The global inhomogeneity index assessed by electrical impedance tomography overestimates PEEP requirement in patients with ARDS: an observational study

Background Electrical impedance tomography (EIT) visualises alveolar overdistension and alveolar collapse and enables optimisation of ventilator settings by using the best balance between alveolar overdistension and collapse (ODCL). Besides, the global inhomogeneity index (GI), measured by EIT, may also be of added value in determining PEEP. Optimal PEEP is often determined based on the best dynamic compliance without EIT at the bedside. This study aimed to assess the effect of a PEEP trial on ODCL, GI and dynamic compliance in patients with and without ARDS. Secondly, PEEP levels from "optimal PEEP" approaches by ODCL, GI and dynamic compliance are compared. Methods In 2015-2016, we included patients with ARDS using postoperative cardiothoracic surgery patients as a reference group. A PEEP trial was performed with four consecutive incremental followed by four decremental PEEP steps of 2 cmH(2)O. Primary outcomes at each step were GI, ODCL and best dynamic compliance. In addition, the agreement between ODCL, GI, and dynamic compliance was determined for the individual patient. Results Twenty-eight ARDS and 17 postoperative cardiothoracic surgery patients were included. The mean optimal PEEP, according to best compliance, was 10.3 (+/- 2.9) cmH(2)O in ARDS compared to 9.8 (+/- 2.5) cmH(2)O in cardiothoracic surgery patients. Optimal PEEP according to ODCL was 10.9 (+/- 2.5) in ARDS and 9.6 (+/- 1.6) in cardiothoracic surgery patients. Optimal PEEP according to GI was 17.1 (+/- 3.9) in ARDS compared to 14.2 (+/- 3.4) in cardiothoracic surgery patients. Conclusions Currently, no golden standard to titrate PEEP is available. We showed that when using the GI, PEEP requirements are higher compared to ODCL and best dynamic compliance during a PEEP trial in patients with and without ARDS.