Discussion
lung injury, derecruitment, ventilator-induced lung injury, ARDS
LUNG HYPERINFLATION, ARDS, RECRUITMENT
The results of the current physiologic study conducted on patients with early moderate-to-severe COVID-19 ARDS can be summarized as follows:Recruitment-to-inflation ratio calculated between 15 and 5 cm HRespiratory mechanics changes induced by PEEP are ineffective estimates of the changes produced on lung strain (Supplementary figure 2, Recruitment-to-inflation ratio, calculated both between PEEP 15 to 5 cm HBecause of the nonuniform behavior of recruitment-to-inflation ratio In ARDS, it is widely accepted that PEEP setting should aim at a balance between alveolar recruitment (A drawback of “universal” strategies driving the setting of high or low PEEP is that the potential for lung recruitment has great interindividual variability, with high PEEP enhancing lung injury in patients with low recruitability, and low PEEP not fully exerting its beneficial effects in recruitable patients.The recently developed recruitment-to-inflation ratio may offer a simple, timely, and reproducible assessment of recruitability at the bedside.Our study demonstrates the reliability of the recruitment-to-inflation ratio measured between PEEP 15 and PEEP 5 cm HHigh recruitment-to-inflation ratio values (The further modeling analysis presented in our manuscript showed a nonlinear fit of the recruitment-to-inflation ratio Our study may have important clinical implications:Dynamic strain is the key mechanism of lung injury during ARDS.The recruitment-to-inflation ratio measured in the 15– to 5–cm HIn selected patients (those with an intermediate recruitment-to-inflation ratio), sequential end-expiratory lung volume measurement enabling recruitment-to-inflation ratio calculation within narrower ranges during a decremental PEEP trial may enable more granular evaluation of PEEP effects on lung strain. This may aid more precise bedside PEEP titration within the evaluated range, setting PEEP to exploit lung recruitment when it is present while minimizing lung hyperinflation when recruitment does not occur.Our study has limitations:In our study, the recruitment-to-inflation ratio was calculated from the lung volume change due to PEEP measured with the dilution technique, and not estimated by the simplified derecruitment maneuver.Patients with airway closure or intrinsic PEEP were excluded from the study: this has helped avoid further confounding factors that could have required even more complex data analysis and interpretation for this proof-of-concept study.We refer to the terms “recruitment” and “recruitability,” although our results should be more properly referred to as “derecruitment” data. This conceptual shift may represent a limitation, considering the hysteretic behavior of pressure–volume curves. Nonetheless, this approach is consistent with the most recent literature on the topic, as derecruitment is simpler to assess and not significantly different from recruitment when a prolonged high-PEEP stabilizing phase precedes the derecruitment procedure, as in our study.Due to the pandemic surge, all studied patients suffered from ARDS caused by COVID-19. There are no reasons to hypothesize that our approach based on end-expiratory lung volume–guided recruitment-to-inflation ratio assessment within narrow PEEP ranges should not be applicable to ARDS from other causes, since respiratory mechanics in COVID-19 ARDS have been shown to share a high degree of similarity with non–COVID-19 cases.No biologic or imaging techniques were used to support the findings of our study. The latter represents a limitation both due to of the lack of data on the regional distribution of ventilation and because computed tomography scan validation of recruitment evaluation is not present. Nevertheless, the role of dynamic strain as key determinant of ventilator-induced lung injury is well-established,Caution is warranted when interpreting linear correlation data from our study: indeed, due to the lack of an external validation of recruitment such as imaging techniques, correlations between recruitment-to-inflation ratio and the recruitment index Crec/FRC and between recruitment-to-inflation ratio and changes in strain must take into account a possible degree of bias, since a number of parameters are involved in the computation of both elements of the correlation.