Objectives: To develop a method for automatic recording of multiple dynamic elastic pressure–volume (Pel/V) loops. To analyse the relationship between multiple dynamic Pel/V loops and static Pel/V loops in a porcine model of acute lung injury/acute respiratory distress syndrome (ALI/ARDS). To test the hypothesis that increasing lung collapse and re-expansion with decreasing positive end expiratory pressure (PEEP) can be characterized by hysteresis of the Pel/V loops.
Material and Interventions: In eight anaesthetized and paralysed pigs, ALI/ARDS was induced by inhalation of dioctyl sodium sulfosuccinate and large tidal volume ventilation.
Measurements and Results: The physiological and histopathological findings indicated a status mimicking an early stage of ALI/ARDS. Automatically, a series of dynamic Pel/V loops from different PEEP levels were recorded with the sinusoidal flow modulation method using a computer-controlled ventilator. During expiration, resistance increased more than twofold. For each step of lower starting pressure, the inspiratory limb was displaced towards lower volume indicating derecruitment. Recruitment occurred between 20 and 40 cm H2O. The expiratory curves, all starting from 50 cm H2O, overlapped. Hysteresis increased significantly in loops recorded from lower PEEP levels. Viscoelasticity explained differences between static and dynamic Pel/V loops.
Conclusions: Automated multiple Pel/V loop determination is feasible and provides comprehensive information on lung derecruitment and recruitment. It requires determination of volume dependence of expiratory resistance. An expiratory curve serves as a reference to inspiratory curves and provides information about hysteresis.