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 CO(2), which is a cerebral vasodilator; indeed, separate studies have found that spontaneous fluctuations in end-tidal CO(2) (PETCO(2)) are correlated with BOLD signal time series. The present study quantifies the degree to which RVT and PETCO(2) 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 PETCO(2), and that both explain remarkably similar spatial and temporal BOLD signal variance across the brain. In addition, end-tidal O(2) is shown to be largely redundant with PETCO(2). Finally, the latency at which PETCO(2) 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.