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The R/I ratio showed limited accuracy in predicting lung recruitability in ARDS but reliably identified low recruiters. 

The recruitment-to-inflation (R/I) ratio was introduced in a study published in June 2025 issue of Critical care as a bedside measure to assess lung recruitability with positive end-expiratory pressure (PEEP), though it had only been validated against pressure–volume curves and lacked a clearly defined threshold for distinguishing low from high recruiters.  

Researchers evaluated the diagnostic performance of the R/I ratio using computed tomography (CT) as the gold standard in individuals with acute respiratory distress syndrome (ARDS).  

They assessed adults with ARDS who were receiving invasive mechanical ventilation. The R/I ratio was measured using the ventilator. Low-dose CT scans were obtained at PEEP levels of 15 and 5 cmH₂O to assess alveolar recruitment. Participants were classified as high recruiters if the PEEP-recruited lung accounted for more than 5% of total lung weight on CT.  

The results showed that 50 individuals were enrolled, and 8 were later excluded (7/50 [14%] due to suspected air leaks and 1/50 [2%] for airway opening pressure (AOP) > 15 cmH₂O). Among the remaining, 33 [79%] were identified as low recruiters and 9 [21%] as high recruiters based on CT. The median R/I ratio was 0.67 [IQR: 0.57–0.74] in high recruiters and 0.42 [IQR: 0.24–0.64] in low recruiters, with no significant difference (P= 0.07). No significant correlation was observed between the R/I ratio and CT-measured recruited lung weight (R² = 0.065; P= 0.10). The area under the curve (AUC) for detecting high recruiters using the R/I ratio was 0.70 [CI_95%: 0.52–0.89], with 0.57 as the optimal threshold. Sensitivity and specificity at this cutoff were 78% [CI_95%: 44%–100%] and 67% [CI_95%: 52%–82%], respectively. The false positive rate was 11/33 [33%], and the false negative rate was 2/9 [22%], with all false negatives occurring in cases with focal ARDS. Misclassification was largely attributed to overestimation of PEEP volume by ventilators, confirmed in a bench study for 2 of the 3 devices used.  

Investigators concluded that the R/I ratio showed limited diagnostic accuracy for predicting lung recruitability in ARDS but reliably identified low recruiters when values were below 0.57 in non-focal cases. 

Source: https://ccforum.biomedcentral.com/articles/10.1186/s13054-025-05453-0