Abstract
BackgroundPotentially harmful lung overstretch can follow intraparenchymal gas redistribution during mechanical ventilation. We hypothesized that inspiratory efforts characterizing spontaneous breathing, positive end-expiratory pressure (PEEP), and high inspiratory resistances influence inspiratory intraparenchymal gas redistribution.MethodsThis was an experimental study conducted on a swine model of mild acute respiratory distress syndrome. Dynamic computed tomography and respiratory mechanics were simultaneously acquired at different PEEP levels and external resistances, during both spontaneous breathing and controlled mechanical ventilation. Images were collected at two cranial–caudal levels. Delta-volume images (ΔVOLs) were obtained subtracting pairs of consecutive inspiratory images. The first three ΔVOLs, acquired for each analyzed breath, were used for the analysis of inspiratory pendelluft defined as intraparenchymal gas redistribution before the start of inspiratory flow at the airway opening. The following ΔVOLs were used for the analysis of gas redistribution during ongoing inspiratory flow at the airway opening.ResultsDuring the first flow-independent phase of inspiration, the pendelluft of gas was observed only during spontaneous breathing and along the cranial-to-caudal and nondependent-to-dependent directions. The pendelluft was reduced by high PEEP (p < 0.04 comparing PEEP 15 and PEEP 0 cm H2O) and low external resistances (p < 0.04 comparing high and low external resistance). During the flow-dependent phase of inspiration, two patterns were identified: (1) gas displacing characterized by large gas redistribution areas; (2) gas scattering characterized by small, numerous areas of gas redistribution. Gas displacing was observed at low PEEP, high external resistances, and it characterized controlled mechanical ventilation (p < 0.01, comparing high and low PEEP during controlled mechanical ventilation).ConclusionsLow PEEP and high external resistances favored inspiratory pendelluft. During the flow-dependent phase of the inspiration, controlled mechanical ventilation and low PEEP and high external resistances favored larger phenomena of intraparenchymal gas redistribution (gas displacing) endangering lung stability.
Highlights
The intraparenchymal gas redistribution occurring in absence of flow at the airway opening has been defined for the first time in 1956 and named pendelluft (Otis et al, 1956)
In the current experimental study, we investigated if spontaneous inspiratory efforts, positive endexpiratory pressure (PEEP), and external resistances influenced onset and magnitude of intraparenchymal gas redistribution: (a) during the first isovolumetric, flow-independent phase of the inspiration defined as pendelluft (Yoshida et al, 2013); (b) during ongoing inspiratory flow at the airway opening
There were no differences in transpulmonary pressure between spontaneous breathing and controlled mechanical ventilation
Summary
The intraparenchymal gas redistribution occurring in absence of flow at the airway opening has been defined for the first time in 1956 and named pendelluft (Otis et al, 1956). The magnitude of this phenomenon is closely related to the heterogeneity of lung injury and to the ventilatory settings. Recent studies show harmful inspiratory pendelluft following high inspiratory efforts when spontaneous breathing is allowed in mechanically ventilated subjects with acute respiratory distress syndrome (ARDS) (Yoshida et al, 2013; Morais et al, 2018). We hypothesized that inspiratory efforts characterizing spontaneous breathing, positive end-expiratory pressure (PEEP), and high inspiratory resistances influence inspiratory intraparenchymal gas redistribution
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