Stress and strain in acute respiratory distress syndrome

Abstract

During mechanical ventilation, stress applied to the lung parenchyma is directly related to maximal transpulmonary pressure, which is computed as the difference between end inspiratory alveolar and pleural pressures. Strain refers to the lung parenchyma deformation and is computed as the ratio of volume increase above functional residual capacity (FRC) over FRC itself. The relationship between stress and strain is proportional. The factor of proportionality between stress and strain is the specific lung elastance, which seems to be constant for a large stress range and not to be altered in case of acute respiratory distress syndrome (ARDS). The stress and strain concept underlines the limitations of the standard parameters (plateau pressure and predicted body weight) used to adapt the ventilator settings in ARDS patients. However, lung heterogeneity during ARDS and the occurrence of lung recruitment are important limitations to the use of the stress and strain concept to individually adapt the ventilator settings at the bedside in ARDS patients.