of 3 days of protective controlled ventilation. The median age was 69 y.o., and P/F at ICU admission was 132 [125–150] mmHg. The ΔPsupport difference between the two observations was 10 [10–10] cmH2O; pCO2 was 41 [37–47] mmHg at high support and 45 [41–50] mmHg at low support (P < 0.05), while tidal volume decreased (10.4 [9.8–11.9] ml/kg high; 8 [7.1–9] ml/kg low, P < 0.01). V/Q matching did not significantly differ from high pressure support (56.1% [46.4–69]) to low-pressure support (61.7% [56.7–69.5], P = 0.847). Still, nine patients improved V/Q matching at lower support, and the improvement between the two study steps was correlated with a higher PEEP level (ρ = 0.539, P = 0.038). Conclusions: Reducing the level of pressure support determined a redistribution of ventilation that did not, on average, result in improved V/Q matching compared to higher support. Our data underline the need for personalized settings during the transition from controlled to assisted mechanical ventilation in patients recovering from ARDS.