Influence of acetazolamide and CO2 on extracranial flow volume and intracranial blood flow velocity

Background and Purpose-The vasomotor response can be tested by means of tra nscranial Doppler sonography. If a constant vessel diameter is assumed, the flow velocity changes will reflect blood flow volume changes. This hypothe sis is difficult to verify. Simultaneous assessment of intracranial flow ve locity and extracranial flow volume changes may solve this problem. Methods-We tested vasomotor response in 32 volunteers (age, 42 +/- 18 years ) with 5% CO2. Acetazolamide (1 g) was tested in 15 volunteers (age, 28 +/- 8 years). To evaluate drug-dependent flow changes in the external carotid artery territory, acetazolamide was administered in 7 patients with unilate ral occlusion of the internal carotid artery without evidence of collateral ization through the ophthalmic artery (age, 67 +/- 12 years). Simultaneous recording included measurements of flow volume in the common carotid arteri es (M-mode color duplex system) and flow velocity in the middle cerebral ar teries. Results-With CO2 and acetazolamide, intracranial flow velocity increased by 31% and 39%, respectively, with a simultaneous increase of common carotid artery flow volume of 47% and 50%, respectively. No change in extracranial flow volume was observed in patients with an occluded internal carotid arte ry. Conclusions-These data show not only the expected increase of flow velocity in the middle cerebral artery but also suggest an increase in cross-sectio nal vessel diameter of 6% and 4% with CO2 and acetazolamide, respectively. It remains unresolved whether this observation is due to a direct effect of me drug on the vessel walls or is simply pressure dependent.