Mechanical Power Correlates With Stress, Strain, and Atelectrauma Only When Normalized to Aerated Lung Size in Patients With Acute Respiratory Distress Syndrome

OBJECTIVES: First, to investigate whether the severity of acute respiratory distress syndrome (ARDS) influences ventilator-induced lung injury (VILI) risk in ventilated patients with similar mechanical power of respiratory system (MP(RS)). Second, to determine whether, under these circumstances, there is a relationship between transpulmonary mechanical power (MP(Tp)) normalized to the aerated lung (specific lung mechanical power or SLMP) and VILI risk, and third, to determine whether normalizing MP(RS) to compliance of respiratory system (CRS) can replace SLMP to bedside. DESIGN: Prospective cohort study. SETTING: The study was conducted in a tertiary academic ICU. PATIENTS: The study included 18 patients with ARDS. INTERVENTIONS: Ventilatory settings were adjusted to achieve a similar MP(RS). MEASUREMENTS AND MAIN RESULTS: Mechanical power was normalized to CRS (specific mechanical power or SMP = MP(RS)/CRS), and SLMP was calculated as the ratio between MP(Tp) and end-expiratory lung volume (SLMP = MP(Tp)/EELV). The strain was defined as the ratio between tidal volume and EELV (strain = Vt/EELV), stress as transpulmonary pressure at the end of inspiration, and atelectrauma as the difference between expiration and inspiration in the nonaerated lung. Although patients had been ventilated with similar MP(RS) = 23.75 (23–24) J/min and MP(Tp) = 11.6 (10.8–12.8) J/min, SLMP increased linearly with the fall in Pao(2)/Fio(2) (R = –0.83, p = 0.0001). MP(RS) only correlated positively with VILI-associated mechanisms when normalized to aerated lung size: correlations between SLMP and stress (R = 0.9, R(2) = 0.84, p = 0.00004), strain (R = 0.97, R(2) = 0.94, p < 0.00001) and atelectrauma (R = 0.82, R(2) = 0.70, p = 0.00002), and correlations between SMP and stress (R = 0.86, R(2) = 0.75, p = 0.00001), strain (R = 0.68, R(2) = 0.47, p = 0.001) and atelectrauma (R = 0.67, R(2) = 0.46, p = 0.002). CONCLUSIONS: The results suggest that normalizing mechanical power to lung-aerated size or CRS may correlate positively with stress, strain, and atelectrauma.