Positive end-expiratory pressure (PEEP) benefits in acute respiratory distress syndrome are driven by lung dynamic strain reduction. This depends on the variable extent of alveolar recruitment. The recruitment-to-inflation ratio estimates recruitability across a 10–cm H2O PEEP range through a simplified maneuver. Whether recruitability is uniform or not across this range is unknown. The hypotheses of this study are that the recruitment-to-inflation ratio represents an accurate estimate of PEEP-induced changes in dynamic strain, but may show nonuniform behavior across the conventionally tested PEEP range (15 to 5 cm H2O).
Twenty patients with moderate-to-severe COVID-19 acute respiratory distress syndrome underwent a decremental PEEP trial (PEEP 15 to 13 to 10 to 8 to 5 cm H2O). Respiratory mechanics and end-expiratory lung volume by nitrogen dilution were measured the end of each step. Gas exchange, recruited volume, recruitment-to-inflation ratio, and changes in dynamic, static, and total strain were computed between 15 and 5 cm H2O (global recruitment-to-inflation ratio) and within narrower PEEP ranges (granular recruitment-to-inflation ratio).
Between 15 and 5 cm H2O, median [interquartile range] global recruitment-to-inflation ratio was 1.27 [0.40 to 1.69] and displayed a linear correlation with PEEP-induced dynamic strain reduction (r = –0.94; P < 0.001). Intraindividual recruitment-to-inflation ratio variability within the narrower ranges was high (85% [70 to 109]). The relationship between granular recruitment-to-inflation ratio and PEEP was mathematically described by a nonlinear, quadratic equation (R2 = 0.96). Granular recruitment-to-inflation ratio across the narrower PEEP ranges itself had a linear correlation with PEEP-induced reduction in dynamic strain (r = –0.89; P < 0.001).
Both global and granular recruitment-to-inflation ratio accurately estimate PEEP-induced changes in lung dynamic strain. However, the effect of 10 cm H2O of PEEP on lung strain may be nonuniform. Granular recruitment-to-inflation ratio assessment within narrower PEEP ranges guided by end-expiratory lung volume measurement may aid more precise PEEP selection, especially when the recruitment-to-inflation ratio obtained with the simplified maneuver between PEEP 15 and 5 cm H2O yields intermediate values that are difficult to interpret for a proper choice between a high and low PEEP strategy.
- The ability to determine the optimal positive end-expiratory pressure (PEEP) for patients with acute respiratory distress syndrome may have important clinical ramifications.
- The optimal PEEP in such patients aims at reducing lung dynamic strain while avoiding baby lung overinflation. The balance between these two effects depends on the variable extent of lung recruitment.
- The recruitment-to-inflation ratio over a 10–cm H2O PEEP range has recently been proposed as a simplified method to bedside estimate lung recruitment, but it is undemonstrated if the recruitment-to-inflation ratio accurately reflects PEEP effects on lung strain.
- Whether the recruitment-to-inflation ratio is uniform or not across a 10–cm H2O PEEP range is unknown.
- In a cohort of patients with moderate to severe COVID-19 acute respiratory distress syndrome, the authors determined respiratory mechanics and end-expiratory lung volume by nitrogen dilution over a 10–cm H2O decremental five-step PEEP trial, and measured the recruitment-to-inflation ratio both globally (10–cm H2O PEEP range) and granularly (narrower PEEP ranges).
- The global effect displayed a linear correlation of both global and granular recruitment-to-inflation ratio with dynamic strain reduction. However, granular recruitment-to-inflation ratio showed high intraindividual variability, suggesting nonuniform recruitability across a 10–cm H2O PEEP range.
- A nonlinear quadratic relationship was noted between granular recruitment-to-inflation ratio and PEEP.
- These pilot data suggest that the recruitment-to-inflation ratio measured across a 10–cm H2O range may be of value clinically for setting PEEP to minimize lung dynamic strain. Granular recruitment-to-inflation ratio assessment may help more precisely assess PEEP effects, with relevant clinical implications for PEEP setting in patients who display a global recruitment-to-inflation ratio close to intermediate values.