Modeling time effects of recruitment and P–V curve characteristics: a simulation study

Introduction

In a multicenter study on respiratory mechanics in ALI and ARDS [1] we observed phenomena most probably related to time effects: change in compliance after a recruitment maneuver, transient end-expiratory volume (Fig. 1) after changing PEEP, and a transient change in shape of the P–V curve after a change in tidal volume (Fig. 2). To gain a better understanding of temporal effects we developed a mathematical model of the respiratory system to investigate these phenomena.

Figure 1

Observed transient volume after switching from ZEEP to PEEP 18 in an ARDS patient. End expiratory volume at ZEEP was defined as 0.

Figure 2

Transient in the shape of the P–V curve after an increase in tidal volume.

Methods

We extended the model of recruitment proposed by Hickling [2] to include chest wall characteristics, FRC, and an initial opening volume of alveolar units once recruited. Alveolar units had a nonlinear compliance and were organized in a bronchial tree structure. Depending on the position in the tree each lung unit was exposed to a gravitational superimposed pressure from 0 in the ventral compartment to a maximal value in the dorsal compartment. Opening and closing pressures of lung units were normally distributed with a modifiable mean and standard deviation. Time dependency was modeled by including timing parameters for recruitment and derecruitment of alveolar units similar to the approach of Bates and colleagues [3].

Results

In contrast to Hickling's model, simulations with the extended model produced realistic transients in volume. Furthermore, transient changes in compliance were predicted by the model. Modeling of recruitment and derecruitment as time-dependent phenomena allowed one to mimic the transients as observed in ARDS patients.

Conclusion

The shape of simulated P–V loops is influenced by time effects only related to closing and opening of alveolar units. Up to now, transient effects have been explained by viscoelasticity and by mechanical inhomogeneity. From the present study we conclude that timing effects during recruitment may be a third mechanism of transient mechanical behaviour of the respiratory system.