The effects of low tidal ventilation on lung strain correlate with respiratory system compliance

Background: The effect of alterations in tidal volume on mortality of acute respiratory distress syndrome (ARDS) is determined by respiratory system compliance. We aimed to investigate the effects of different tidal volumes on lung strain in ARDS patients who had various levels of respiratory system compliance. Methods: Nineteen patients were divided into high (C-high group) and low (C-low group) respiratory system compliance groups based on their respiratory system compliance values. We defined compliance >= 0.6 ml/(cmH(2)O/kg) as C-high and compliance <0.6 ml/(cmH(2)O/ kg) as C-low. End-expiratory lung volumes (EELV) at various tidal volumes were measured by nitrogen wash-in/washout. Lung strain was calculated as the ratio between tidal volume and EELV. The primary outcome was that lung strain is a function of tidal volume in patients with various levels of respiratory system compliance. Results: The mean baseline EELV, strain and respiratory system compliance values were 1873 ml, 0.31 and 0.65 ml/ (cmH(2)O/kg), respectively; differences in all of these parameters were statistically significant between the two groups. For all participants, a positive correlation was found between the respiratory system compliance and EELV (R = 0.488, p = 0.034). Driving pressure and strain increased together as the tidal volume increased from 6 ml/kg predicted body weight (PBW) to 12ml/kg PBW. Compared to the C-high ARDS patients, the driving pressure was significantly higher in the C-low patients at each tidal volume. Similar effects of lung strain were found for tidal volumes of 6 and 8 ml/kg PBW. The "lung injury" limits for driving pressure and lung strain were much easier to exceed with increases in the tidal volume in C-low patients. Conclusions: Respiratory system compliance affected the relationships between tidal volume and driving pressure and lung strain in ARDS patients. These results showed that increasing tidal volume induced lung injury more easily in patients with low respiratory system compliance.