Abstract Rationale Repetitious recruitment and derecruitment (R/D) of small airways and alveoli are time- and pressure-dependent phenomena which intensify during states of lung injury. R/D is often described as cyclic, driven by periodic forcing of a mechanical ventilator. However, imaging of small airways has revealed irregular patterns in R/D over long time scales 1. We hypothesize that intratidal R/D dynamics are irregular and aperiodic, exhibiting both short- and long-term transients during periodic mechanical ventilation. Methods Subsegmental lobules were carefully dissected from isolated porcine lungs and mechanically ventilated in a volume-controlled mode with zero end-expiratory pressure and tidal volume set to reach 25 cmH2O peak inspiratory pressure. A microphone was used to record audible crackles associated with recruitment events at a sampling rate of 11025 Hz. Inspiratory crackles were detected via high-pass filtration (2 kHz), envelope filtering, and peak detection. Results In four samples from different pigs, the number of inspiratory crackles per breath (median minimum, maximum) was 6 1,17, 1 0,5, 8 3,15, and 1 0,5. Over short timescales, varying degrees of crackle organization were present, ranging from virtually periodic every breath, to quasi-periodic every two or three breaths, to near random fluctuations. Some crackles appear to exhibit bifurcations in behavior, shifting between periodic, quasi-periodic, and random patterns. Over longer timescales, drifts in both crackle timing and number were observed. Conclusions Alveolar and airway instability may give rise to highly complex R/D dynamics consisting of both short- and long-term transients. The observed aperiodicity may be due to local shifts in dynamical behavior between stable, and less stable modes of recruitment observed in the lung crackle waveforms (Figure 1). Over much longer timescales (i.e., 2000 breaths, data not shown), samples exhibited convergence to more stable patterns of crackle timing, potentially corresponding to surfactant degradation in ex vivo lung samples and/or evolution of lung mechanics during mechanical ventilation. Further investigation is warranted to discover mechanisms of aperiodicity in R/D occurrence and timing, which may lead to targeted strategies to mitigate associated harm. References: 1 Broche et al., Crit Care Med 45(4):687-694 This abstract is funded by: Carver Charitable Trust
Meggo et al. (2026) studied this question.