Brain tissue oxygenation: more than a number

Introduction

The study objective was to analyse what kind of dynamic interrelations exist between brain tissue oxygenation (PbtO₂) and corresponding fast modifications of arterial blood pressure (ABP), cerebral perfusion pressure (CPP) and intracranial pressure (ICP) in transient events.

Methods

We reviewed retrospectively 325 computer recordings of PbtO₂, invasive ABP and ICP waveforms from 23 head-injured patients. All patients were sedated, paralysed and ventilated. All signals were digitised, and recorded using ICM+ software. We divided the events into two groups, depending on whether ABP (Group 1) or ICP (Group 2) was the first parameter to change. Group 1 was further subdivided based on whether the vascular autoregulation was intact (ABP-ICP negative correlation) or was impaired (ABP-ICP positive correlation).

Results

Group 1 (n = 255): intact cerebral autoregulation (n = 179): during hypotension PbtO₂ decreased with delay with respect to CPP (48.5 s; SEM 92.1) and ICP (39.9 s; SEM 91.4), and during hypertension PbtO₂ increased with a delay of 58.1 seconds (71.9 SEM) with respect to CPP and 52.2 seconds (72.2 SEM) with respect to ICP; impaired cerebral autoregulation (n = 76): PbtO₂ modified following ABP changes, with a delay of 56.8 seconds (SEM 59.3) with respect to CPP and 54.2 seconds (58.8 SEM) with respect to ICP. Group 2 (n = 61): plateau waves and isolated gradual increases in ICP caused CPP to lower, followed by a PbtO₂ decrease. The delay in PbtO₂ reaction was 23.1 seconds (55.7 SEM, n = 23) with respect to ICP and 18.4 seconds (54.9 SEM, n = 24) to CPP.

Conclusion

Transient events were observed in PbtO₂ related to ABP or ICP modifications. Changes in PbtO₂ were present irrespective of the state of autoregulation or the origin of the event (haemodynamic or ICP related). Generally PbtO₂ followed the CPP direction. PbtO₂ usually changed with a delay relative to the pressure parameters. The CPP-PbtO₂ delay was significantly shorter in the events characterized by primary ICP modification (Group 2) in comparison with the ABP-led events (Group 1), irrespective of the state of autoregulation. These findings should be taken into account to evaluate the validity of indices assessing cerebral autoregulation using PbtO₂.