The recruitment-to-inflation ratio might help characterize lung recruitability at the bedside in patients with acute respiratory distress syndrome (ARDS), according to study results published in the American Journal of Respiratory and Critical Care Medicine.

Researchers proposed a bedside approach to estimate recruitability accounting for the presence of complete airway closure by validating a single-breath method for measuring recruited volume and tested whether it differentiates patients with different responses to positive end expiratory pressure (PEEP).

A total of 45 patients with ARDS underwent ventilation at 15 and 5 cm H2O of PEEP and multiple pressure-volume curves were compared with a single-breath technique. The researchers found that by abruptly releasing PEEP (15-5 cm H2O) over a single breath maneuver, the recruited volume over a given range of PEEP could be measured. Recruited volume measured by the experimental and reference methods was strongly correlated (P <.001; R2=0.798) with small bias (-21 mL). At PEEP 15, patients who were “high recruiters” had better oxygenation vs patients with who were “low recruiters” who experienced lower systolic arterial pressure.

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Taking into account the possibility of airway closure and therefore the effective change on pressure, the measurement of recruited volume over a given range of PEEP allows the calculation of the compliance of the recruited lung; the latter is compared with the “baby lung” (ie, the lung tissue that remains aerated at low PEEP or at functional residual capacity) compliance using the recruitment-to-inflation ratio, which helps to differentiate between recruiters and nonrecruiters.

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“By comparing [the recruited lung] with the compliance of the baby lung, the [recruitment-to-inflation] ratio differentiates patients with, on average, different oxygen and circulation responses to PEEP,” the researchers wrote. “This ratio correlates with both oxygenation and alveolar dead space, indicating further validity of this index.”


Chen L, Del Sorbo L, Grieco DL, et al. Potential for lung recruitment estimated by the recruitment-to-inflation ratio in acute respiratory distress syndrome [published on October 2, 2019]. Am J Respir Crit Care Med. doi:10.1164/rccm.201902-0334OC