1. Transcranial flow velocity waves were measured via Doppler sonography of the middle cerebral artery during hypo-, hyper- and normo-capnia. Applying the principle of vascular impedance, flow velocity waves were analysed in 30 young subjects, 37 elderly subjects and 18 patients with high-grade unilateral internal carotid artery disease.

2. There was evidence that the relative peak-to-peak velocity in the middle cerebral artery could serve as an index of peripheral wave reflection and cerebral resistance (CRi). The response of CRi to changes in arterial CO2 concentration (CRi reactivity) showed a clear age-dependency. However, the absolute side-to-side asymmetry of CRi reactivity (ΔR) did not vary with age and could be used to define a normal range (0–4%CRi/vol.%CO2).

3. Selective angiography demonstrated no cerebral cross-flow through the anterior part of the circle of Willis in nine patients with carotid artery stenosis whose absolute ΔR was above the normal range and whose CRi reactivity of the affected hemisphere was lower than that of the healthy opposite hemisphere. Conversely, another group of nine patients, whose ipsilateral CRi reactivity was higher than the contralateral CRi reactivity, demonstrated cross-flow through the anterior part of the circle of Willis.

4. ΔR may be used to identify patients who have high-grade internal carotid artery stenosis and present with low cerebral vascular resistance owing to poor intracerebral collaterals.

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