Inverse correlation of fluctuations of cerebral blood and water concentrations in humans

Abstract

Near-infrared spectroscopy (fNIRS) measures concentrations of oxygenated (HbO) and deoxygenated (HbR) hemoglobin in the brain. Recently, we demonstrated its potential also for measuring concentrations of cerebral water (hbox {cH}_{2}hbox {O}). We performed fNIRS measurements during rest to study fluctuations in concentrations of hbox {cH}_{2}hbox {O}, HbO and HbR in 33 well-rested healthy control subjects (HC) and 18 acutely sleep-deprived HC. Resting-state fNIRS signal was filtered in full-band, cardiac, respiratory, low-, and very-low-frequency bands. The sum of HbO and HbR constitutes the regional cerebral blood volume (CBV). CBV and hbox {cH}_{2}hbox {O} concentrations were analyzed via temporal correlation and phase synchrony. Fluctuation in concentrations of hbox {cH}_{2}hbox {O} and CBV was strongly anti-correlated across all frequency bands in both frontal and parietal cortices. Fluctuation in concentrations of {hbox {cH}}_{2}{hbox {O}} and CBV showed neither a completely synchronous nor a random phase relationship in both frontal and parietal cortices. Acutely sleep-deprived subjects did not show significant differences in temporal correlation or phase synchrony between fluctuations in {hbox {cH}}_{2}{hbox {O}} and CBV concentrations compared with well-rested HC. The reciprocal interrelation between fluctuations in CBV and {hbox {cH}}_{2}{hbox {O}} concentrations is consistent with the Munro–Kellie doctrine of constant intracranial volume. This coupling may constitute a functional mechanism underlying glymphatic circulation, which persists despite acutely disturbed sleep patterns.