Separate determination of inspiratory and expiratory dynamic lung mechanics using expiratory flow control

Introduction

During mechanical ventilation, the volume-dependent mechanical behaviour of the respiratory system may differ between inspiration and expiration due to expiratory collapse of small airways and due to repeated intratidal recruitment. Up to now, no method is available for separate determination of inspiratory and expiratory resistance (R) and compliance (C) during mechanical ventilation. We hypothesized that the control of the expiratory flow rate (expiratory flow control [EFC]) allows a discrete analysis of R and C.

Methods

Different flow profiles were applied in two mechanical lung models. These profiles included standard ventilator and EFC modes. The volume dependency of dynamic respiratory mechanics was calculated using the SLICE method [1]. The algorithm was extended for a separate analysis of the inspiration and expiration phase. We validated the efficacy of EFC in six sheep using a modified standard ventilator (Evita 4; Draeger Medical, Lübeck, Germany) with EFC in the pressure-control mode.

Results

It is mainly the passive expiration with the linear dependency of flow, volume and pressure that inhibits the separate inspiratory and expiratory analysis of respiratory mechanics. Figure 1 shows the effect of EFC on the breathing pattern in a healthy animal. At early expiration with EFC, flow is drastically reduced as compared with passive expiration (grey line). With EFC (solid line), the change in flow is independent of the change in volume, allowing the determination of R and C. Figure 2 displays the analysis of compliance in a lung model (C = 24) during constant flow ventilation without EFC. Without EFC, expiratory compliance is indeterminable (solid line). With EFC (Fig. 3) excellent agreement of inspiratory and expiratory compliance with the reference compliance was found. In six healthy sheep compliance determined during EFC was computed in inspiration and expiration. Dependent on the extent of EFC, we found a high agreement of inspiratory and expiratory compliances compared with mixed (inspiratory and expiratory) data.

Figure 1

Figure 2

Figure 3

Conclusion

The control of expiratory flow allows the application of volume-dependent multiple linear regression analysis during inspiration respectively expiration alone. With EFC, the separate mechanical analysis reveals accurate results in well-controlled mechanical lung models and in experimental animals.