A linearized expiration flow homogenizes the compartmental pressure distribution in a physical model of the inhomogeneous respiratory system.

Objective: Flow-controlled expiration (FLEX) and flow-controlled ventilation (FCV) imply a linearized expiration, and were suggested as new approaches for lung-protective ventilation, especially in the case of an inhomogeneous lung. We hypothesized that a linearized expiration homogenizes the pressure distribution between compartments during expiration, compared to volume-controlled (VCV) and pressure-controlled (PCV) ventilation.
Approach: We investigated the expiratory pressure decays in a physical model of an inhomogeneous respiratory system. The model contained four compartments of which two had a high (25 ml cmH ₂ O ^-1 ) and two a low compliance (10 ml cmH ₂ O ^-1 ). These were combined with either a high (6.5 cmH ₂ O s l ^-1 ) or low resistance (2.8 cmH ₂ O s l ^-1 ), respectively. The model was ventilated in all modes at various tidal volumes and peak pressures, and we determined in each compartment the expiratory time at which the pressure declined to 50% (t ₅₀ ) of the end-inspiratory pressure, and the maximal differences of t ₅₀ (Δt ₅₀ ) and pressure (Δp _max ) between all compartments.
Main Results: During FLEX and FCV, t ₅₀ was 6- to 7-fold higher compared to VCV and PCV (all P < 0.001). During VCV and PCV, Δt ₅₀ was higher (128 ± 18 ms) compared to FLEX and FCV (49 ± 19 ms; all P < 0.001). Δp _max reached up to 3.8 ± 0.2 cmH ₂ O during VCV and PCV, but only 0.6 ± 0.1 cmH ₂ O during FLEX and FCV (P < 0.001).
Significance: FLEX and FCV provide a more homogeneous expiratory pressure distribution between compartments with different mechanical properties compared with VCV and PCV. This may reduce shear stress within inhomogeneous lung tissue.