Background
The aim of this study was to investigate whether two newly developed indices of brain tissue oxygen (PtiO2) pressure reactivity, named ORx and bPtiO2, allow an estimation of the status of cerebrovascular autoregulation after head injury? This was accomplished by validating these new indices against an established parameter for the assessment of autoregulation, the cerebrovascular pressure reactivity index (PRx).
Methods
In 27 patients after severe head injury continuous monitoring of mean arterial blood pressure (MAP), intracranial pressure (ICP), cerebral perfusion pressure (CPP), and partial pressure of brain tissue oxygen (PtiO2) was performed for an average period of 6.5 days. ORx was calculated as a moving correlation coefficient between values of CPP and PtiO2. The bPtiO2 was calculated as a moving value of the slope of the linear regression function between CPP and PtiO2. PRx was calculated as described by Czosnyka et al. as a moving correlation coefficient between values of ICP and MAP. Outcome was assessed at six months after trauma (Glasgow Outcome Scale).
Results
ORx and bPtiO2 correlated significantly with PRx (r = 0. 55 for ORx, r = 0.53 for bPiO2, p<0.01). PRx and ORx showed a significantly negative correlation to the monitored PtiO2 values (r = −0.42 for PRx, r = −0.41 for ORx, p<0.05) and patient outcome (r = −0.52 for PRx, r = −0.62 for ORx, p<0.01), whereas bPtiO2 did not. ICP and CPP showed no correlation to outcome.
Conclusion
The results of this study suggest, that the newly developed indices ORx and bPtiO2, which describe the relationship between changes of CPP and PtiO2, might provide additional information on the status of cerebrovascular autoregulation after TBI, as both correlated well with PRx. The data furthermore indicate that patients with impaired autoregulation are at increased risk for secondary cerebral hypoxia and poor outcome. In contrast, patients with better reactivities presented with better outcome.