Single-breath method for assessing the viscoelastic properties of the respiratory system

In order to explain the time dependency of resistance and elastance of the respiratory system, a linear viscoelastic model (Maxwelll body) has been pr oposed, In this model the maximal viscoelastic pressure (Pvisc,max) developed withi n the tissues of the lung and chest wall at the end of a constant-flow (V') inflation of a given time (tr) is given by: Pvisc,max = R2V'(1-e(-tI/tau 2 )), where R2 and tau 2 are, respectively, the resistance and time constant of the Maxwell body. After rapid airway occlusion at tI, tracheal pressure (Ptr) decays according to the following function: Ptr(t) = Pvisc(t) + Prs,s t = Pvisc,max e(tocc/tau 2)+ Prs,st, where tocc is time after occlusion and Prs,st is static re-coil pressure of the respiratory system. By fitting Pt r after occlusion to this equation, tau 2 and Pvisc,max are obtained. Using these values, together with the lr and tr pertaining to the constant-how i nflation preceding the occlusion, R2 can be calculated from the former equa tion, Thus, from a single breath, the constants tau 2, R2 and E2 (R2/tau 2) can be obtained, This method was used in 10 normal anaesthetized, paralysed, mechanically ve ntilated subjects and six patients with acute lung injury, The results were reproducible in repeated tests and similar to those obtain ed from the same subjects and patients with the time-consuming isoflow, mul tiple-breath method described previously.