Ventilatory Variables and Mechanical Power in Patients with Acute Respiratory Distress Syndrome

Mortality in Acute Respiratory Distress Syndrome (ARDS) has decreased after the adoption of lung-protective strategies. Lower tidal-volumes, lower driving pressures, lower respiratory rates, and higher end-expiratory pressures have all been suggested as key components of lung protection strategies. A unifying theoretical explanation has been proposed that attributes lung injury to the energy transfer rate (mechanical power) from ventilator to the patient, calculated from a combination of several ventilator variables. We aimed to assess the impact of mechanical power on mortality in patients with ARDS as compared to that of primary ventilator variables such as driving pressure, tidal volume, and respiratory rate. We obtained data on ventilatory variables and mechanical power from a pooled database of ARDS patients who had participated in six randomized clinical trials of protective mechanical ventilation, and one large observational cohort of ARDS patients. The primary outcome was mortality at 28 or 60 days. We included 4,549 patients (38% women; mean age, 55±23 years). Average mechanical power was 0.32±0.14 J.min-1.Kg-1 predicted body weight, driving pressure was 15.0±5.8 cmH2O, and respiratory rate was 25.7±7.4 breaths/minute. Driving pressure, respiratory rate, and mechanical power were significant predictors of mortality in adjusted analyses. The impact of driving pressure on mortality was four times as large as that of respiratory rate. Mechanical power was associated with mortality during controlled mechanical ventilation in ARDS but a simpler model using only driving pressure and respiratory rate was equivalent.