Hyperthermia modulates regional differences in cerebral blood flow to changes in CO sub(2)

The purpose of this study was to assess blood flow responses to changes in carbon dioxide (CO2) in the internal carotid artery (ICA), external carotid artery (ECA), and vertebral artery (VA) during normothermic and hyperthermic conditions. Eleven healthy subjects aged 22 plus or minus 2 (SD) yr were exposed to passive whole body heating followed by spontaneous hypocapnic and hypercapnic challenges in normothermic and hyperthermic conditions. Right ICA, ECA, and VA blood flows, as well as left middle cerebral artery (MCA) mean blood velocity (V sub(mean)), were measured. Esophageal temperature was elevated by 1.53 plus or minus 0.09x-deg-xC before hypocapnic and hypercapnic challenges during heat stress. Whole body heating increased ECA blood flow and cardiac output by 130 plus or minus 78 and 47 plus or minus 26%, respectively (P < 0.001), while blood flow (or velocity) in the ICA, MCA, and VA was reduced by 17 plus or minus 14, 24 plus or minus 18, and 12 plus or minus 7%, respectively (P < 0.001). Regardless of the thermal conditions, ICA and VA blood flows and MCA V sub( mean) were decreased by hypocapnic challenges and increased by hypercapnic challenges. Similar responses in ECA blood flow were observed in hyperthermia but not in normothermia. Heat stress did not alter CO sub(2) reactivity in the MCA and VA. However, CO sub(2) reactivity in the ICA was decreased (3.04 plus or minus 1.17 vs. 2.23 plus or minus 1.03%/mmHg; P = 0.039) but that in the ECA was enhanced (0.45 plus or minus 0.47 vs. 0.95 plus or minus 0.61%/mmHg; P = 0.032). These results indicate that hyperthermia is capable of altering dynamic cerebral blood flow regulation.