To study the impaired cerebral autoregulation mechanism of postural orthostatic tachycardia syndrome (POTS) patients under carbon dioxide, a nonlinear regression model was applied to evaluate their cerebrovascular responses to carbon dioxide (CO 2 ). The cerebral blood flow (CBFV), end-tidal carbon dioxide (${{\text{P}}_{{\text{ETC}}{{\text{O}}_2}}}$), and other cardiopulmonary signals were processed for each subject with a tilt table experiment and hyperventilation. The sigmoidal model for cerebrovascular responses to CO 2 during the hyperventilation phase for POTS patients demonstrated an observable dissimilarity in comparison with control groups of healthy elders and healthy youths, especially in its range of change in CBFV.