Introduction: Lung protective ventilation requires low tidal volume and suitable positive end-expiratory pressure (PEEP). To date, few methods have been accepted for clinical use to set the appropriate PEEP. The aim of this study was to test the feasibility of PEEP titration guided by ventilation homogeneity using the global inhomogeneity (GI) index based on electrical impedance tomography (EIT) images. Methods: In a retrospective study, 10 anesthetized patients with healthy lungs mechanically ventilated under volume-controlled mode were investigated. Ventilation distribution was monitored by EIT. A standardized incremental PEEP trial (PEEP from 0 to 28 mbar, 2 mbar per step) was conducted. During the PEEP trial, "optimal" PEEP level for each patient was determined when the air was most homogeneously distributed in the lung, indicated by the lowest GI index value. Two published methods for setting PEEP were included for comparison based on the maximum global dynamic compliance and the intra-tidal compliance-volume curve. Results: No significant differences in the results were observed between the GI index method (12.2 ± 4.6 mbar) and the dynamic compliance method (11.4 ± 2.3 mbar, P > 0.6), or between the GI index and the compliance-volume curve method (12.2 ± 4.9 mbar, P > 0.6). Conclusions: According to the results, it is feasible and reasonable to use the GI index to select the PEEP level with respect to ventilation homogeneity. The GI index may provide new insights into the relationship between lung mechanics and tidal volume distribution and may be used to guide ventilator settings.

Double-lumen tubes (DLTs) are frequently used to establish one-lung ventilation (OLV). Their correct placement is crucial. We hypothesized that electrical impedance tomography (EIT) reliably displays distribution of ventilation between left and right lung and may thus be used to verify correct DLT placement online. Regional ventilation was studied by EIT in 40 patients requiring insertion of left-sided DLTs for OLV during thoracic surgery. EIT was recorded during two-lung ventilation before induction of anaesthesia and after DLT placement, and during OLV in the supine and subsequently in the lateral position. EIT measurements were made before and after verification of correct DLT placement by fibreoptic bronchoscopy (FOB). EIT accurately displayed distribution of ventilation between left and right lung online. All cases (n=5) of initially misplaced DLTs in the contralateral right main bronchus were detected by EIT. However, EIT did not allow prediction of FOB-detected endobronchial cuff misplacement requiring DLT repositioning. Furthermore, after DLT repositioning, distribution of ventilation, as assessed by EIT, did not change significantly (all P>0.5). This study demonstrates that EIT enables accurate display of left and right lung ventilation and, thus, non-invasive online recognition of misplacement of left-sided DLTs in the contralateral main bronchus. However, as distribution of ventilation did not correlate with endobronchial cuff placement, EIT cannot replace FOB in the routine control of DLT position.

Introduction: Limiting the energy transfer between ventilator and lung is crucial for ventilatory strategy in acute respiratory distress syndrome (ARDS). Part of the energy is transmitted to the viscoelastic tissue components where it is stored or dissipates. In mechanically ventilated patients, viscoelasticity can be investigated by analyzing pulmonary stress relaxation. While stress relaxation processes of the lung have been intensively investigated, non-linear interrelations have not been systematically analyzed, and such analyses have been limited to small volume or pressure ranges. In this study, stress relaxation of mechanically ventilated lungs was investigated, focusing on non-linear dependence on pressure. The range of inspiratory capacity was analyzed up to a plateau pressure of 45 cmH₂O. Methods: Twenty ARDS patients and eleven patients with normal lungs under mechanical ventilation were included. Rapid flow interruptions were repetitively applied using an automated super-syringe maneuver. Viscoelastic resistance, compliance and time constant were determined by multiple regression analysis using a lumped parameter model. This same viscoelastic model was used to investigate the frequency dependence of the respiratory system's impedance. Results: The viscoelastic time constant was independent of pressure, and it did not differ between normal and ARDS lungs. In contrast, viscoelastic resistance increased non-linearly with pressure (normal: 8.4 (7.4-11.9) [median (lower - upper quartile)] to 35.2 (25.6-39.5) cmH₂O·sec/L; ARDS: 11.9 (9.2-22.1) to 73.5 (56.8-98.7)cmH₂O·sec/L), and viscoelastic compliance decreased non-linearly with pressure (normal: 130.1(116.9-151.3) to 37.4(34.7-46.3) mL/cmH₂O; ARDS: 125.8(80.0-211.0) to 17.1(13.8-24.7)mL/cmH₂O). The pulmonary impedance increased with pressure and decreased with respiratory frequency. Conclusions: Viscoelastic compliance and resistance are highly non-linear with respect to pressure and differ considerably between ARDS and normal lungs. None of these characteristics can be observed for the viscoelastic time constant. From our analysis of viscoelastic properties we cautiously conclude that the energy transfer from the respirator to the lung can be reduced by application of low inspiratory plateau pressures and high respiratory frequencies. This we consider to be potentially lung protective.