Cerebral hemodynamic response to CO2 tests in patients with internal carotid artery occlusion: Modeling study and in vivo validation

The aim of this work was to analyze the cerebral hemodynamics in patients w ith internal carotid artery (ICA) occlusion during CO2 dynamical tests. The study was performed using an original mathematical model of cerebral hemod ynamics. The model includes the circle of Willis, the microvascular circula tion in the territories of each cerebral artery, cortical anastomoses, intr acranial pressure, the cerebrospinal fluid circulation, the brain Venous ci rculation and two regulation mechanisms: autoregulation and CO2 reactivity. Model predictions are compared with real data obtained on 20 healthy subje cts (group A) and 14 patients with unilateral ICA stenosis (group B) during CO2 tests, The percent changes in Doppler middle cerebral artery velocity per mm Hg of CO2 pressure variations (reactivity index, RI) were separately computed during hypocapnia and hypercapnia. In group A, the reactivity ind ex was practically the same in both sides. Patients in group B had a signif icantly lower CO2 reactivity in the side ipsilateral to the occlusion (hypo capnia RI: 1.06 +/- 1.6 vs. 2.3 +/- 0.8%/mm Hg; hypercapnia: 0.9 +/- 0.8 vs , 2.4 +/- 1.0%/mm Hg). Our model can explain these results very well, assum ing values for the diameter of the communicating arteries in the range repo rted in the clinical literature. Moreover, computer simulations suggest tha t patients with a small diameter of both communicating arteries (as low as 0.4-0.5 mm) exhibit a dramatic fall in CO2 reactivity in the ipsilateral si de compared with the contralateral one, with the appearance of paradoxical responses. A decrease in ipsilateral RI reactivity, however, may also depen d on the presence of a significant stenosis of the contralateral ICA. Copyr ight (C) 2000 S. Karger AG, Basel.