Changes in the Intracranial Rheoencephalogram at Lower Limit of Cerebral Blood Flow Autoregulation

Cerebral blood flow (CBF) reactivity monitoring is an appropriate primary parameter to evaluate cerebral resuscitation due to a systemic or regional cerebral injury leading to possible irreversible brain injury. Use of the electrical impedance method to estimate CBF is rare, as the method's anatomical background is not well understood. Use of intracranial rheoencephalography (iREG) during hemorrhage and comparison of iREG to other CBF measurements have not been previously reported. Our hypothesis was that iREG would reflect early cerebrovascular alteration (CBF autoregulation). Studies comparing iREG, laser Doppler flowmetry and ultrasound were undertaken on anesthetized rats to define CBF changes during hemorrhage. Blood was removed at a rate required to achieve a mean arterial blood pressure (MABP) of 40 mm Hg over 15 min. Estimation of CBF was taken with intracranial, bipolar REG (REG I; n = 14), laser Doppler flowmetry (LDF; n = 3) and carotid flow by ultrasound (n = 11). Data were processed off-line. During the initial phase of hemorrhage, when MABP was close to 40 mm Hg, intracranial REG amplitude transiently increased (80.94%); LDF (77.92%) and carotid flow (52.04%) decreased and changed with systemic arterial pressure. Intracranial REG amplitude change suggests classical CBF autoregulation, demonstrating its close relationship to arteriolar changes. The studies indicate that iREG might reflect cerebrovascular responses more accurately than changes in local CBF measured by LDF and carotid flow. REG may indicate promise as a continuous, non-invasive life-sign monitoring tool with potential advantages over ultrasound, the CBF measurement technique normally applied in clinical practice. REG has particular advantages in non-hospital settings such as military and emergency medicine.