Model based analysis reveals differences in viscoelasticity between acute respiratory distress syndrome and healthy lungs

Introduction

We hypothesized that the time course of the slow pressure drop after interruption of inspiratory flow contains information about the underlying lung disease. Respiratory data obtained from repetitive inspiratory flow interruption manoeuvres was compared between mechanically ventilated patients without pulmonary disease and with acute respiratory distress syndrome (ARDS) and was analyzed using a model describing the Newtonian and viscoelastic properties of the lung.

Methods

Inspiratory airflow was repetitively interrupted for 3 seconds after inflation of 100 ml volume steps up to a maximum plateau pressure of 45 mbar by means of an automated super syringe manoeuvre (Evita 4Lab; Dräger Medical, Lübeck, Germany). Twelve patients with healthy lungs and 20 patients suffering from ARDS were investigated. We determined the Newtonian and viscoelastic unit of a model (Figure 1a) [1, 2] by mathematical fitting the model to segments of the pressure curve (Figure 1b). The slow pressure drop was described by viscoelasticity (resistance R2, compliance C2).

Figure 1

(a, left) Newtonian (R1, C1) and viscoelastic unit (R2, C2). (b, right) Data fit.

Results

Analysis of time constant T of the viscoelastic unit revealed no differences between ARDS and healthy patients (Figure 2a). However, compliance C2 (Figure 2b) and resistance R2 (Figure 2c) of the viscoelastic unit were significantly different. C2 was lower and R2 was higher in ARDS patients. The time constant as well as C2 and R2 showed a significant dependency on pressure.

Figure 2

Results for the viscoelastic unit. (a, left) Time constant T; (b, middle) compliance C; (c, right) resistance R.

Conclusion

The time constant of the viscoelastic unit determines the decay of the pressure curve after airflow interruption. Identical time constants mean that there is no significant difference in the decay between health and ARDS. Only the model-based analysis revealed the significant difference in viscoelasticity that is associated with ARDS.