Diagnostic performance of the recruitment-to-inflation ratio to assess lung recruitability by PEEP in ARDS. a computed tomography study

The recruitment-to-inflation (R/I) ratio is a new bedside tool with potential to evaluate lung recruitability by positive end-expiratory pressure (PEEP) but was only validated using pressure-volume curves and the optimal threshold separating low and high recruiters by PEEP is not precisely known. The aim of the study is to evaluate the diagnostic performance of the R/I ratio using computed tomography (CT) as a gold standard in patients with acute respiratory distress syndrome (ARDS).

This study is a single-center prospective observational study performed on adults ARDS patients under invasive mechanical ventilation. The R/I ratio was measured with the ventilator, and low-dose CT scans were acquired at PEEP 15 and 5 cmH₂O to compute alveolar recruitment. Patients were classified as high recruiters if the recruited lung by PEEP was greater than 5% of lung weight on CT.

Fifty patients were included, and eight patients were secondarily excluded (7/50 [14%] for suspected air leaks, and 1/50 [2%] for airway opening pressure > 15 cmH₂O). Thirty-three patients [79%] were classified as low recruiters and 9 [21%] as high recruiters with CT. The median R/I ratio was not significantly higher in high recruiters (0.67 [IQR: 0.57-0.74]), than in low recruiters (0.42 [IQR: 0.24-0.64]), p = 0.07. There was no significant correlation between the R/I ratio and the weight of the recruited lung on CT (R² = 0.065; p = 0.10). The AUC of the R/I ratio to identify high recruiters was 0.70 (CI_95%: [0.52-0.89]), with an optimal threshold amounting to 0.57. Sensitivity and specificity at this threshold were 78% [CI_95%:44%-100%] and 67% [CI_95%: 52%-82%], respectively. The false positive and false negative rates were 11/33 [33%] and 2/9 [22%], respectively, and all false negative patients had focal ARDS. Misclassification was mainly related to overestimation of PEEP volume by the ventilators, that was confirmed by a bench study for two out of three ventilators used in the study.

In a population of ARDS patients with low recruitment potential, the R/I ratio had a poor diagnostic performance to predict lung recruitability, but R/I ratio values below 0.57 reliably identified ARDS patients with low recruitability, provided they did not present focal ARDS.