Abstract
A close monitoring of the cerebrovascular parameters is essential in some neurological diseases to prevent secondary brain insults. Persistent rises in intracranial pressure caused by oedema, tumours, or haematomas may decrease the cerebral blood flow (CBF) to values below the minimum required for neuronal survival (Lang & Chesnut, 1994). As a method to evaluate CBF, Polzer and Schuhfried proposed in 1950 to apply the well-known impedance plethysmography techniques to the head, which was specifically referred to as Rheoencephalography (REG) (Polzer & Schuhfried, 1950). For this purpose, bipolar and tetrapolar electrode arrangements, named as REG I and REG II respectively, were used to measure the impedance changes of the head synchronized to the heartbeat. In REG I, a low-amplitude current is injected through two electrodes attached to the scalp, and the related electric potential difference is measured between these electrodes, whereas in REG II, a second pair of electrodes is used to measure the electric potential difference (Geddes & Baker, 1989). The latter technique reduces eventual artifacts caused by mechanical alterations in the electrode-skin interface of the current injection electrodes.
Key Terms in this Chapter
Rheoencephalography: Electromedical technique used to assess the cerebral blood flow (CBF) by noninvasive electrical impedance methods using electrodes attached to the scalp surface.
Electrical Impedance: Measure of the opposition to the flow of a sinusoidal alternating electric current.
Blind Source Separation: A part of Signal Processing that studies algorithms to extract a set of unknown signal sources from the analysis of a set of observations, each one of which is a mixture in unknown proportions of the unknown signal sources.
Intracranial REG Component: Part of a REG signal that is caused by the pulsatility character of the blood flow in the intracranial tissues.
Bipolar Rheoencephalogram (REG I): Pulsatile impedance changes of the head related to the cardiac cycle that are measured by injecting a small current between two electrodes attached to the scalp surface and recording the electric potential difference generated between them.
Photoplethysmography: Technique to estimate the pulsatile changes of the blood volume in a tissue by means of the measurement of the light reflected on or transmitted through that tissue.
Tetrapolar Rheoencephalogram (REG II): Pulsatile impedance changes of the head related to the cardiac cycle that are measured by injecting a small current between two electrodes attached to the scalp surface and taking the electric potential difference between another pair of electrodes.
Extracranial REG Component: Part of a REG signal that is caused by the pulsatility character of the blood flow in the extracranial tissues.