We showed that the oscillations of breath-by-breath O2-CO2 exchange ratio (bER) were superior to those of end-tidal CO2 in correlating with the low frequencies (0.008-0.03Hz) of resting state cerebral hemodynamic fluctuations (CHF). Brain regions showing significant association of ΔBOLD with bER overlapped with many regions of default mode network. Transcranial Doppler sonography and fMRI were used to measure CHF and time series of partial pressure of O2 and CO2 were collected. bER-CHF coupling is a novel metric to measure brain-body interaction that may provide some answers to the physiological contributions to low frequencies of CHF.
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