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Background: Amplitude responsiveness of cerebral blood flow velocity (CBFV) to CO2 is an established test of cerebrovascular reserve capacity. We aimed to analyze the time course of CBFV during CO2 reactivity testing in patients with severe carotid stenosis. Methods: 87 patients were assigned to three different groups according to the degree of stenosis (A 75–89%, B 90–94%, C 95–100%) and CBFV dynamics was assessed by transcranial Doppler sonography during hypercapnia induced by inhalation of 7% CO2. Time constants for the duration of CBFV and the end-tidal CO2 partial pressure (PetCO2) increase and decrease were calculated. In addition, time delays between subsequent changes of PetCO2 and CBFV during on- and offset of hypercapnia were determined. Results: There was a significant reduction of conventional CO2 reactivity on the side ipsilateral to the stenosis in all groups. Time constants of CBFV showed no major differences between ipsi- and contralateral sides or between different groups. The off delay between the decrease in PetCO2 and the consequent decrease in CBFV was the only out of 6 different dynamic parameters which significantly differentiated critical stenosis and occlusion from severe stenosis. Retest variability showed a highly significant correlation. Conclusions: Analysis of the dynamic CBFV response during the applied CO2 inhalation protocol revealed a significant and reproducible delay parameter which has power to detect cerebral hemodynamic compromise in patients with carotid artery stenosis similar to the conventional CO2 reactivity parameter.

Background: The anterior communicating artery is a key collateral pathway in patients with internal carotid artery (ICA) occlusions. Diagnostic balloon or manual occlusion of the ICA is a qualified test for the collateral function of the circle of Willis. The aim of our study was to test whether transcranial Doppler sonography (TCD) with manual carotid compression is able to predict the degree of cross-over flow and tolerance of ICA occlusion. Methods: A prospective study was performed on 88 patients suffering from neck tumors or inoperable ICA aneurysms with planned ligation or permanent balloon occlusion of the ICA. Preceding a balloon occlusion lasting 20 min we performed TCD with manual carotid compression test. Insufficient cross-over flow during transient carotid occlusion was defined angiographically as missing opacification of the middle cerebral artery (MCA) after selective injection of contrast media in the contralateral ICA, and sonographically as a decrease greater than 60% in flow velocity of the MCA ipsilateral to the carotid occlusion. Results: TCD and angiography could be compared in 70 of 88 patients. Angiography showed no or insufficient collateral flow in 26%. All patients (n = 10) with a decrease in MCA velocity greater than 60% on angiography showed missing or poor cross-over flow. A greater than 50% increase in the anterior cerebral artery (ACA) predicted moderate or good cross-over flow in 98%. Clinical observation revealed 11 transient neurological deficits. Eight of them showed insufficient cross-over flow angiographically and sonographically. In 2 of 11 only TCD revealed such low collateral supply. Neither technique predicted hemodynamic ischemia in only 1 of the 11. Conclusion: TCD with carotid compression test is a valid method for predicting interhemispheric flow and clinically relevant hemodynamic compromise in cases of occlusion of the ICA.