Dynamic compliance in flow-controlled ventilation

Abstract

The recent paper by Wittenstein et al. "Comparative effects of flow-vs.volume-controlled one-lung ventilation on gas exchange and respiratory system mechanics in pigs" [1] contains some intriguing observations we would like to discuss. Alveolar pressure amplitudeIn flow-controlled ventilation (FCV), the gas flow is constant during both inspiration and expiration [2,3].This significantly differs from volume-controlled ventilation (VCV) where inspiratory flow is constant, but exhalation is passively driven by lung-chest elasticity resulting in a decelerating flow profile.Furthermore, in contrast to VCV (and any other ventilation mode) in FCV gas is always moving either into or out of the lungs without any pause phases in an accurately controlled way.This causes a continuous pressure drop across the airway resistance.Consequently, during inspiration tracheal pressure must be higher than alveolar pressure, whereas during expiration the opposite pertains.The alveolar driving pressure (ΔP) in FCV is therefore lower than the measured tracheal ΔP.Compliance (which necessarily means dynamic compliance considering the nature of FCV) calculated from tracheal ΔP will lead to an underestimation of actual (alveolar) lung compliance because tracheal ΔP is higher than the alveolar pressure swing.Figure 1 shows how it is possible to estimate the difference between tracheal ΔP and aggregate alveolar pressure swing using the measured airway resistance reported by Wittenstein et al. [1] for FCV.The calculated difference between the two pressures (partially) accounts for the difference of the compliance in both groups of the Wittenstein study.Using alveolar rather than tracheal ΔP for the calculation of the dynamic compliance therefore results in similar compliance in both groups.

Topics & Concepts