Reply to van den Berg and van der Hoeven: In Patients with ARDS, Optimal PEEP Should Not Be Determined Using the Intersection of Relative Collapse and Relative Overdistention.

From the Authors:

We thank van den Berg and van der Hoeven for the opportunity to further discuss our research letter in which positive end-expiratory pressure (PEEP) was titrated at the level of lowest relative alveolar overdistention and collapse based on electrical impedance tomography (EIT) (1). In their comment, the authors argue that PEEP should not be set at the minimum level of both alveolar overdistention and collapse, as alveolar overdistention is potentially more harmful.

We fully agree that alveolar overdistention is harmful to our patients. The Alveolar Recruitment Trial showed us that systematically performed recruitment maneuvers, known to cause alveolar overdistention, increased mortality rate in patients with acute respiratory distress syndrome (ARDS) (2). However, the amount of alveolar overdistention or collapse prior to the application of high airway pressures was unknown. Determining alveolar overdistention and collapse is crucial, as PEEP titration approaches are based on the assumption that there is an optimal compromise between alveolar recruitment (i.e., limit the amount of collapse) and minimizing alveolar overdistention.

Numerous bedside PEEP titration approaches have been described, but none have shown to improve patient survival in large randomized controlled trials. In addition, correlation between different approaches is poor. The explanation is that most bedside PEEP titration approaches have at least one of the following three limitations: 1) the approach does not quantify alveolar recruitment; 2) the respiratory system is assessed as a whole, and local lung inhomogeneities remain undetected; and 3) alveolar overdistention is not quantified.

EIT is a functional imaging tool that continuously assesses regional ventilation and lung volume changes at the bedside. As such, EIT is a bedside PEEP titration approach that quantifies both alveolar recruitment and alveolar overdistention and is able to detect local lung inhomogeneities. However, the amount of studies that used EIT to titrate PEEP in critically ill patients with ARDS is limited. In addition, there is no consensus on how to interpret EIT data.

Blankman and colleagues (3) compared several EIT-derived PEEP titration approaches in patients after cardiac surgery and proposed the intratidal gas distribution index to identify alveolar overdistention in the nondependent lung regions and to titrate PEEP. In a case series, Yoshida and colleagues (4) used a ventral-dorsal ventilation distribution of 50–50% to reach homogeneous ventilation and limit alveolar overdistention. In contrast, Franchineau and colleagues (5) aimed to limit the amount of relative collapse to 15% while maintaining the lowest percentage of overdistention in patients with extracorporeal membrane oxygenation. Alternatively, we could have aimed for the greatest amount of ventilated pixels or calculate the global inhomogeneity index. We chose to titrate PEEP at the lowest level of relative alveolar overdistention and collapse, as it is a simple and intuitive approach that has proven to be beneficial in mechanically ventilated patients during surgery (6). This approach resulted in low driving pressures and low transpulmonary pressures in all our patients.

We share the concerns of van den Berg and van der Hoeven that alveolar overdistention is harmful to the lungs. Therefore, we quantified the amount of alveolar overdistention before applying higher PEEP in our patients with coronavirus disease (COVID-19)–related ARDS. The Pleural Pressure Working Group’s planned RECRUIT (Recruitment Assessed by Electrical Impedance Tomography: Feasibility, Correlation with Clinical Outcomes and Pilot Data on Personalised PEEP Selection) project (https://www.plugwgroup.org/), which aims to compare the results of different bedside methods to titrate PEEP based on EIT, might provide us with some answers on how to titrate PEEP using EIT data. In the meantime, we agree with our colleagues to limit the amount of alveolar overdistention in patients with COVID-19–related ARDS by applying prone positioning and quantifying the amount of alveolar overdistention during a PEEP trial.