Revisit on the cerebrovascular responses to end-tidal CO2 fluctuations during spontaneous breathing as a surrogate of regional cerebrovascular reactivity assessment under hypercapnic challenge

Regional cerebraovascular reactivity (CVR) to endogenous carbon dioxide (CO₂) during spontaneous breathing (i.e. end-tidal partial pressure of CO2 (P_ETCO₂) measured at rest), was substantially different from that obtained under externally applied CO₂ challenge. Individual maps of CVR to endogenous CO₂ showed significant inter-subject variability while CVR maps under hypercapnic challenge did not. Such inter-subject variability was not reduced by correction of respiratory effects. In addition, the CVR to end-tidal partial pressure of oxygen (P_ETO₂) during spontaneous breathing shows less inter-subject variability. Our findings question the compatibility of using CVR during spontaneous breathing as a surrogate of CVR under hypercapnic challenge.