Components of Mechanical Power on the Functional Lung in ARDS

Overview

Mechanical power (MP) is a composite variable that integrates static and dynamic respiratory parameters and has been associated with ventilator-induced lung injury (VILI). MP is predominantly delivered to the reduced functional lung size, commonly referred to as the "baby lung" (BL). Functional lung size, in turn, is closely related to respiratory system compliance (Crs). Previous studies have demonstrated an association between MP normalized to compliance (MP/Crs) and mortality. Moreover, evidence suggests that VILI associated with MP/Crs results primarily from the combined effect of its components rather than from any single component in isolation. The objective of this study is to evaluate both the overall and the relative contribution of each component of mechanical power, normalized to compliance (MP/Crs), to in-hospital mortality in patients with acute respiratory distress syndrome (ARDS).

In patients with acute respiratory distress syndrome (ARDS) who require mechanical ventilation (MV), each respiratory cycle transfers a quantifiable amount of mechanical energy to the lung. This energy is necessary both to expand the respiratory system and to overcome airway resistance. The total mechanical energy delivered per breath, when multiplied by the respiratory rate (RR), defines the concept of mechanical power (MP). From a pathophysiological perspective, MP is considered a key determinant of ventilator-induced lung injury (VILI), as it reflects the dynamic interaction of pressure, volume, and flow variables over time. However, this transferred energy does not act uniformly throughout the lung. In ARDS, the functional portion of ventilated lung tissue is reduced due to edema, atelectasis, and consolidation. This remaining ventilatable lung volume is often referred to as the "baby lung" (BL). When mechanical energy is delivered to a smaller BL, the energy density (energy per unit of available tissue) increases, which may amplify local stress and strain, thereby exacerbating lung injury. Respiratory system compliance (Crs) has been widely used as a surrogate for BL size, given that lower compliance indicates a smaller proportion of functional lung available for ventilation. Protective ventilation strategies, therefore, should aim not only to minimize the absolute amount of mechanical energy delivered but also to adapt it to the size of the BL in order to avoid excessive energy transfer and its potential for injury. For this reason, normalizing mechanical power to compliance (MP/Crs) has been proposed as a physiologically sound parameter, as it accounts for both the intensity of mechanical ventilation and the size of the lung that receives it. Several studies have identified an association between MP/Crs and mortality in ARDS, supporting its role as a potential prognostic marker. Furthermore, emerging evidence suggests that the detrimental effect of MP/Crs on lung tissue is not explained by a single variable in isolation, but rather by the combined contribution of its components, including elastic dynamic power (related to driving pressure), resistive power (related to airflow resistance), and static elastic power (related to positive end-expiratory pressure). This highlights the need for a more granular evaluation of how each of these elements contributes to overall patient outcomes. Therefore, to test the hypothesis that MP/Crs is independently associated with patient-centered outcomes in a well-characterized cohort of patients with ARDS, we will analyze detailed ventilatory parameters required for the calculation of mechanical power and its components. These data will be obtained from a prospectively collected cohort of intensive care unit (ICU) patients, allowing for accurate computation of MP/Crs and comprehensive evaluation of its relationship with clinically relevant outcomes.