Toward optimal mechanical ventilation of the injured lung: the role of expiratory duration

Positive pressure mechanical ventilation is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), but it can also increase mortality by causing ventilator-induced lung injury (VILI) if applied inappropriately. Although strategies like low-tidal volume ventilation and prone positioning have been shown to reduce mortality, the optimal patient-specific approach to mechanical ventilation in ARDS has yet to be identified. The worst manifestations of acute lung injury arise when fluid and proteins from the blood leak through a damaged blood-gas barrier, accumulating in the airspaces and impairing the ability of pulmonary surfactant to lower surface tension. This amplifies the ventilatory stresses in the lung tissues, which further damages the blood-gas barrier, leading to a vicious cycle of worsening injury. Studies suggest that VILI may be most effectively avoided by preventing the atelectrauma caused during inspiration by the forced reopening of lung units that close during each expiration. Atelectrauma is conventionally mitigated with positive end-expiratory pressure (PEEP), but it remains unclear if the algorithmic selection of PEEP leads to mortality reductions in ARDS. Animal studies, however, support the efficacy of exploiting the time-dependent nature of recruitment and derecruitment, for example through the use of brief expiratory durations that continually adapt to changing lung mechanics. Despite decades of research, it remains unclear how to minimize VILI in any given ARDS patient. Animal studies coupled with energy dissipation analysis indicate that the prevention of VILI requires, above all, avoidance of the atelectrauma caused by cyclic recruitment and derecruitment in the lung. In addition, the path to optimal mechanical ventilation in ARDS must be based not only on the amplitudes of the pressures applied to the lung but also on their temporal natures.