Imbalances in regional lung ventilation, with gravity-dependent collapse and overdistention of nondependent zones, are likely associated to ventilator-induced lung injury. Electric impedance tomography is a new imaging technique that is potentially capable of monitoring those imbalances. The aim of this study was to validate electrical impedance tomography measurements of ventilation distribution, by comparison with dynamic computerized tomography in a heterogeneous population of critically ill patients under mechanical ventilation. Multiple scans with both devices were collected during slow-inflation breaths. Six repeated breaths were monitored by impedance tomography, showing acceptable reproducibility. We observed acceptable agreement between both technologies in detecting right–left ventilation imbalances (bias = 0% and limits of agreement = −10 to +10%). Relative distribution of ventilation into regions or layers representing one-fourth of the thoracic section could also be assessed with good precision. Depending on electrode positioning, impedance tomography slightly overestimated ventilation imbalances along gravitational axis. Ventilation was gravitationally dependent in all patients, with some transient blockages in dependent regions synchronously detected by both scanning techniques. Among variables derived from computerized tomography, changes in absolute air content best explained the integral of impedance changes inside regions of interest (r2 ⩾ 0.92). Impedance tomography can reliably assess ventilation distribution during mechanical ventilation.

The open lung approach is a mechanical ventilation strategy involving lung recruitment and a decremental positive end-expiratory pressure trial. We compared the Acute Respiratory Distress Syndrome network protocol using low levels of positive end-expiratory pressure with open lung approach resulting in moderate to high levels of positive end-expiratory pressure for the management of established moderate/severe acute respiratory distress syndrome. A prospective, multicenter, pilot, randomized controlled trial. A network of 20 multidisciplinary ICUs. Patients meeting the American-European Consensus Conference definition for acute respiratory distress syndrome were considered for the study. At 12-36 hours after acute respiratory distress syndrome onset, patients were assessed under standardized ventilator settings (FIO2≥0.5, positive end-expiratory pressure ≥10 cm H2O). If Pao2/FIO2 ratio remained less than or equal to 200 mm Hg, patients were randomized to open lung approach or Acute Respiratory Distress Syndrome network protocol. All patients were ventilated with a tidal volume of 4 to 8 ml/kg predicted body weight. From 1,874 screened patients with acute respiratory distress syndrome, 200 were randomized: 99 to open lung approach and 101 to Acute Respiratory Distress Syndrome network protocol. Main outcome measures were 60-day and ICU mortalities, and ventilator-free days. Mortality at day-60 (29% open lung approach vs. 33% Acute Respiratory Distress Syndrome Network protocol, p = 0.18, log rank test), ICU mortality (25% open lung approach vs. 30% Acute Respiratory Distress Syndrome network protocol, p = 0.53 Fisher's exact test), and ventilator-free days (8 [0-20] open lung approach vs. 7 [0-20] d Acute Respiratory Distress Syndrome network protocol, p = 0.53 Wilcoxon rank test) were not significantly different. Airway driving pressure (plateau pressure - positive end-expiratory pressure) and PaO2/FIO2 improved significantly at 24, 48 and 72 hours in patients in open lung approach compared with patients in Acute Respiratory Distress Syndrome network protocol. Barotrauma rate was similar in both groups. In patients with established acute respiratory distress syndrome, open lung approach improved oxygenation and driving pressure, without detrimental effects on mortality, ventilator-free days, or barotrauma. This pilot study supports the need for a large, multicenter trial using recruitment maneuvers and a decremental positive end-expiratory pressure trial in persistent acute respiratory distress syndrome.

Rationale: Current American-European Consensus Conference definitions for acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are inadequate for inclusion into clinical trials due to the lack of standardization for measuring the oxygenation defect. Objectives: We questioned whether an early assessment of oxygenation on specific ventilator settings would identify patients with established ARDS (persisting over 24 h). Methods: At the time of meeting ARDS criteria (Day 0) and 24 hours later (Day 1), arterial blood gases were obtained on standard ventilator settings, Vt 7 ml/kg predicted body weight plus the following positive end-expiratory pressure (PEEP) and FiO2 settings in sequence: (1) PEEP ≥ 5 cm H2O and FiO2 ≥ 0.5, (2) PEEP ≥ 5 cm H2O and FiO2 1.0, (3) PEEP ≥ 10 cm H2O and FiO2≥0.5, and (4) PEEP ≥ 10 cm H2O and FiO2 1.0. Measurements and Main Results: One hundred seventy patients meeting ARDS criteria (PaO2/FiO2 128 ± 33 mm Hg) were enrolled. Overall hospital mortality was 34.1%. The standard ventilator settings that best identified patients with established ARDS and predicted differences in intensive care unit (ICU) mortality were PEEP ≥ 10 cm H2O and FiO2 ≥ 0.5 at Day 1 (P = 0.0001). Only 99 (58.2%) patients continued to meet ARDS criteria (PaO2/FiO2, 155.8 ± 29.8 mm Hg; ICU mortality, 45.5%), whereas 55 patients were reclassified as having ALI (PaO2/FiO2, 246.5 ± 25.6 mm Hg; ICU mortality, 20%) and 16 patients as having acute respiratory failure (PaO2/FiO2, 370 ± 54 mm Hg; ICU mortality, 6.3%) (P = 0.0001) on these settings. Conclusions: Patients meeting current American-European Consensus Conference ARDS criteria may have highly variable levels of lung injury and outcomes. A systematic method of assessing severity of lung injury is required for enrollment of patients with ARDS into randomized controlled trials. Clinical trial registered with www.clinicaltrials.gov (NCT00435110).

We tested whether the continuous monitoring of dynamic compliance could become a useful bedside tool for detecting the beginning of collapse of a fully recruited lung. Prospective laboratory animal investigation. Clinical physiology research laboratory, University of Uppsala, Sweden. Eight pigs submitted to repeated lung lavages. Lung recruitment maneuver, the effect of which was confirmed by predefined oxygenation, lung mechanics, and computed tomography scan criteria, was followed by a positive end-expiratory pressure (PEEP) reduction trial in a volume control mode with a tidal volume of 6 mL/kg. Every 10 mins, PEEP was reduced in steps of 2 cm H2O starting from 24 cm H2O. During PEEP reduction, lung collapse was defined by the maximum dynamic compliance value after which a first measurable decrease occurred. Open lung PEEP according to dynamic compliance was then defined as the level of PEEP before the point of collapse. This value was compared with oxygenation (Pao2) and CT scans. Pao2 and dynamic compliance were monitored continuously, whereas computed tomography scans were obtained at the end of each pressure step. Collapse defined by dynamic compliance occurred at a PEEP of 14 cm H2O. This level coincided with the oxygenation-based collapse point when also shunt started to increase and occurred one step before the percentage of nonaerated tissue on the computed tomography exceeded 5%. Open lung PEEP was thus at 16 cm H2O, the level at which oxygenation and computed tomography scan confirmed a fully open, not yet collapsed lung condition. In this experimental model, the continuous monitoring of dynamic compliance identified the beginning of collapse after lung recruitment. These findings were confirmed by oxygenation and computed tomography scans. This method might become a valuable bedside tool for identifying the level of PEEP that prevents end-expiratory collapse.