Lung recruitability in ARDS: from recruited volume to functional response

Lung recruitability in acute respiratory distress syndrome (ARDS) is commonly characterized by the potential of nonaerated lung to become aerated when distending pressure is increased. However, the amount of lung that can be recruited does not establish whether achieving and maintaining that recruitment results in physiological benefit. This distinction explains why anatomical estimates of recruitability do not translate directly into decisions about recruitment-oriented interventions or positive end-expiratory pressure (PEEP). This Perspective proposes a physiological approach that distinguishes recruitment potential, achieved recruitment, and the physiological response to recruitment. Anatomical recruitability describes the potential for previously nonaerated lung to become aerated during a defined change in distending pressure, whereas anatomical recruitment describes the aeration achieved by that intervention. Functional recruitability is an interpretative concept that considers the physiological benefit of achieved recruitment in relation to its mechanical and hemodynamic consequences; it is not a new physiological variable, measurement, threshold, or method for PEEP titration. Accordingly, the significance of recruitment cannot be inferred from aeration alone but requires integration with respiratory mechanics, gas exchange, and cardiovascular tolerance under the conditions in which recruitment is maintained. Because both recruitability and its physiological consequences can evolve over the course of ARDS, reassessment under comparable conditions can be informative when clinically warranted. When additional recruitment produces little or transient physiological benefit, or when that benefit is offset by the mechanical or hemodynamic burden required to achieve or maintain it, we describe the response as recruitment futility. This term refers to the physiological response to a recruitment-oriented intervention and should not be equated with low anatomical recruitability or medical futility. The physiological significance of lung recruitability can be interpreted along a sequence from recruitment potential to achieved recruitment and physiological response, extending beyond the amount of lung recruited alone. This approach supports a more physiologically informed interpretation of recruitment-oriented interventions and provides a rationale for prospectively evaluating anatomical recruitability as a stratification variable in physiology-enriched ARDS trials.