Time delay as a parameter for cerebrovascular reactivity in patients with severe carotid stenosis

Amplitude responsiveness of cerebral blood flow velocity (CBFV) to CO(2) is an established test of cerebrovascular reserve capacity. We aimed to analyze the time course of CBFV during CO(2 )reactivity testing in patients with severe carotid stenosis.

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% CO(2. )Time constants for the duration of CBFV and the end-tidal CO(2 )partial pressure (Pet(CO2)) increase and decrease were calculated. In addition, time delays between subsequent changes of Pet(CO2) and CBFV during on- and offset of hypercapnia were determined.

There was a significant reduction of conventional CO(2) 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 Pet(CO2) 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.

Analysis of the dynamic CBFV response during the applied CO(2) 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 CO(2 )reactivity parameter.