Optimum electrical impedance tomography-based PEEP and recruitment-to-inflation ratio in patients with severe ARDS on venovenous ECMO

The significance of the Recruitment to Inflation (R/I) ratio in identifying PEEP recruiters in patients undergoing ultra-protective lung ventilation during venovenous ECMO is not well established. To compare the concordance of the R/I ratio and Electrical Impedance Tomography (EIT) in determining optimum PEEP settings in severe ARDS patients on ECMO and ventilated with very low tidal volumes. Initially, a low-flow insufflation was performed to detect and measure the airway opening pressure (AOP). Subsequently, the R/I ratio was calculated from PEEP 15–5 cmH2O, followed by a decremental PEEP trial (20–6 cmH2O in 2 cmH2O steps) monitored by EIT. The optimum EIT-based PEEP was defined as the intersection of the collapse and overdistension curves. Among 54 ECMO patients (tidal volume: 4.8 [3.0–6.0] mL/kg), 13 (24%) exhibited an airway opening pressure (AOP) of 11 (8–14) cmH2O. The cohort’s median R/I ratio was 0.43 (0.28–0.61). A tertile-based analysis of the R/I ratio (≤ 0.34; 0.34–0.54; > 0.54) revealed median optimum EIT-based PEEP of 8 [8–10], 10 [8–14], and 14 [12–16] cmH2O, respectively. The R/I ratio demonstrated weak inverse correlations with lung overdistension (R2 = 0.19) and positive correlations with lung collapse (R2 = 0.26) measured by EIT (p < 0.01). The R/I ratio is feasible during ultra-protective ventilation and provides valuable indications for guiding PEEP titration. Specifically, an R/I ratio > 0.34 may help identify patients likely to benefit from further individualized PEEP optimization using EIT. In contrast, when the R/I ratio is ≤ 0.34, a moderate PEEP level (8–10 cmH₂O) may suffice.