Cerebral vasodilatory capacity and reactivity in young African Americans : a comparison of rebreathing and steady-state hypercapnia to assess cerebrovascular health
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Evaluation of changes in cerebral blood flow (CBF) in response to manipulation of arterial carbon dioxide tension (P[subscript CO₂]) is a non-invasive technique to assess cerebrovascular function. Hypercapnia is achieved using respiratory maneuvers that produce either transient or step changes in P[subscript CO₂]. Although the CBF response is similar between methods, transient hypercapnia through rebreathing provides additional data points and allows for a more dynamic assessment of the relationship between CBF and end-tidal CO₂ tension (P[subscript ET,CO₂]). The aim of this study was to compare rebreathing and steady-state methodologies to evaluate population differences in the cerebral hemodynamic response to hypercapnia. We hypothesized that during rebreathing, African American (AA) individuals would exhibit a lower maximal increase in cerebrovascular conductance (CVCI) relative to Caucasian Americans (CA) despite comparable cerebrovascular reactivity to CO₂ (CVR), whereas during steady-state hypercapnia, there would be no difference between groups during inhalation of 3% or 6% CO₂, but AA would display reduced CVCI when breathing 9% CO₂. Middle cerebral artery blood flow velocity (CBFV) was measured using transcranial Doppler ultrasound in young, healthy AA (n = 12) and CA (n = 11) participants during two conditions (randomized): (i) transient, breath-by-breath increases in P[subscript ET,CO₂] induced by rebreathing; and (ii) steady-state increases in P[subscript ET,CO₂] induced by inhalation of 3%, 6%, and 9% CO₂. The maximal increase in CVCI (CBFV/mean arterial pressure) with hypercapnia was lower in AA compared to CA in both methods (rebreathing, P = 0.018; steady-state, P = 0.049). Linear regression of CVCI vs. P[subscript ET,CO₂] during steady-state hypercapnia suggested that CVR was reduced in AA when the slopes from baseline (P = 0.044) or 3% (P = 0.039) through 6% CO₂ were considered, whereas logistic regression of the response to rebreathing indicated no difference in maximal CVR between the two groups (P = 0.59). These results indicate that both rebreathing and steady-state techniques were sufficient to detect differences in the cerebral vasodilatory reserve capacity between populations, whereas linear regression to estimate CVR is influenced by the extent of hypercapnia and may inaccurately describe CVR when data from the asymptotic region of the response is included in analysis.