AN IMPEDANCE INDEX IN NORMAL SUBJECTS AND IN SUBARACHNOID HEMORRHAGE

The impedance of a hemodynamic system is defined as the ratio of each harmonic component of blood pressure to that of flow, Calculation of i mpedance curves has been extensively performed in the systemic circula tion, leading to the recognition of reflected pressure and flow waves and clarifying the shape of ultrasound waveforms, Impedance in the hum an cerebral circulation has not been measured primarily because of the relative inaccessibility of simultaneous flow and pressure data in th e human cerebral circulation, We defined an impedance index using the transcranial Doppler waveform for that of flow and a noninvasive appla nation measure of the carotid artery pressure waveform, Middle cerebra l artery velocities and carotid artery pressure waveforms were simulta neously recorded in 16 vessels from 10 normal volunteers, 42 vessels i n 14 patients with aneurysmal subarachnoid hemorrhage, and 14 vessels in 7 subjects during conditions of hypocapnia, normocapnia and hyperca pnia, Impedance was calculated by dividing the harmonic associated wit h pressure divided by that of flow, and averaging 10 to 20 such calcul ations, Relative impedance curves were calculated by dividing by the i mpedance at the first harmonic, Impedance was also studied in an elect rical model consisting of a Windkessel element containing inductance i n series with a second Windkessel to model the large vessel and vascul ar bed, respectively, Model parameters were taken from the literature for these calculations, For the normal subjects, the shape of the impe dance index curve was similar to those found in the systemic circulati on, The impedance index curves for patients in vasospasm (middle cereb ral velocity greater than 180) showed a peak at the second or third ha rmonic, which appeared more frequently than the nonspasm group (p < 0. 01), Furthermore, the ratio of the second harmonic to the first harmon ic was significantly > 1.0 in the spasm group but significantly < 1.0 in the normal group (p < 0.05), Calculations from the electrical model replicated the appearance of these peaks at the second or third harmo nic for vasospasm parameters, A statistically significant peak appeare d at the second or third harmonic in the impedance index curves for pa tients in vasospasm, which was replicated quantitatively by our electr ical model, Although such peaks can be explained in the systemic circu lation by the presence of reflected waves, the distance to the reflect ion site is larger than expected for the cerebral circulation, This su ggests the importance of the inertia of blood as a stenosis worsens an d as the origin for the observed changes in the impedance index curves .