The effect of increased positive end expiratory pressure on brain tissue oxygenation and intracranial pressure in acute brain injury patients.

Cerebral hypoxia is an important cause of secondary brain injury. Improving systemic oxygenation may increase brain tissue oxygenation (PbtO ₂ ). The effects of increased positive end-expiratory pressure (PEEP) on PbtO ₂ and intracranial pressure (ICP) needs to be further elucidated. This is a single center retrospective cohort study (2016-2021) conducted in a 34-bed Department of Intensive Care unit. All patients with acute brain injury under mechanical ventilation who were monitored with intracranial pressure and brain tissue oxygenation (PbtO ₂ ) catheters and underwent at least one PEEP increment were included in the study. Primary outcome was the rate of PbtO ₂ responders (increase in PbtO ₂  > 20% of baseline) after PEEP increase. ΔPEEP was defined as the difference between PEEP at 1 h and PEEP at baseline; similarly ΔPbtO ₂ was defined as the difference between PbtO ₂ at 1 h after PEEP incrementation and PbtO ₂ at baseline. We included 112 patients who underwent 295 episodes of PEEP increase. Overall, the median PEEP increased form 6 (IQR 5-8) to 10 (IQR 8-12) cmH ₂ O (p = 0.001), the median PbtO ₂ increased from 21 (IQR 16-29) mmHg to 23 (IQR 18-30) mmHg (p = 0.001), while ICP remained unchanged [from 12 (7-18) mmHg to 12 (7-17) mmHg; p = 0.42]. Of 163 episode of PEEP increments with concomitant PbtO ₂ monitoring, 34 (21%) were PbtO ₂ responders. A lower baseline PbtO ₂ (OR 0.83 [0.73-0.96)]) was associated with the probability of being responder. ICP increased in 142/295 episodes of PEEP increments (58%); no baseline variable was able to identify this response. In PbtO ₂ responders there was a moderate positive correlation between ΔPbtO ₂ and ΔPEEP (r = 0.459 [95% CI 0.133-0.696]. The response in PbtO ₂ and ICP to PEEP elevations in brain injury patients is highly variable. Lower PbtO ₂ values at baseline could predict a significant increase in brain oxygenation after PEEP increase.
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