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 of 12 cm H 2 O for 30 min, which was then abruptly reduced to 2 cm H 2 O. Finally, a decremental PEEP trial was performed to measure collapse and overdistension at each level. Gas exchange, end-tidal carbon dioxide, 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. The R/I was calculated during the single-breath derecruitment maneuver from ventilator data. RESULTS: The R/I (median = 0.41, range = 0 to 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 of less than or equal to 0.40 (n = 10), those with R/I greater than 0.40 (n = 10) required higher PEEP to optimize regional mechanics and balance collapse and overdistension (median [quartile 1, quartile 3]: 10 [8, 11] cm H 2 O vs . 8 [7, 9]; P = 0.03) and 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, the R/I reflects potential for lung recruitment. The R/I may help identify patients in whom higher PEEP improves physiology and may favor less injurious ventilation.