Effects of clamping end‐tidal CO₂ on neurofluidic low‐frequency oscillations

Abstract

In recent years, low‐frequency oscillations (LFOs) (0.01–0.1 Hz) have been a subject of interest in resting‐state functional magnetic resonance imaging research. They are believed to have many possible driving mechanisms, from both regional and global sources. Internal fluctuations in the partial pressure of CO2 (PCO2) has long been thought of as one of these major driving forces, but its exact contributions compared with other mechanisms have yet to be fully understood. This study examined the effects of end‐tidal PCO2 (PetCO2) oscillations on LF cerebral hemodynamics and cerebrospinal fluid (CSF) dynamics under “clamped PetCO2” and “free‐breathing” conditions. Under clamped PetCO2, a participant's PetCO2 levels were fixed to their baseline average, whereas PetCO2 was not controlled in free breathing. Under clamped PetCO2, the fractional amplitude of hemodynamic LFOs in the occipital and sensorimotor cortex and temporal lobes were found to be significantly reduced. Additionally, the fractional amplitude of CSF LFOs, measured at the fourth ventricle, was found to be reduced by almost one‐half. However, the spatiotemporal distributions of blood and CSF delay times, as measured by cross‐correlation in the LF domain, were not significantly altered between conditions. This study demonstrates that, while PCO2 oscillations significantly mediate LFOs, especially those observed in the CSF, other mechanisms are able to maintain LFOs, with high correlation, even in their absence.

Widespread Autonomic Physiological Coupling Across the Brain-Body Axis

“…Further, global BOLD signals are similarly present in both conditions, as reflected in the explained variance estimates of the first principal component between the two conditions (PC1 free r 2 = 0.28; PC1 clamped r 2 = 0.25; Supplementary Figure 10). Consistent with a previous report comparing overall time-lag structure of BOLD signals between clamped and free breathing conditions (Kish et al, 2023), we found that the spatiotemporal structure of global BOLD signals was similarity maintained in clamped CO2 conditions (Supplementary Figure 4).…”