Lung recruitability determines the impact of PEEP on mechanical power in ARDS.

BACKGROUND

Mechanical power increases with positive end-expiratory pressure (PEEP). However, its injurious potential may depend on the available lung gas volume, which can be modified by alveolar recruitment. We investigated how PEEP-induced recruitment affects mechanical power.

METHODS

We analyzed previously collected data on 20 patients with acute respiratory distress syndrome who underwent a decremental PEEP trial (15-5 cmH₂O). End-expiratory lung volume and respiratory mechanics were measured to quantify recruited volume, functional residual capacity (FRC), and the recruitment-to-inflation (R/I) ratio. Absolute power and power normalized to aerated lung volume (FRC + recruited volume) were calculated at each PEEP level. Patients were classified as having higher or lower recruitability according to the cohort median recruited volume accrued between PEEP 5 and 15 cmH₂O, expressed as a fraction of FRC (median 0.42).

RESULTS

Absolute mechanical power increased linearly with rising PEEP (approximately + 1 J/min per cmH₂O), from 20 [16-23] J/min at 5 cmH₂O, to 31 [28-33] J/min at 15 cmH₂O, irrespective of recruitability (low recruitability: + 1.12 J/min per cmH₂O, p < 0.001; high recruitability: + 0.96 J/min per cmH₂O, p < 0.001, p for interaction = 0.12). Normalized power increased in patients with lower recruitability (+ 0.43 J/min/L per cmH2O, p < 0.001) but decreased in those with higher recruitability (- 0.33 J/min/L per cmH2O, p < 0.001; p for interaction < 0.001). The reduction in normalized power was strongly related to PEEP-induced recruitment, expressed as recruited volume/FRC (- 102% per unit, R2 = 0.75, p < 0.001), and to R/I ratio (- 38% per unit, R2 = 0.69, p < 0.001). Associations with PEEP-related changes in compliance (R2 = 0.40, p = 0.003) and PaO₂/FiO₂ (R2 = 0.33, p = 0.008) were weaker. In the multivariate model, PEEP-induced recruitment (p = 0.002) and compliance changes (p = 0.011) remained independent predictors of normalized power changes.

CONCLUSIONS

Absolute mechanical power increases with higher PEEP, but power per aerated lung decreases when PEEP produces substantial recruitment. PEEP-induced increases in absolute power do not necessarily imply a higher mechanical load per alveolar unit. Recruited volume and compliance changes are the main physiological determinants of this effect. Among bedside tools, the R/I ratio best identifies whether and to what extent PEEP will reduce or increase mechanical power per alveolar unit.