title: Pressure-dependent stress relaxation in acute respiratory distress syndrome and healthy lungs: an investigation based on a viscoelastic model.
abstract: BACKGROUND
  Limiting the energy transfer between ventilator and lung is crucial for ventilatory strategy in acute respiratory distress syndrome (ARDS). Part of the energy is transmitted to the viscoelastic tissue components where it is stored or dissipates. In mechanically ventilated patients, viscoelasticity can be investigated by analyzing pulmonary stress relaxation. While stress relaxation processes of the lung have been intensively investigated, non-linear interrelations have not been systematically analyzed, and such analyses have been limited to small volume or pressure ranges. In this study, stress relaxation of mechanically ventilated lungs was investigated, focusing on non-linear dependence on pressure. The range of inspiratory capacity was analyzed up to a plateau pressure of 45 cmH2O.
  
  
  METHODS
  Twenty ARDS patients and eleven patients with normal lungs under mechanical ventilation were included. Rapid flow interruptions were repetitively applied using an automated super-syringe maneuver. Viscoelastic resistance, compliance and time constant were determined by multiple regression analysis using a lumped parameter model. This same viscoelastic model was used to investigate the frequency dependence of the respiratory system's impedance.
  
  
  RESULTS
  The viscoelastic time constant was independent of pressure, and it did not differ between normal and ARDS lungs. In contrast, viscoelastic resistance increased non-linearly with pressure (normal: 8.4 (7.4-11.9) [median (lower - upper quartile)] to 35.2 (25.6-39.5) cmH2O.sec/L; ARDS: 11.9 (9.2-22.1) to 73.5 (56.8-98.7)cmH2O.sec/L), and viscoelastic compliance decreased non-linearly with pressure (normal: 130.1(116.9-151.3) to 37.4(34.7-46.3) mL/cmH2O; ARDS: 125.8(80.0-211.0) to 17.1(13.8-24.7)mL/cmH2O). The pulmonary impedance increased with pressure and decreased with respiratory frequency.
  
  
  CONCLUSIONS
  Viscoelastic compliance and resistance are highly non-linear with respect to pressure and differ considerably between ARDS and normal lungs. None of these characteristics can be observed for the viscoelastic time constant. From our analysis of viscoelastic properties we cautiously conclude that the energy transfer from the respirator to the lung can be reduced by application of low inspiratory plateau pressures and high respiratory frequencies. This we consider to be potentially lung protective.
authors.affiliation: Department of Anesthesiology and Critical Care Medicine, University Medical Center, Freiburg, D-79106 Freiburg, Germany. steven.ganzert@uniklinik-freiburg.de
authors.forename: Steven
authors.initials: S
authors.lastname: Ganzert
authors.forename: Knut
authors.initials: K
authors.lastname: Möller
authors.forename: Daniel
authors.initials: D
authors.lastname: Steinmann
authors.forename: Stefan
authors.initials: S
authors.lastname: Schumann
authors.forename: Josef
authors.initials: J
authors.lastname: Guttmann
mesh_terms: D004548:Elasticity; D007320:Inspiratory Capacity; D008168:Lung / Q000502:physiology* / Q000503:physiopathology; D010101:Oxygen Consumption; D010992:Plethysmography, Impedance; D012016:Reference Values; D012121:Respiration, Artificial / Q000379:methods; D012128:Respiratory Distress Syndrome / Q000503:physiopathology*; D015656:Respiratory Mechanics; D014783:Viscosity