Fatigue-aware alveolar stress and strain tracking: a theoretical biomechanics framework for regional mechanical dose accumulation in ventilator-induced lung injury

Ventilator-induced lung injury remains incompletely explained by global bedside variables that do not resolve regional load, deformation, time, repetition, and recovery. FAAST (Fatigue-Aware Alveolar Stress and Strain Tracking) and its deterministic implementation, FAAST2, describe regional mechanical exposure and fatigue. The framework maps a shared central-airway boundary into 20 compartments, defines functional-residual-capacity-referenced volumetric strain, closes generalized constitutive stress through regional specific elastance, and tracks Time-Accumulated Stress History (TASH), the complete-cycle load-time channel, and Strain-Accumulated Strain History (SASH), the cumulative positive loading-branch deformation-work channel. A cycle-indexed Continuum of Damage Index (CDI) combines elemental load-time and deformation-work increments with inherited fatigue. CDI is irreversible and bounded; a continuous recovery-opportunity factor accelerates decay of reversible fatigue memory rather than subtracting accumulated damage. Applying selected ventilation values from Park’s ALS report with declared PEEP and normal-lung mechanics produced a chronic-stability comparator with 1-h Strain-Accumulated Strain History 88.9 kPa, dynamic strain 0.194, per-cycle deformation work 0.098 kPa, recovery opportunity 0.827, and bounded steady fatigue 0.015; CDI was not extrapolated across chronic support. Relative to this comparator, 1-h regional SASH across Cases 1–4H was 99.5, 485.1, 723.8, and 1,510.4 kPa, or 1.12, 5.46, 8.14, and 17.0 times the comparator, respectively; illustrative 24-h maximum compartment CDI was 0.280, 0.478, 0.618, and 0.851. Higher tidal volume increased burden despite fewer breaths, while a preferentially loaded compliant region in heterogeneous ARDS carried the greatest burden. A matched OSCILLATE sensitivity applied reported day-1 arm settings to the same Panwar-H-anchored 20-compartment lung for 24 h. In the compartment attaining maximum CDI at 24 h, control ventilation produced Time-Accumulated Stress History 28.080 kPa·h, SASH 15,496 kPa, and maximum compartment CDI 0.596. Across declared HFOV tracheal oscillatory tidal-volume sensitivities of 0.5–2.0 mL/kg predicted body weight, TASH was approximately 31.299 kPa·h, SASH ranged from 13,513 to 54,053 kPa, and CDI ranged from 0.575 to 0.777. Modeled SASH arm equivalence occurred at 0.573 mL/kg; this is a mechanical comparison point, not an injury threshold. These are ecological sensitivity outputs, not measured HFOV tidal volumes, patient reconstructions, time-weighted exposures, mortality predictions, or evidence that pulmonary mechanics exclude hemodynamic harm. FAAST2 requires regional construct validation and biological calibration before clinical use.