Significance of transcranial Doppler CO2. Reactivity measurements for the diagnosis of hemodynamically relevant carotid obstructions.

Transcranial Doppler ultrasonography can be used to determine CO2 reactivity in the large basal cerebral arteries. CO2 reactivity is expressed as percentage increase of mean flow velocity above one volume percent of CO2, using a reference value of 40 mmHg pCO2 normalized autoregulatory reserve. A normalized autoregulatory value of 15 as the lower limit of the normal range clearly separates patients with internal carotid artery obstructions greater than or equal to 70% from the control group. Stenoses of the ipsilateral internal carotid artery of greater than or equal to 70% result in a significant decrease of normalized autoregulatory reserve in the middle cerebral artery, which can be normalized by removal of the upstream flow obstacle using a carotid thromboendarterectomy. The scatter of normalized autoregulatory values in severe internal carotid obstructions indicates the variability of collateral circulation A retrospective comparison of normalized autoregulatory reserve and ipsilateral ischemic symptoms in the supply area of the internal carotid artery reveals a significant correlation between clinical symptoms and reduced normalized autoregulatory reserve.