Quantitative assessment of cerebral vascular reserve by means of transcranial Doppler ultrasound and rebreathing maneuver: bedside test and mathematical modeling.

Cerebral vascular reserve was measured by means of the transcranial Doppler (TCD) technique from carbon dioxide (CO2) tests, in healthy individuals and patients with unilateral internal carotid artery (ICA) occlusion. The percentage changes in middle cerebral arteries blood flow velocity (VMCA) per mmHg of pCO2 variations (reactivity index, RI) were separately computed during hypocapnia and hypercapnia, the latter obtained by a rebreathing maneuver. Clinical data have been compared with predictions obtained using an original mathematical model of intracranial dynamics, in order to search a theoretical explanation of the hemodynamic events observed during clinical testing. This analysis has also been considered in order to support the choice between different CO2 tests for quantitative assessment of vascular reactivity. Clinical data and model simulations agree in showing that side-to-side reactivity differences (Irel) measured from hypercapnia test allow a good discrimination of patients with poor compensatory capacity. They suggest that an Irel significantly greater than 30% after hypercapnia test can be considered indicative of patients with poor compensatory capacity. These preliminary results encourage a long-term follow-up of cerebral vascular reserve by means of TCD during "rebreathing test", for selecting patients with poor vascular reserve that may benefit from brain revascularization.