Regional viscoelastic properties of thoracic tissues in acute respiratory distress syndrome (ARDS) and their change with position and positive end-expiratory pressure (PEEP) are unknown. In an experimental porcine ARDS, dorsal and ventral lung (R2,L and E2,L) and chest wall (R2,cw and E2,cw) viscoelastic resistive(R) and elastic(E) parameters were measured at 20, 15, 10 and 5 cmH2O PEEP in supine and prone position. E2 and R2 were obtained by fitting the decay of pressure after end-inspiratory occlusion to the equation: Pviscmax(t)=R2 e(-t/τ2), where t is length of occlusion and τ2 time constant. E2 was = R2/τ2. R2,cw and E2,cw were measured from esophageal, dorsal and ventral pleural pressures. Global R2,L and E2,L were obtained from the global trans-pulmonary pressure (airway pressure-esophageal pressure), and regional R2,L and E2,L from the dorsal and ventral airway pressure-pleural pressure difference. Lung ventilation was measured by electrical impedance tomography (EIT). Global R2,cw and E2,cw did not change with PEEP or position. Global R2,L (median(Q1-Q3)) was 37.1(11.0-65.1), 5.1(4.3-5.5), 12.1(8.4-19.5), and 41.0 (26.6-53.5) cmH2O/L/s in supine, and 15.3 (9.1-41.9), 7.9 (5.7-11.0), 8.0 (5.1-12.1) and 12.9 (6.4-19.4) cmH2O/L in prone from 20 to 5 cmH2O PEEP (P=0.06 for PEEP and P=0.06 for position). Dorsal R2,L significantly and positively correlated with amount of collapse measured with EIT. Global and regional lung and chest wall viscoelastic parameters can be described by a simple rheological model. Regional E2 and R2 were uninfluenced by PEEP and position except for PEEP on dorsal E2,L and position on dorsal E2,cw.
Dynamics of regional lung aeration determined by electrical impedance tomography in patients with acute respiratory distress syndrome
