Relationship between respiration, end-tidal CO2, and BOLD signals in resting-state fMRI

A significant component of BOLD fMRI physiological noise is caused by variations in the depth and rate of respiration. It has previously been demonstrated that a breath-to-breath metric of respiratory variation (respiratory volume per time; RVT), computed from pneumatic belt measurements of chest expansion, has a strong linear relationship with resting-state BOLD signals across the brain. RVT is believed to capture breathing-induced changes in arterial CO₂, which is a cerebral vasodilator; indeed, separate studies have found that spontaneous fluctuations in end-tidal CO₂ (PETCO₂) are correlated with BOLD signal time series. The present study quantifies the degree to which RVT and PETCO₂ measurements relate to one another and explain common aspects of the resting-state BOLD signal. It is found that RVT (particularly when convolved with a particular impulse response, the “respiration response function”) is highly correlated with PETCO₂, and that both explain remarkably similar spatial and temporal BOLD signal variance across the brain. In addition, end-tidal O₂ is shown to be largely redundant with PETCO₂. Finally, the latency at which PETCO₂ and respiration belt measures are correlated with the time series of individual voxels is found to vary across the brain and may reveal properties of intrinsic vascular response delays.