Cerebrospinal compensation of pulsating cerebral blood volume in hydrocephalus.

OBJECTIVE The aim of the study was to develop a computational method for the assessment of brain pressure compensation of cerebrospinal arterial blood inflow. The method was verified using clinical recordings performed during infusion studies in a group of patients diagnosed with hydrocephalus. MATERIALS AND METHODS We studied 27 patients suspected of having normal pressure hydrocephalus. The infusion test was used to measure the resistance to cerebrospinal fluid outflow, and the elastance coefficient was performed together with recording of the blood flow velocity in the middle cerebral artery. From the blood flow velocity waveform, the pulsatile pattern of increasing cerebral blood volume during one heart cycle was evaluated as a time integral of the arterial blood flow velocity minus the mean arterial blood flow. Cerebrospinal 'compliance index' (C(i)) was calculated as the amplitude of change in blood volume divided by the amplitude of intracranial pressure pulse waveform. RESULTS Compliance index C(i) decreased during the infusion test, proportionally inverse to the rise in intracranial pressure controlled by the external infusion of saline (R=-0.76; p<0.005). A relative change in compliance (from baseline to the plateau phase of the study) was positively associated with greater brain elasticity (R=0.61; p<0.005) and poorer compensatory reserve at the phase of infusion (R=0.51; p=0.009) CONCLUSION: C(i) decreases during the infusion study and seems to well replicate the relative changes in cerebrospinal compensatory reserve in hydrocephalus.