Reactivity of brain tissue oxygen to change in cerebral perfusion pressure in head injured patients

It has been reported recently that correlation between brain tissue oxygen (PbtO2) and cerebral perfusion pressure (CPP) may serve as an indicator of cerebral autoregulation after subarachnoid hemorrhage. We aimed to compare similar indices describing interaction between changes in intracranial pressure (ICP), arterial blood pressure (ABP), and brain tissue oxygen to verify their clinical utility in patients after traumatic brain injury.

Retrospective analysis of multimodal monitoring of 32 patients suffering from head injury, admitted in the Neurosciences Critical Care Unit, Addenbrooke's Hospital, Cambridge, UK. Initial 24 h intervals of continuous ABP, ICP, and PbtO2 recordings were analyzed. Index of tissue oxygen reactivity ORx was evaluated as the correlation coefficient between PbtO2 and CPP over a period of 60 min and compared to the index of pressure reactivity PRx. "Optimal CPP" and a hypothetical "optimal PbtO2" were defined as the ranges of CPP and PbtO2 at which PRx or ORx were indicating best cerebrovascular milieu.

PRx and ORx mean values did not show any correlation with each other (R = 0.012; P = 0.95) between patients. There was also no correlation between ORx and PbtO2 (R = 0.098; P = 0.61) and between PRx and PbtO2 (R = 0.019; P = 0.923). No clear and consistent value of "optimal CPP" minimizing ORx or of hypothetical "optimal PbtO2" were found analyzing PbtO2 or ORx trend over the 24 h of monitoring. However, in most of patients 'optimal CPP' has been found for PRx index. The same has been confirmed when the data from whole monitoring period were analyzed. There was no correlation between values of 'optimal CPP' assessed using ORx and 'optimal CPP' assessed with PRx.

The relationships between PbtO2, ORx, and CPP in head injury appear less useful than reported before for patients after subarachnoid hemorrhage.