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 an d patients with unilateral internal carotid artery (ICA) occlusion. The per centage changes in middle cerebral arteries blood flow velocity (V-MCA) per mmHg of pCO(2) variations (reactivity index, RI) were separately computed during hypocapnia and hypercapnia, the latter obtained by a rebreathing man euver. 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 cho ice between different CO2 tests for quantitative assessment of vascular rea ctivity. Clinical data and model simulations agree in showing that side-to- side reactivity differences (I-rel) measured from hypercapnia test allow a good discrimination of patients with poor compensatory capacity. They sugge st that an I-rel 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 r eserve by means of TCD during "rebreathing test", for selecting patients wi th poor vascular reserve that may benefit from brain revascularization.