Recruitment-To-Inflation Ratio to Assess the Impact of Peep on Dynamic Lung Strain

Abstract

Abstract Background: In ARDS, the beneficial effects of positive end-expiratory pressure (PEEP) on ventilator-induced lung injury (VILI) are proportional to the extent of dynamic lung strain reduction. This depends on the variable recruitment produced by PEEP. Current quantification of alveolar recruitment and PEEP-induced changes in dynamic lung strain relies on computed tomography (CT), which lacks bedside accessibility. The recruitment to-inflation ratio (R/I) is a bedside-available tool proposed to assess recruitability. We performed an experimental study on an animal model of ARDS to establish whether R/I accurately reflects the impact of PEEP on dynamic lung strain assessed through CT. Methods: Fourteen anesthetized, mechanically ventilated pigs with ARDS (lipopolysaccharide infusion) underwent CT-scans, respiratory mechanics, gas exchange and ventilatory ratio assessment at PEEPLOW (5 cmH2O) and PEEPHIGH (PEEP adjusted to reach a plateau airway pressure of 28-30 cmH2O). CT-scans were analyzed to quantify recruitability (potential for lung recruitment in terms of tissue recruitment) and PEEP-induced dynamic lung strain reduction. R/I was calculated through a one-breath derecruitment maneuver between the two PEEP experimental conditions. Results: Median R/I was 1.08 [0.88-1.82]. At PEEPHIGH, dynamic lung strain decreased form 0.59 [0.46 - 0.80] to 0.37 [0.29-0.44]: however, the extent of PEEP-induced dynamic strain reduction due to PEEP was highly inter-individually variable, with a coefficient of variation (standard deviation/mean) 69%. The entity of PEEP-induced reduction in dynamic lung strain exhibited a strong correlation with R/I (R2=0.87; p< 0.0001), while it was independent from changes in oxygenation (R2<0.01, p=0.97) and ventilatory ratio (R2<0.01, p>0.99). No correlation was found between R/I and potential for lung tissue recruitment assessed by CT-scan (R2=0.08; p=0.34). Conclusions: PEEP-induced changes in lung dynamic strain are inter-individually variable. R/I, despite not reflecting the extent of potential for lung tissue recruitment, allows to estimate the amount of PEEP-induced reduction in dynamic lung strain at bedside, proving its useful for setting PEEP to minimize the harmful effects of PEEP during mechanical ventilation.