Validation studies on a noninvasive neuromonitoring method, rheoencephalography - A review.

The loss of cerebral blood flow autoregulation is experienced after severe neurological injuries. Its impairment leads to a cascade of secondary neurological injuries that often follow the primary neurological insult. However, although its maintenance is important, cerebral autoregulation monitoring remains and does not follow consistent clinical standards. The traditional neurocritical monitoring method is invasive intracranial pressure monitoring. Several methods have also been tested for noninvasive neuromonitoring. Rheoencephalography is a form of non-invasive monitoring based on bioimpedance measurement; however, it is not used in routine clinical practice today. This review presents selected results of in-vitro and in-vivo studies evaluating the rheoencephalogram pulse waveform quantification during cerebral blood flow alterations, correlations with the intracranial pressure and other cerebral blood flow measuring modalities, and calculation of the rheoencephalogram-based autoregulation index. A total number of 1027 human and 638 animal measurements were involved. An in-vitro study documented the correlation between Doppler ultrasound flow and bioimpedance. In-vivo animal and human studies established a correlation with other modalities such as laser Doppler flow, carotid flow, near-infrared spectroscopy, and intracranial pressure as well as invasive and noninvasive cerebral blood flow autoregulation indexes. Studies documented that rheoencephalogram reflects cerebral volume change, cerebrovascular reactivity, intracranial pressure, and cerebral blood flow autoregulation. Human measurements documented that rheoencephalogram pulse wave morphology changes (peak 2) and that cerebral blood flow autoregulation's active/ passive status and peak 2 increases during the Trendelenburg position. These studies support the idea that rheoencephalography can be considered as a future non-invasive neuromonitoring modality.