Evaluation of the potential for lung recruitment with the Recruitment-to-Inflation ratio during general anesthesia

Background: During general anesthesia, the effect of positive end-expiratory pressure (PEEP) on lung injury depends on the potential for lung recruitment, which is variable among patients. The recruitment-to-inflation ratio (R/I) is measured from ventilator data during a single-breath PEEP-release maneuver, and has been proposed to assess recruitability without the need for additional equipment during acute respiratory distress syndrome. This study hypothesized that R/I reliably estimates the individual potential for lung recruitment during general anesthesia. Methods: Twenty patients undergoing open abdominal surgery received mechanical ventilation with PEEP=12 cmH2O for thirty minutes, which was then abruptly reduced to 2 cmH2O. Finally, a decremental PEEP trial was performed to measure collapse and overdistension at each level. Gas exchange, end-tidal CO2, respiratory mechanics by ventilator waveform analysis, regional overdistension and collapse by electrical impedance tomography, end-expiratory lung volume and functional residual capacity by nitrogen dilution were measured. R/I was calculated during the single-breath derecruitment maneuver from ventilator data. Results: R/I (median=0.41, range=0-0.86) was linearly correlated with recruited volume normalized to functional residual capacity measured by gas dilution (r=0.48, slope=0.27 [0.03, 0.52]), and recruited volume scaled to PEEP-induced inflation volume measured by tomography (r=0.82, slope=1.2 [0.77, 1.64]). Compared to patients with R/I≤0.40 (n=10), those with R/I>0.40 (n=10) required higher PEEP to optimize regional mechanics and balance collapse and overdistension (median [Q1, Q3]: 10 [8, 11] cmH2O vs. 8 [7, 9], p=0.03), showed decreases in dead space (-2 [-5, 1] % vs. 3 [0, 5] %, p=0.013) and greater reductions in collapse (-44 [-31, -47] % vs. -30 [-20, -38] %, p=0.016) and dynamic lung strain (-0.06 [-0.09, -0.05] vs. -0.04 [-0.05, -0.02], p=0.04) with higher PEEP. Conclusions: During general anesthesia, R/I reflects potential for lung recruitment. R/I may help identify patients in whom higher PEEP improves physiology and may favor less injurious ventilation.