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Cerebral CO2-reactivity was tested by transcranial Doppler sonography (Doppler CO2 test) in 232 patients. Time averaged flow velocity in the middle cerebral artery at the 40 mm Hg blood pCO2 level was taken as a reference point, and the relative increase of flow in hypercapnia of 46.5 mm Hg pCO2 was defined as "Normalized Autoregulatory Response" (NAR). A total of 82 patients with no evidence of cerebrovascular disease gave "normal" values for NAR (23.2 +/- 5.2 SD). In 150 patients with 233 stenoses and occlusions of the internal carotid artery NAR was significantly decreased in higher-grade stenoses (P = 0.01 for 80% diameter reduction, P less than 10(-6) for 90% or more). In such stenoses, patients with NAR less than 14 had suffered more frequently (P less than 0.01) from ipsilateral transient ischemic attacks and/or stroke during the previous 6 months than patients with "normal" NAR. Preoperative NAR less than 14 always improved to "normal" values following carotid surgery, while preoperative NAR greater than 19 remained unchanged (60 cases). The transcranial Doppler CO2 test is thought to be a reliable noninvasive method to detect hemodynamically critical carotid stenoses and occlusions. This may be of interest in selecting patients for superficial temporal artery-middle cerebral artery bypass and carotid surgery. For practical use 4 categories of NAR are suggested.

In patients with an internal carotid artery (ICA) occlusion the CO2 reactivity (autoregulatory reserve) is supposed to give information about the function of the collateral supply. To prove this hypothesis we compared the CO2 reactivity measured by transcranial Doppler sonography to the ipsilateral clinical symptoms and the patterns of infarction in cranial computed tomography (CCT). We studied 251 cases of ICA occlusion. Of the 141 cases with normal autoregulatory reserve, 37 (27%) had recently developed an ipsilateral neurological deficit. Of the 44 cases with exhausted CO2 reactivity, 28 (64%) had experienced an event. The difference is highly significant (p less than 0.0001). In 59 of the 75 patients for whom CCT images were available, we found signs of vascular-ischemic lesions. Of the 30 patients with hemodynamic infarctions, 13 showed an exhausted autoregulatory reserve, while of 19 cases with territorial infarctions only 1 and of 10 with lacunar infarctions none had an exhausted CO2 reactivity. The difference is significant (p less than 0.01).

Comparisons with intraoperative findings suggest that Duplex scanning may be of value in predicting the morphology of carotid artery stenoses. Such studies, however, are based on the results obtained by experienced investigators. To clarify whether sonographic criteria can be standardized, 6 investigators from different hospitals each documented 30 carotid artery stenoses of greater than or equal to 40% diameter reduction which could be imaged in at least 2 planes. Three sonographic criteria (plaque surface, echo density, echo structure), each with 8 categories, were assessed from the image documentations by the 6 as well as by 2 independent further investigators. Depending on the experience and on the device used 29-81% of all stenoses greater than or equal to 40% examined in the different laboratories could be included in the study. The +/- 1 category concordance between the different investigators averaged 50-60% for all sonographic criteria independent from the degree of stenosis and from the image quality. Because of lack of sufficient reproducibility the subjective assessment of sonographic criteria is found to be not suitable for use in multicentre studies.

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