Hannes Widing: The open lung in assisted spontaneous breathing

Abstract

Background: Mechanical ventilation is essential for managing acute respiratory distress syndrome (ARDS). The implementation of assisted spontaneous breathing may improve diaphragm function and blood oxygenation. However, the optimal protective settings for assisted spontaneous breathing and positive end-expiratory pressure (PEEP) remain uncertain. During the weaning phase from ventilation, assisted spontaneous breathing is commonly used; however, the relationship between applied PEEP levels, transpulmonary pressure, and atelectasis formation during this process is not well understood. We hypothesised that 1) higher PEEP levels may mitigate injurious lung phenomena and 2) atelectasis formation is linearly correlated with transpulmonary pressure during ventilator weaning. 

Method: The effects of six different PEEP levels on atelectasis, tidal recruitment–derecruitment (R/D), lung stress and tidal volume distribution were tested in two studies involving experimental ARDS (Studies I and II). Furthermore, the effects of PEEP on inspiratory efforts and the effort-to-drive ratio were analysed retrospectively (Study III). The correlation between atelectasis volume and transpulmonary pressure was assessed during a ramp procedure in patients weaning from mechanical ventilation (Study IV). 

Results: In experimental ARDS, lung weight affected by atelectasis and tidal R/D decreased significantly with increasing PEEP levels (PEEP 0 vs 15 cmH2O, p < 0.01). Higher PEEP levels shifted the tidal volume distribution dorsally. The oesophageal pressure swing and the effort-to-drive ratio showed a significant association with the applied PEEP level (p < 0.01). Shifting PEEP from 0 to 15 cmH2O resulted in a decrease in the mean (SD) oesophageal pressure swing from -4.2 (3.1) to -1.9 (1.5) cmH2O (p < 0.01). In humans, the end-expiratory volume of atelectasis decreased significantly with reduced positive pressures (p < 0.05) and was linearly correlated with end-expiratory transpulmonary pressure (p < 0.05).

Conclusion: Higher PEEP levels limit atelectasis and tidal recruitment–derecruitment. PEEP improves ventilation homogeneity by distributing tidal volume dorsally and modulates large inspiratory efforts through the effort-to-drive ratio, potentially protecting against further lung injury. These results were found in experimental ARDS using NAVA ventilation. Further studies are required to determine their applicability in human ARDS. Additionally, the volume of end-expiratory atelectasis is linearly correlated with end-expiratory transpulmonary pressure during a ramp procedure in humans weaning from mechanical ventilation.