Neuromonitoring of patients with severe neurological illness are detailed elsewhere. In the setting of cerebral edema, ICP monitoring is a staple of neurocritical care. Pressure AR is an important hemodynamic mechanism that protects the brain against inappropriate fluctuations in CBF in the face of changing CPP. Both static and dynamic AR have been monitored in neurocritical care to aid prognostication and contribute to individualizing optimal CPP targets in patients. Theoretically, failure of cerebral AR is associated with poor outcomes in various acute neurological diseases. Continuous bedside monitoring of autoregulation is now feasible and should be considered as a part of multimodality monitoring including measurement of pressure reactivity. A previous study documented that REG (REGx) and ICP (PRx) has high correlation in order to detect the lower limit of CBF AR. The fundamental relationships between SAP, vessel tone, cerebral blood volume and ICP form the basis for the pressure reactivity index (PRx). PRx is analogous to other time domain AR indices and is calculated as the continuous correlation between thirty consecutive time-averaged (10 s) SAP and ICP values. A positive index (positive correlation) implies impaired passive CBF AR, while a negative index (inverse correlation) implies intact, active AR. The utility and feasibility of REG as a monitoring modality is previously demonstrated and validated as a reflection of cerebrovascular reactivity. The bioimpedance amplifier was used previously at Walter Reed Army Institute of Research (WRAIR) and Naval Medical Research Center (Silver Spring, MD); and has an FDA safety clearance. It is expected that REG can predict evolving vasospasm and expanding intracranial bleeding amongst several other clinical applications.
