EIT-based ventilation phenotypes of left-to-right asymmetry and ventral-to-dorsal center in PEEP titration in ARDS

BACKGROUND: Ventilation distribution assessed by electrical impedance tomography (EIT) has great interests in acute respiratory distress syndrome (ARDS). The aim of the study was to explore ARDS phenotypes based on left-right and ventral-dorsal ventilation distribution and to investigate their clinical characteristics and outcomes. METHOD: This retrospective study included ARDS patients from two ICUs who underwent mechanical ventilation and EIT monitoring. Asymmetry index (AI) was defined as the right-to-left ventilation difference in percentage. Based on the AI at low PEEP (0-3 cmH₂O), patients were classified as asymmetric (|AI| > 20%) or symmetric (|AI| ≤ 20%) phenotype. Asymmetric phenotype was divided into right (R, AI > 20%) and left (L, AI < -20%) subphenotypes. Based on the median center of ventilation (CoV) of the study population at low PEEP, symmetric phenotype was further divided into ventral (V, CoV < 42.2%) and non-ventral (NV, CoV ≥ 42.2%) subphenotypes. RESULT: A total of 217 patients with ARDS during positive end-expiratory pressure (PEEP) titration was analyzed. At low PEEP, 95 patients were defined as asymmetric phenotype and 122 were symmetric (|AI| 36.0% [28.0, 48.0] vs. 8.0% [4.0, 16.5]; p < 0.001). Among asymmetric phenotype, 69 were R subphenotype and 26 were L. R Subphenotype had higher BMI than L (p = 0.037). Among symmetric phenotype, 61 were V subphenotype and 61 were NV. V subphenotype had higher BMI (p = 0.027), more extrapulmonary ARDS (p = 0.010), and better lung recruitability (p = 0.021) than NV. From low to high PEEP (15-18 cmH₂O), 47 patients remained asymmetric phenotype, 48 transitioned from asymmetric to symmetric, 97 remained symmetric, 25 transitioned from symmetric to asymmetric. Patients who remained asymmetric phenotype had fewer 28-day ventilator-free days than those who transitioned to symmetric (p = 0.009). CONCLUSION: Based on AI and CoV, EIT enabled rapid phenotyping of ARDS. In symmetric ARDS, V subphenotype had higher BMI, extrapulmonary ARDS incidence, and lung recruitability. In asymmetric ARDS, improvment of symmetry during PEEP titration was related to better outcome. The asymmetry of lung ventilation might be a potential lung injury target in ARDS.