Incremental PEEP Increases Peak Inspiratory Flow Disproportionately to Inspiratory Pressure Swing Across Respiratory Exposure Groups in Spontaneously Breathing Volunteers

Background: How spontaneously breathing individuals modulate inspiratory pressure and peak inspiratory flow (PIF) during incremental positive end-expiratory pressure (PEEP) loading, and whether this differs across respiratory exposure groups, are incompletely characterised. Methods: In a secondary analysis of a public dataset, 78 volunteers (Normal, Asthmatic, Smoker, Vaper) underwent PEEP titration from 4.0 to 12.0 cmH2O in 0.5 cmH2O steps. Per-breath mask inspiratory pressure swing (MIPS), PIF, and an exploratory flow-effort ratio (FER = PIF/MIPS) were analysed with PEEP-random-slope mixed-effects models; three electrical impedance tomography (EIT) metrics (pendelluft, Center of Ventilation, and Global Inhomogeneity) provided a mechanistic assessment. Results: PIF increased with PEEP across all cohorts (β = +0.068 L·s−1·cmH2O−1, 95% CI 0.064–0.072, and p < 0.001; +68% over the range) and persisted after adjustment for respiratory rate and duty cycle (Ti/Ttot), whereas MIPS increased modestly (β = +0.026, p = 0.004). Asthmatic and Vaper subjects showed significantly steeper PIF responses than Normal subjects (interaction p = 0.005 and 0.002, respectively). FER rose uniformly (β = +0.023, p < 0.001), and all EIT metrics were group-independent (no cohort × PEEP interaction, p ≥ 0.35). Conclusions: Progressive PEEP loading raises PIF disproportionately to mask pressure swing, with group-independent regional ventilation. No clear regional redistribution was detected in the EIT indices examined; the specific mechanism, however, remains unidentified.