ACETAZOLAMIDE AND CO2 - ACUTE EFFECTS ON CEREBRAL AND RETROBULBAR HEMODYNAMICS

Purpose: Acetazolamide and CO2 are cerebral vasodilators whose specifi c effects in various brain regions have not been carefully defined. We investigated the effects of these agents in both larger cerebral and smaller, retrobulbar arteries, to compare their general cerebral vasod ilatory influence with their specific ocular vascular effects. Methods : Twelve young adults with healthy eyes were studied under normocapnic and hypercapnic (6% CO2, 94% O-2 tanked gas) conditions after receivi ng either placebo or 1,000 mg acetazolamide (3 h before study). Color Doppler imaging was used to measure peak systolic and end-diastolic ve locities (PSV and EDV) in the internal carotid, middle cerebral, ophth almic, and central retinal arteries under each condition. Results: Ace tazolamide and CO2 each lowered intraocular pressure; combining the ag ents provided no additive ocular hypotensive effect. Hypercapnia or ac etazolamide per se failed to alter PSV, EDV, or the derived resistance index [RI; (PSV - EDV)/PSV] in the internal carotid or in either orbi tal artery. However, when hypercapnia was superimposed upon acetazolam ide, the resistance index fell in the internal carotid and central ret inal arteries (each p < 0.05). In contrast, the middle cerebral artery was responsive to either vasodilator and to their combination: PSV an d EDV rose, and RI fell with each experimental treatment. Conclusions: In the brain, the middle cerebral artery exhibits substantial depende nce of flow velocity on the vasodilators CO2 and acetazolamide, In con trast, the ophthalmic and central retinal arteries appear less respons ive. Nonetheless, the combination of carbonic anhydrase inhibition (ac etazolamide) with CO2 augmentation did lower vascular resistance dista l to the central retinal artery, suggesting that this mechanism vasodi lates critical ocular tissues.