The authors reply.

Abstract

Accurate quantification of lung recruitment in patients with the acute respiratory distress syndrome (ARDS) may be essential for understanding the patient subphenotype and applying personalized mechanical ventilation strategy. Patients with higher potential for lung recruitment (PLR) may be more severe (1) and application of higher positive end-expiratory pressure (PEEP) may be beneficial. Traditionally, PLR was assessed as the difference in weight of the nonaerated lung fraction measured by CT scans between two pressure levels (e.g., 5–45 cm H2O) divided by the weight of nonaerated lung at lower pressure level. By this method, CT-based PLR (PLRCT) represents the fraction of collapsed lung units becoming reaerated at highest airway pressure (1). Application of higher PEEP in patients with larger PLRCT may decrease the risk of ventilator-induced lung injury (VILI) by reducing alveolar atelectrauma and lung strain (i.e., the tidal volume/lung volume at zero PEEP ratio). More recently, bedside noninvasive radiation-free methods to quantify PLR through changes in end-expiratory lung inflation at different PEEP levels (PLRinfl) have become more widely available. By using electrical impedance tomography (EIT) (2), nitrogen washin/washout technique (3), or the difference between release and tidal volume during sudden decrease of PEEP in volume control ventilation (4), the actual change in end-expiratory lung volume (∆EELV) between two PEEP levels can be measured. Then, the expected ∆EELV based on respiratory mechanics can be computed as the compliance of the respiratory system at lower PEEP multiplied by the PEEP increase. If part of the lung units improve their mechanical properties by becoming reaerated or better inflated at higher PEEP, the actual ∆EELV will be higher than the expected and recruitment can be calculated as the difference between the two ∆EELVs. Finally, the recruitment to inflation (R/I) ratio was recently introduced to quantify PLRinfl. Briefly, the size of the recruited lung can be measured by the compliance of the recruited volume, calculated by dividing recruitment by the PEEP change, while the size of the starting baby lung is represented by the respiratory system compliance at lower PEEP. The ratio between these two compliances is the R/I ratio, which represents the potential increase in the size of the baby lung by application of higher PEEP. At variance from PLRCT, both reaeration of previously collapsed units and improved mechanics of partially deaerated units concur to determine the intensity of PLRinfl. Thus, application of higher PEEP in patients with higher PLRinfl (i.e., ≥ 0.5) may be protective through decreased atelectrauma and lung strain (as for PLRCT) but also through reduced lung stress (i.e., transpulmonary pressure) due to improved regional mechanics. PLRinfl, in addition to being bedside and avoiding exposure to radiation, might be regarded as a more comprehensive assessment of the beneficial effects of higher PEEP on the mechanisms leading to VILI. As correctly pointed out by He et al (5) in their letter, PLRinfl has intrinsic limitations: in the presence of airway opening pressure (AOP) above the lower PEEP level and if significant overdistension occurs at higher PEEP, PLRinfl might underestimate the real PLR. Thus, as working clinical compromises: Tommaso Mauri, MD1,2