PART ONE - EFFECTS OF SUSTAINED LOW-LEVEL ELEVATIONS OF CARBON-DIOXIDE ON CEREBRAL BLOOD-FLOW AND AUTOREGULATION OF THE INTRACEREBRAL ARTERIES IN HUMANS

Cerebral blood flow velocity (CBFv) was measured by insonating the mid dle cerebral arteries of four subjects using a 2 Mhz transcranial Dopp ler. Ambient CO2 was elevated to 0.7% for 23 d in the first study and to 1.2% for 23 d in the same subjects in the second study. By nonparam etric testing CBFv was elevated significantly by +35% above preexposur e levels during the first 1-3 d at both exposure levels, after which C BFv progressively readjusted to pre-exposure levels. Despite similar C BFv responses, headache was only reported during the initial phase of exposure to 1.2% CO2. Vascular reactivity to CO2 assessed by rebreathi ng showed a similar pattern with the CBFv increases early in the expos ures being greater than those elicited later. An increase in metabolic rate of the visual cortex was evoked by having the subjects open and close their eyes during a visual stimulus. Evoked CBFv responses measu red in the posterior cerebral artery were also elevated in the first 1 -3 d of both studies returning to pre-exposure levels as hypercapnia c ontinued. Cerebral vascular autoregulation assessed by raising head pr essure during 10 degrees head-down till both during the low level expo sures and during rebreathing was unaltered. There were no changes in t he retinal microcirculation during serial fundoscopy studies. The time -dependent changes in CO2 vascular reactivity might be due either to r etention of bicarbonate in brain extracellular fluid or to progressive increases in ventilation, or both. Cerebral vascular autoregulation a ppears preserved during chronic exposure to these low levels of ambien t CO2.