Mechanical ventilation with different alveolar pressures improves alveolar recruitment: a model study

Introduction

In patients with acute respiratory distress syndrome (ARDS) tidal ventilation is always inhomogeneous, with atelectasis in dependent lung and overdistention in nondependent lung. The aim of our study was to develop a flow pattern that simultaneously produces locally different alveolar pressures, in order to recruit collapsed alveoli and at the same time to prevent hyperinflation of already open alveoli.

Methods

We modified Horsfield's model of the canine lung [1] to predict alveolar pressure differences between the collapsed and opened alveoli. The morphometric model has an asymmetrical branching airway system with 47 orders, whereby order 47 corresponds to the trachea. Each branch is terminated with a viscoelastic tissue unit and there are in total 150,077 acini for the whole model. Using this model, atelectasis of alveoli was simulated by reducing the diameter of small airways (order 1, 2 and 3) up to 90% of their original sizes. The tissue damping factor G was 10⁴ times higher in the collapsed alveoli than that in normal alveoli. The shunt gas compression compliance C_g for the alveolus was 10⁶ times smaller. Different percentages of atelectatic area from 0 to 90% were simulated. Flow distribution and airway input impedance were calculated at different frequencies from 0.1 to 5 Hz. Alveolar pressure differences were obtained by analyzing the products of flow distribution and alveolar input impedance.

Results

The alveolar pressure P_alv reduced as the frequency increased (for both collapsed and open alveoli). Compared with the situation at 0.2 Hz, P_alv reduced to 9.1 ± 1.0% at 4 Hz. On the other hand, P_alv increased as the percentage of atelectatic area increased (for all frequencies). P_alv increased to 267 ± 6% when 70% of alveoli collapsed compared with 10% of collapse. The pressure differences between collapsed and open alveoli increased as the frequency increased. At 0.2 Hz the differences were <3%, while at 4 Hz the differences were >20% (relative to the lower value).

Conclusions

Increasing the high-frequency contents of the inspiratory flow pattern may help to develop a pressure difference between collapsed and open alveoli. The low-frequency contents of the flow pattern, however, may be still needed to guarantee the minimum ventilation.