Cerebral vascular responsiveness in chronic hypercapnia.

To assess the responsiveness of the cerebral vessels to chronic hypercapnia, we measured middle cerebral artery flow velocity by transcranial Doppler ultrasound in 20 normal subjects and in 14 COLD patients before and after stimulation by progressive hypercapnia (rebreathing test) or by intravenous administration of an acetazolamide bolus. The results showed no statistically significant difference in baseline flow velocity between the normal subjects and the COLD patients. The COLD patients showed a reduced cerebral vascular responsiveness to both stimuli. Cerebral blood flow is normal in chronic hypercapnia and the mechanism by which compensation is achieved leads to a decrease in cerebral vascular responsiveness.