Bedside assessment of respiratory viscoelastic properties in ventilated patients

Viscoelasticity represents an important component of respiratory mechanics, being responsible, in some cases, for most of the pressure dissipated during breathing. Hitherto the methods available for determining the viscoelastic properties have been simplified, but are still time-demanding and depend on a great deal of calculation. In this study, a simple means of determining respiratory viscoelastic properties during mechanical ventilation was introduced. The viscoelastic constants of the respiratory system, modelled as a Maxwell body, were studied in 17 normal subjects and seven patients with acute lung injury (ALI) using two end-inspiratory occlusions; one with a short inspiratory time (tI) to determine the elastic component of viscoelasticity and the other with a long tI to assess the resistive component of viscoelasticity. The results were reproducible and similar to those provided by the previously described multiple-breath method (MB). The mean+/-SD viscoelastic resistance was 5.31+/-1.50 cm H2O x L(-1) x s with the proposed method and 5.71+/-1.87 cm H2O x L(-1) x s with the MB method in normal subjects, and 8.93+/-2.82 cm H2O x L(-1) x s and 10.36+/-3.13 cm H2O x L(-1), respectively in ALI patients. The mean+/-SD viscoelastic elastance was 3.92+/-0.84 cm H2O x L(-1) and 4.94+/-1.01 cm H2O x L(-1) in normal subjects and 7.08+/-2.01 cm H2O x L(-1) and 8.21+/-1.16 cm H2O x L(-1) in ALI patients, respectively. The mean+/-SD viscoelastic time constant was 1.36+/-0.24 s and 1.17+/-0.34 s in normal subjects and 1.26+/-0.35 s and 1.24+/-0.23 in ALI patients, respectively. The method was easy to perform and applicable at the bedside in clinical routine.