• Morten Andresen, Joseph Donnelly, Marcel Aries, Marianne Juhler, David Menon, Pja Hutchinson, and Peter Smielewski.
• Division of Neurosurgery, Department of Clinical Neurosciences, Cambridge University Hospital, Cambridge, UK. andresen@gmail.com.
• Neurocrit Care. 2018 Apr 1; 28 (2): 162-168.

BackgroundContinuous monitoring of cerebral autoregulation is considered clinically useful due to its ability to warn against brain ischemic insults, which may translate to a relationship with adverse outcome. It is typically performed using the pressure reactivity index (PRx) based on mean arterial pressure and intracranial pressure. A new ORx index based on brain tissue oxygenation and cerebral perfusion pressure (CPP) has been proposed that similarly allows for evaluation of cerebrovascular reactivity. Conflicting results exist concerning its clinical utility.MethodsRetrospective analysis was performed in 85 patients with traumatic brain injury (TBI). ORx was calculated using three time windows of 5, 20, and 60 min. Correlation coefficients and individual "optimal CPP" (CPPopt) were calculated using both PRx and ORx, and relation to patient outcome investigated.ResultsCorrelation coefficients for all comparisons between PRx and ORx indicated poor association between these indices (range from -0.04 to 0.07). PRx was significantly lower in patients with good outcome (p = 0.01), while none of the ORx indices proved to be significantly different in the two outcome groups. Higher mortality related to average CPP < CPPopt was found regardless of which index was used to calculate CPPopt.ConclusionIn the TBI setting, ORx does not appear to correlate with vascular pressure reactivity as assessed with PRx. Its potential use for individualizing CPP thresholds remains unclear.

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