Dynamic brain-body coupling of breath-by-breath O(2)-CO(2) exchange ratio with resting state cerebral hemodynamic fluctuations

BACKGROUND: The origin of low frequency cerebral hemodynamic fluctuations (CHF) in the resting state remains unknown. Breath-by breath O(2)-CO(2) exchange ratio (bER) has been reported to correlate with the cerebrovascular response to brief breath hold challenge at the frequency range of 0.008–0.03Hz in healthy adults. bER is defined as the ratio of the change in the partial pressure of oxygen (ΔPO(2)) to that of carbon dioxide (ΔPCO(2)) between end inspiration and end expiration. In this study, we aimed to investigate the contribution of respiratory gas exchange (RGE) metrics (bER, ΔPO(2) and ΔPCO(2)) to low frequency CHF during spontaneous breathing. METHODS: Twenty-two healthy adults were included. We used transcranial Doppler sonography to evaluate CHF by measuring the changes in cerebral blood flow velocity (ΔCBFv) in bilateral middle cerebral arteries. The regional CHF were mapped with blood oxygenation level dependent (ΔBOLD) signal changes using functional magnetic resonance imaging. Temporal features and frequency characteristics of RGE metrics during spontaneous breathing were examined, and the simultaneous measurements of RGE metrics and CHF (ΔCBFv and ΔBOLD) were studied for their correlation. RESULTS: We found that the time courses of ΔPO(2) and ΔPCO(2) were interdependent but not redundant. The oscillations of RGE metrics were coherent with resting state CHF at the frequency range of 0.008–0.03Hz. Both bER and ΔPO(2) were superior to ΔPCO(2) in association with CHF while CHF could correlate more strongly with bER than with ΔPO(2) in some brain regions. Brain regions with the strongest coupling between bER and ΔBOLD overlapped with many areas of default mode network including precuneus and posterior cingulate. CONCLUSION: Although the physiological mechanisms underlying the strong correlation between bER and CHF are unclear, our findings suggest the contribution of bER to low frequency resting state CHF, providing a novel insight of brain-body interaction via CHF and oscillations of RGE metrics.