ARDS: ventilator management

In the continuum of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS), the lungs are damaged following an insult that may be of pulmonary (e.g., pneumonia, aspiration pneumonitis) or extrapulmonary (e.g., sepsis, trauma, transfusion) origin. Regardless of the source of injury, ARDS is characterized by an acute onset, bilateral airspace infiltrates on chest X-ray, and hypoxemia (PaO₂/FIO₂ < 300), assuming no evidence of left atrial hypertension. With damage to the alveolar epithelium, alveolar-capillary membrane, and endothelium, lung compliance progressively worsens, and hypoxemia becomes refractory. Consequently, mechanical ventilation is necessary.

Unfortunately, while respiratory support is needed in ARDS, mechanical ventilation itself can worsen lung injury. Thus, the goals of mechanical ventilation should include achieving adequate gas exchange while limiting additional injury. This “lung-protective” strategy incorporates low tidal volume (VT) ventilation and low airway plateau pressure (Pplat) with permissive hypercapnia, if needed. The National Institutes of Health and ARDS Network trial included 861 patients and demonstrated a 9% absolute mortality reduction in the group randomized to receive 6ml/kg (predicted body weight) VT and Pplat less than 30cmH₂O compared with 12ml/kg VT and Pplat less than 50cmH₂O.

Additionally, CT imaging of patients with ARDS has shown that the lung consolidation is heterogeneous. As such, the preserved, highly compliant, lung may be more prone to be exposed to higher tidal volumes and inflation pressures. It has been suggested to use positive end-expiratory pressure (PEEP) and possibly recruitment maneuvers to expand collapsed alveoli, which may also help to redistribute lung water and perhaps reduce injury related to the repeated expansion and collapse of alveoli. Trials suggest greater ventilator-free days and possibly a mortality benefit with high PEEP; however, an optimal level of PEEP remains unknown.

The current ARDSnet protocol does not specify any particular ventilator mode but recommends settings to achieve an initial VT of 8ml/kg predicted body weight that is then progressively decreased to 6ml/kg or lower (minimum of 4ml/kg) if Pplat is not less than 30cmH₂O. Respiratory rate is adjusted to achieve a pH goal of 7.30-7.45. A minimum PEEP of 5cmH₂O is used and adjusted (along with FIO₂) to achieve an oxygenation goal of PaO₂ 55-80mmHg or SpO₂ 88-95%.