Abstract
BackgroundAlthough PEEP and inversed I:E ratio have been shown to improve gas exchange in ARDS, both can adversely affect systemic hemodynamics and cerebral perfusion. The goal of this study was to assess how changes in PEEP and I:E ratio affect systemic and cerebral oxygenation and perfusion in normal and injured lung.MethodsEight anesthetized Chinchilla-Bastard rabbits were ventilated at baseline with pressure-regulated volume control mode, VT = 6 ml/kg, PEEP = 6 cmH2O, FIO2 = 0.4; respiratory rate set for ETCO2 = 5.5%, and I:E = 1:2, 1:1 or 2:1 in random order. Ultrasonic carotid artery flow (CF), arterial (PaO2), jugular venous blood gases and near infrared spectroscopic cerebral oxygenation (∆HBO2) were recorded for each experimental condition. After induced lung injury, the animals were ventilated with PEEP = 9 followed by 6 cmH2O.ResultsAt baseline, inverse-ratio ventilation (IRV) significantly reduced cerebral oxygenation (∆O2HB; − 27 at 1:2; − 15 at 1:1 vs. 0.27 μmol/L at 2:1; p < 0.05), due to a significant reduction in mean arterial pressure and CF without modifying gas exchange. In injured lung, IRV improved gas exchange but decreased cerebral perfusion without affecting brain oxygenation. The higher PEEP level, however, improved PaO2 (67.5 ± 19.3 vs. 42.2 ± 8.4, p < 0.05), resulting in an improved ∆HBO2 (− 13.8 ± 14.7 vs. –43.5 ± 21.3, p < 0.05), despite a drop in CF.ConclusionsOur data suggest that unlike moderate PEEP, IRV is not effective in improving brain oxygenation in ARDS. In normal lung, IRV had a deleterious effect on brain oxygenation, which is relevant in anesthetized patients.
Highlights
positive end-expiratory pressure (PEEP) and inversed I:E ratio have been shown to improve gas exchange in acute respiratory distress syndrome (ARDS), both can adversely affect systemic hemodynamics and cerebral perfusion
Statistical analysis Data are presented as mean ± standard error (SEM)
We found that the mechanical ventilation strategy in the presence of lung injury significantly impacted brain oxygenation
Summary
PEEP and inversed I:E ratio have been shown to improve gas exchange in ARDS, both can adversely affect systemic hemodynamics and cerebral perfusion. Brain tissue oxygenation is directly affected by the changes in both systemic hemodynamics and blood oxygenation. It closely depends on blood perfusion and cellular metabolism in the brain, which are intricately and continuously. Available data suggest that cerebrovascular autoregulation may be impaired in a significant number of patients with ARDS [16]. It is not well understood how the potentially conflicting effects of positive pressure ventilation settings, namely PEEP and ratio of inspiratory to expiratory time (I:E), on cerebral perfusion and blood oxygenation affect brain tissue oxygenation. The importance of understanding the impact of ventilator settings on brain oxygenation is further underscored by the high prevalence of cognitive impairment in ARDS survivors [17], where both mechanical ventilation [18] and brain tissue hypoxia [19] may play a causative role
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