Intra-individual variability in cerebrovascular and respiratory chemosensitivity: Can we characterize a chemoreflex...

Intra-individual variability in the magnitude of human cerebrovascular and respiratory chemoreflex responses is largely unexplored. By comparing response magnitudes of cerebrovascular CO₂ reactivity (CVR; middle and posterior cerebral arteries; MCA, PCA), central (CCR; CO₂) and peripheral respiratory chemoreflexes (PCR; CO₂ and O₂), we tested the hypothesis that a within-individual reactivity magnitude profile could be characterized. The magnitudes of CVR and CCR were tested with hyperoxic rebreathing and PCR magnitudes were tested through transient respiratory tests (TT-CO₂, hypercapnia; TT-N₂, hypoxia). No significant intra-individual relationships were found between CCR vs. CVR (MCA and PCA), CCR vs. PCR (TT-N₂ or TT-CO₂) (r<0.2, P>0.3) response magnitudes. Statistically significant relationships were found between MCA vs. PCA reactivity (r=0.45, P<0.01) and PCR TT-N₂ vs. PCR TT-CO₂ (r=0.79, P<0.001) responses. Using qualitative and quantitative comparisons, we conclude that an intra-individual chemoreflex reactivity magnitude profile cannot be characterized. These data highlight the considerable between- and within-individual variability that exists in human cerebrovascular and respiratory chemoreflexes.