Cerebrovascular CO₂ responsiveness (CCR) in SCD patients and healthy controls. (A) Cerebrovascular CO₂ responsiveness expressed as relative change in mean CBF velocity (% ΔV_mean) per millimeter of mercury change in P_ETCO₂ (CCR-TCD) was lower in SCD patients (■, n = 23) than in healthy controls (, n = 16). (B) Cerebrovascular CO₂ responsiveness expressed as absolute changes (Δ μmol/L) in cerebral [O₂Hb], [dHb], and [t-Hb] per millimeter of mercury P_ETCO₂ (CCR-NIRS) were also significantly lower in SCD patients (■, n = 14) than in healthy controls (, n = 13). Data are mean ± SEM.