Noninvasive optical evaluation of spontaneous low frequency oscillations in cerebral hemodynamics

Spontaneous low frequency oscillations (LFOs) around 0.1Hz have been observed in mean arterial pressure (MAP) and cerebral blood flow velocity (CBFV). Previous studies have shown that cerebral autoregulation in major arteries can be assessed by quantification of the phase shift between LFOs of MAP and CBFV. However, many cerebral diseases are associated with abnormal microvasculature and tissue dysfunction in brain, and quantification of these abnormalities requires direct measurement of cerebral tissue hemodynamics. This pilot study used a novel hybrid near-infrared diffuse optical instrument to noninvasively and simultaneously detect LFOs of cerebral blood flow (CBF) and cerebral oxygenation (i.e., oxygenated/deoxygenated/total hemoglobin concentration: [HbO₂]/[Hb]/THC) in human prefrontal cortex. Using the hybrid instrument and a finger plethysmograph, the dynamic changes of CBF, [HbO₂], [Hb], THC and MAP were concurrently measured in 15 healthy subjects at rest, during 70° head-up-tilting (HUT) and during enforced breathing at 0.1Hz. The LFOs were extracted from the measured variables using power spectral analysis, and the phase shifts and coherences of LFOs between MAP and each of the measured hemodynamic variables were calculated from the corresponding transfer functions. Levels of coherence (>0.4) were used to judge the success of LFO measurements. We found that CBF, [HbO₂] and THC were reliable hemodynamic parameters in detecting LFOs and HUT was the most robust and stable protocol for quantifying phase shifts of hemodynamic LFOs. Comparing with other relevant studies, similar success rates for detecting cerebral LFOs have been achieved in our study. The phase shifts of LFOs in CBF were also close to those in CBFV reported by other groups, although the results in cerebral oxygenation measurements during enforced breathing varied across studies. Future study will investigate cerebral LFOs in patients with cerebral impairment and evaluate their cerebral autoregulation capabilities and neurocognitive functions via the quantification of LFO phase shifts.