[Information content of the rheoencephalogram and rheoencephaloplethysmogram for assessing cerebral hemodynamics]

In acute experiments in cats, separate measurement of impedance of extra and intracranial tissues and skull bones was carried out by passing 30-150 kHz electrical current. Extra and intracranaial tissue impedances are approximately equal and one order less than that of skull bones which excludes electrical current penetration into the cranial cavity directly through the bone. The changes on electroplethysmograms recorded with extra and intracranial electrodes during simultaneous clipping of both common carotids or external and internal jugular veins were compared. In part of experiments jugular veins were clipped before and after switching off extracranial blood flow. Intracranial impedance changes thus achieved are several times greater than extracranial ones and are not registered in extracranial bitemporal electrode location. The only way to register them without intracranial electrode insertion is by use of ocular-occipital lead. Yet in this case the intracranial component makes only about 30% of the registered signal. This value increases significantly with artifical stabilization of extracranial hemodynamics.