An impedance index in normal subjects and in subarachnoid hemorrhage

An impedance index is defined using the transcranial Doppler waveform for that of flow and a noninvasive applanation measure of the carotic artery pressure waveform. Middle cerebral artery velocities and carotid artery pressure waveforms are simultaneously recorded in 16 vessels from 10 normal volunteers, 42 vessels in 14 patients with aneurysmal subarachnoid hemorrhage, and 14 vessels in 7 subjects during conditions of hypocapnia, normocapnia and hypercapnia. Impedance is calculated by dividing the harmonic associated with pressure divided by that of flow, and averaging 10 to 20 such calculations. Relative impedance curves are calculated by dividing the impedance at the first harmonic. Impedance is also studied in an electrical model consisting of a Windkessel element containing inductance in series with a second Windkessel to model large vessel and vascular bed, respectively.