Intracranial pressure monitoring: relationship between indices of cerebrovascular reserve, system bandwidth, and cerebral perfusion

Loss of cerebrovascular reserve with the resultant increase of cerebral perfusion, and disruption of capillary fluid balance generally leads to the development of cerebral edema, a serious secondary complication of traumatic brain injury. Two indices of cerebrovascular reserve derived from recordings of intracranial pressure (ICP) and arterial blood pressure (ABP) have been proposed. The purpose of this study was to examine the relationship between changes of arteriolar resistance, cerebral perfusion, system bandwidth, and the indices of cerebrovascular reserve during dilatory challenge induced by ventilation with CO₂. Steady state values of arteriolar diameter significantly increased during hypercapnia resulting in a decrease of arteriolar resistance by more than 90%. Cerebral perfusion pressure (CPP) was found to range from -18% to 12%. Significant correlation between percent change of relative flow and either indices of cerebrovascular reserve, arteriolar resistance, or system bandwidth were not determined. System bandwidth between ABP and ICP and the Correlation Index of cerebrovascular reserve (CorrX), were found to be exponentially correlated with values ranging from 0.85 to 0.99 with a group (n=5) mean (+S.D.) of 0.94 (+0.06). Such a relationship suggests that near minimal tone, further losses of tone lead to changes of stiffness that produce large changes in system bandwidth.