RHEOENCEPHALOGRAPHY

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Abstract
Rheoencephalo-graphy is the term applied to the study of the changing electrical impedance of the head which results from the pulsatile flow of blood. An impedance change occurs because blood is a conductor having different properties than the average of the other tissues composing the head. The rheoencephalogram is the analog recording of the impedance change and resembles in appearance a pressure-pulse or volume-pulse curve. Rheoencephalography is a painless, non-dangerous, relatively simple technique with the potential of yielding information about intra-cerebral hemodynamics. Contributing to the R. E. G. curve is the circulatory inflow from the internal carotid, vertebral and external carotid arteries. The degree of contribution from the blood flow internal to the skull, as opposed to that from the blood flow external to the skull, is somewhat obscure. Indications are, however, that the principal contribution is from the intra-cerebral blood flow. There are a number of factors which are of importance in recording the rheoencephalogram. Some are purely electronic problems, such as the inherent difficulty with noise resulting from the low signal level. Other physical factors affect the nature of information derived: whether bipolar or tetrapolar detection is used; variations in the size and position of electrodes; and the frequency of the measuring current which traverses the head. Variation in individual physiological factors will also effect the R. E. G. Body position and relative head position cause curve variations, as does anything which effects the cardio-vascular system.