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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.

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Radolovich, D., Timofeev, I., Lavinio, A. et al. Brain tissue oxygenation: more than a number. Crit Care 12, P109 (2008). https://doi.org/10.1186/cc6330

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Keywords

  • Arterial Blood Pressure
  • Intracranial Pressure
  • Study Objective
  • Cerebral Perfusion Pressure
  • Transient Event