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Preemptive mechanical ventilation based on dynamic physiology in the alveolar microenvironment: Novel considerations of time-dependent properties of the respiratory system.

Identification of acute respiratory distress syndrome (ARDS) as a syndrome and the possibility that a mechanical ventilator setting could reduceARDSmortality led to a large number of physiologic studies investigating the optimal ventilator setting for the ARDS patient. What these studies found was that mechanical ventilation can significantly reduce ARDS morbidity and mortality when set properly, but can exacerbate lung damage by causing a secondary ventilator-induced lung injury (VILI) when set improperly. Dreyfuss and Saumon described the transpulmonary pressure (Ptp) gradient across the alveolus caused alveolar strain (i.e., alveolar volume change) as the major mechanism of VILI rather than just the magnitude of airway pressure (volutrauma vs. barotrauma). Further work from Gattinoni et al. in ARDS patients demonstrated that dependent lung areas demonstrate preferential loss of lung volume, whereas nondependent lung areas have relatively normal ventilation. The term “baby lung” was coined for this remaining normal residual lung tissue, and it was postulated that overdistension would occur in the more compliant “baby lung” if a normal size tidal volume (Vt) were delivered to the ARDS patient. The mechanism of atelectrauma is excessive shear-stress as the collapsed alveolar walls pull apart during inflation and stress-concentration, which occurs between patent and totally collapsed or edema-filled alveoli. These physiologic and clinical studies suggested that VILI might be minimized if (a) Ptp was reduced in order to lower alveolar strain; (b) Vt and plateau pressure (Pplat) were lowered to prevent lung overdistension; and (c) adequate positive end-expiratory pressure (PEEP) was used to minimize atelectrauma. Using these data as proof of concept, several small clinical trials were conducted, with some studies showing no improvement in lung protection with low Vt (LVt) strategy