Monitoring cerebrovascular pressure reactivity with rheoencephalography

Determining optimal perfusion pressure for patients with traumatic brain injury can be accomplished by monitoring the pressure reactivity index, or PRx, which requires an intracranial pressure monitor. We hypothesized that pressure reactivity could be quantified using a rheoencephalography index, or REGx. We measured the REGx and PRx as repetitive, low-frequency linear correlation between arterial blood pressure and intracranial pressure (PRx) or arterial blood pressure and REG pulse amplitude (REGx) in a piglet model of progressive hypotension. We compared the PRx and REGx against a gold standard determination of the lower limit of autoregulation using laser-Doppler measurements of cortical red cell flux. The PRx produced an accurate metric of vascular reactivity in this cohort, with area under the receiver-operator characteristic curves of 0.91. REGx was moderately correlated to the PRx, (Spearman r = 0.63, p < 0.0001; Bland-Altman bias-0.13). The area under the receiver-operator curve for the REGx was 0.86. Disagreement occurred at extremes of hypotension.