Optimizing positive end-expiratory pressure by oscillatory mechanics minimizes tidal recruitment and distension: an experimental study in a lavage model of lung injury

Abstract

Introduction: It is well established that during mechanical ventilation of patients with acute respiratory distress syndrome cyclic recruitment/derecruitment and overdistension are potentially injurious for lung tissues. We evaluated whether the forced oscillation technique (FOT) could be used to guide the ventilator settings in order to minimize cyclic lung recruitment/derecruitment and cyclic mechanical stress in an experimental model of acute lung injury.

Methods: We studied six pigs in which lung injury was induced by bronchoalveolar lavage. The animals were ventilated with a tidal volume of 6 ml/kg. Forced oscillations at 5 Hz were superimposed on the ventilation waveform. Pressure and flow were measured at the tip and at the inlet of the endotracheal tube respectively. Respiratory system reactance (Xrs) was computed from the pressure and flow signals and expressed in terms of oscillatory elastance (EX5). Positive end-expiratory pressure (PEEP) was increased from 0 to 24 cm H2O in steps of 4 cm H2O and subsequently decreased from 24 to 0 in steps of 2 cm H2O. At each PEEP step CT scans and EX5 were assessed at end-expiration and end-inspiration.

Results: During deflation the relationship between both end-expiratory and end-inspiratory EX5 and PEEP was a U-shaped curve with minimum values at PEEP = 13.4 ± 1.0 cm H2O (mean ± SD) and 13.0 ± 1.0 cm H2O respectively. EX5 was always higher at end-inspiration than at end-expiration, the difference between the average curves being minimal at 12 cm H2O. At this PEEP level, CT did not show any substantial sign of intra-tidal recruitment/derecruitment or expiratory lung collapse.

Conclusions: Using FOT it was possible to measure EX5 both at end-expiration and at end-inspiration. The optimal PEEP strategy based on end-expiratory EX5 minimized intra-tidal recruitment/derecruitment as assessed by CT, and the concurrent attenuation of intra-tidal variations of EX5 suggests that it may also minimize tidal mechanical stress.

Figure 1

Left panel: Respiratory system resistance (Rrs), oscillatory elastance (E_X5, the inverse of compliance), percentage volume of aerated (VtissA%, blue), poorly aerated (VtissPA%, green) and nonaerated (VtissNA%, red) tissue at end-expiration (closed symbols) and at end-inspiration (open symbols) for one representative animal. Squared symbols are used for incremental positive end-expiratory pressure (PEEP), circles for decremental. Right panel: a representative computed tomography (CT) slice (selected approximately 1 cm above the diaphragmatic dome) of the same animal at end-expiration (EE) and at end-inspiration (EI), at the different PEEP levels during the incremental (UP) and decremental (D) PEEP trial. Colour code for regions as above.

Figure 2

Respiratory system resistance (Rrs), oscillatory elastance (E_X5), percentage volume of non-aerated tissue (VtissNA%), lung gas volume (Vgas) at end-expiration (closed symbols) and at end-inspiration (open symbols) during the incremental and decremental positive end-expiratory pressure (PEEP) trial as a function of PEEP. Data are reported as mean and SD. Significance of differences between inspiration and expiration: *P <0.01, +P <0.05.

Figure 3

End-inspiratory (open symbols) and end-expiratory (closed symbols) oscillatory elastance (E_X5) and dynamic elastance (Edyn). Edyn has been computed over the whole breath (grey triangles, dashed line) and intra-tidal, at the highest (open circles, solid lines) and at the lowest (closed circles, solid line) portion of tidal volume. Data are reported as mean and SD. Significance of differences between inspiration and expiration: *P <0.01, +P <0.05.

Figure 4

Oscillatory elastance (E_X5), gas volume (Vgas) and percentage volume of non-aerated tissue (VtissNA%) as a function of pressure during the incremental (left panel) and decremental (right panel) positive end-expiratory pressure (PEEP) trial. Open symbols refer to end-inspiratory data points, closed symbols to end-expiratory ones. Dashed lines connect end-inspiration and end-expiration at the same PEEP step.