LeTourneau, William M.

Background/Objectives: Retained airway secretions are a common complication of mechanical ventilation and a major contributor to ventilator-associated pneumonia. Airway Pressure Release Ventilation following a Time-Controlled Adaptive Ventilation protocol (APRV-TCAV) has been anecdotally observed to improve secretion clearance, but the mechanism is not well established. Because TCAV sets the release time (TLow) from each patient’s own expiratory flow decay, we hypothesized that the resulting expiratory-dominant flow profile would generate a net antegrade force on secretions. This bench study compared secretion movement between APRV-TCAV and conventional Volume-Control Continuous Mandatory Ventilation (VC-CMV), with and without an in-line oscillatory device. Methods: A critical care ventilator was connected to a 3 L test lung via a 7 mm endotracheal tube (ETT). Simulated mucus (1% guar gum) was instilled into the ETT, and secretion movement was measured under VC-CMV (positive end-expiratory pressure (PEEP) 5, 10, 15 cmH2O) and APRV-TCAV (PHigh 20, 25, 30 cmH2O; PLow 0 cmH2O). Each condition was repeated with an in-line oscillatory device. Results: All VC-CMV settings produced retrograde secretion movement toward the test lung; all APRV-TCAV settings produced antegrade movement. The largest antegrade movement was observed at the widest PHigh-to-PLow differential. Adding the oscillatory device raised peak expiratory flow above peak inspiratory flow in every condition, yet abolished antegrade movement under APRV-TCAV. Conclusions: In this bench study, APRV-TCAV reversed the direction of secretion movement within the ETT, with larger antegrade movement at wider PHigh-to-PLow differentials. This suggests a potential mechanism by which the ventilator flow profile itself may influence secretion movement. The directional effect depended on the intact release-phase waveform and was abolished by in-line oscillatory therapy despite preserved peak flow asymmetry.

Mechanical ventilation remains a cornerstone of acute respiratory failure management, but ventilator-induced lung injury persists despite lung-protective strategies focused on limiting tidal volume (V T ), plateau pressure, and driving pressure. Mechanical power has emerged as a comprehensive metric that quantifies the total energy delivered to the lungs per unit time by integrating V T , airway pressures, breathing frequency, flow, and PEEP into a single physiologic measure. Observational studies through 2025 show that elevated mechanical power is associated with worse outcomes, including increased mortality, fewer ventilator-free days, and longer ICU stays, with breathing frequency identified as a major contributor even when other settings remain within protective ranges. Although no universal injurious threshold exists because of patient variability, normalization strategies such as indexing mechanical power to predicted body weight or respiratory system compliance have shown promise in improving prognostic value, although consensus on the optimal method is lacking. Currently, mechanical power should be considered a complementary bedside tool that integrates multiple ventilator variables, helps identify key contributors to lung stress and strain, and may support individualized ventilator adjustments to reduce cumulative injury risk as further evidence and technology evolve.

Identifying the mechanisms of ventilator/ventilation-induced lung injury requires an understanding of the pulmonary physiology involved in the mechanical properties of the lung along with the involvement of the inflammatory cascade. Accurately measuring parameters that represent physiologic lung stress and lung strain at the bedside can be clinically challenging. Although surrogates for lung stress and strain have been proposed, such as plateau pressure and driving pressure, these values only represent a static variable in the ventilator breath. It has been proposed that a single variable could be used as a unifying parameter to identify a threshold for the safe application of mechanical ventilation. The concept of “mechanical power” applies an energy load transfer designation to the ventilator settings and output of tidal volume, airway pressures, and flow. However, there is a potential disconnect between the use of “absolute” mechanical power and the variability of body weight throughout a mixed medical population. Using ideal body weight as an influential factor to express mechanical power can potentially allow for a more accurate depiction of energy applied to the lungs and a potentially reliable injurious mechanical ventilation threshold indicator.