Fig. 2

Relative changes in vertebral artery (VA, top) and internal carotid artery (ICA, bottom) blood flow at each LBNP stage (Pre-LBNP, Severe LBNP and End of LBNP), with (open) or without (solid) the hypercapnic gas. Application of the hypercapnic gas mixture increased both ICA and VA blood flows. LBNP decreased blood flow to both regions (i.e., Severe LBNP). However, at Severe LBNP in the hypercapnic trial, both ICA and VA blood flows were at a similar level relative to normothermic baseline (indicated by the dashed horizontal line), whereas in the Air trial those were values were below normothermic baseline. At the end of LBNP, due to presyncope, both ICA and VA blood flows were equivalent and both were lower than normothermic baseline. *P < 0.05 different from the Air trial; ^†P < 0.05 different from Pre-LBNP; ^‡P < 0.05 different from Severe LBNP