Cerebrovascular CO₂ responsiveness (CCR) in SCD patients with and without hydroxyurea. (A) Cerebrovascular CO₂ responsiveness expressed as relative change in mean CBF velocity (percentage ΔV_mean) per millimeter of mercury change in P_ETCO₂ (CCR-TCD) was higher in SCD patients using hydroxyurea (Hydroxy⁺, , n = 10) than in those not using hydroxyurea (Hydroxy⁻, ■, n = 13), although the difference was not statistically significant. (B) Cerebrovascular CO₂ responsiveness expressed as absolute changes (Δ μmol/L) in cerebral [O₂Hb], [dHb], and [t-Hb] per millimeter of mercury P_ETCO₂ were also higher in SCD patients using hydroxyurea (n = 7) than in those not using hydroxyurea (n = 7), although the difference was only statistically significant for dHb-derived cerebrovascular CO₂ responsiveness. Data are mean ± SEM.