Electrical Impedance Tomography-Derived Dorsal Recruitment-to-Inflation Ratio Identifies PEEP Responsiveness and Risk Phenotypes in ARDS

• Background and Rationale ARDS is associated with heterogeneous lung collapse. The recruitment-to inflation (R/I) ratio has emerged as an index of recruitability, but is usually assessed globally. Electrical impedance tomography (EIT) enables regional assessment of ventilation and may provide more precise determinations of recruitable lung regions, particularly dorsal areas.
• Objectives Primary objective: To compare the predictive value of regional (dorsal and ventral) versus global R/I for recruitment success in moderate-severe ARDS. Secondary objectives: To classify recruitment phenotypes and evaluate associations with oxygenation, compliance improvement, ventilator-free days, ICU length of stay, and 28-day mortality.
• Study Design Prospective, single-centre observational cohort study in ICU patients with moderate-severe ARDS. All eligible patients undergo standardised recruitment maneuver and EIT monitoring.
• Study Setting Ruijin Hospital, Shanghai Jiao Tong University School of Medicine ICU, December 2022 - December 2024.
• Intervention Standardized PEEP step maneuver from 5 → 15 cmH₂O, maintained for 3 minutes. Continuous EIT monitoring of regional compliance, oxygenation, hemodynamics. Dynamic strain cutoff (>0.6) or adverse events terminate the maneuver. Ventilation mode: volume control, tidal volume 4-6 mL/kg predicted body weight, plateau ≤28-30 cmH₂O, FiO₂ titrated for SpO₂ 88-92%.
• Measurements EIT-derived ΔEELI in dorsal and ventral regions Regional and global compliance (Crec and Crs) R/I calculated: Crec/Crs at baseline PEEP Airway opening pressure by low-flow inflation method Primary outcome: recruitment success (≥10% compliance improvement + ≥20 mmHg PaO₂/FiO₂ increase) Secondary: ventilator-free days, ICU LOS, 28-day mortality, dynamic strain
• Statistical Analysis Plan Distribution check by Shapiro-Wilk. Paired tests for within-patient comparisons. ROC curves and Youden index for cut-off selection. Multivariate regression for mortality risk factors. Bonferroni correction for multiple comparisons.
• Safety Monitoring Terminate RM if: MAP <65 mmHg, SpO₂ <88%, arrhythmia onset, dynamic strain >0.6.

Inclusion criteria

• Patients who meet the Berlin definition of ARDS and have been treated with mechanical ventilation (clinical diagnosis of ARDS by the attending ICU physician)
• PaO₂/FiO₂ ≤200 mmHg

Exclusion criteria

• Age< 18 or >90 years
• Pneumothorax, pneumomediastinum
• Hemodynamic instability (rising vasopressor need in <6h)
• Contraindications to RM or EIT