Key result
Respiratory measures (ENV, RV, RVT) can miss respiratory events like deep breaths, with RVT missing them more frequently than ENV or RV, and all measures decrease systematically over 14.4-minute scans.
Why the study?
Little was known about how different respiratory measures compare to each other or how they perform beyond small studies when gauging respiratory contributions to resting-state fMRI signals.
Observational
Different respiratory measures derived from abdominal belts during fMRI have varying sensitivities to respiratory events and show systematic decreases over time, which has implications for modeling respiratory influences on fMRI.
May miss deep breaths in fMRI; leaves open optimal respiratory correction for confounder modeling.
Breathing rate and depth influence the concentration of carbon dioxide in the blood, altering cerebral blood flow and thus functional magnetic resonance imaging (fMRI) signals. Such respiratory fluctuations can have substantial influence in studies of fMRI signal covariance in subjects at rest, the so-called “resting state functional connectivity” technique. If respiration is monitored during fMRI scanning, it is typically done using a belt about the subject’s abdomen to record abdominal circumference. Several measures have been derived from these belt records, including the windowed envelope of the waveform (ENV), the windowed variance in the waveform (respiration variation, RV), and a measure of the amplitude of each breath divided by the cycle time of the breath (respiration volume per time, RVT). Any attempt to gauge respiratory contributions to fMRI signals requires a respiratory measure, but little is known about how these measures compare to each other, or how they perform beyond the small studies in which they were initially proposed. In this paper, we examine the properties of these measures in hundreds of healthy young adults scanned for an hour each at rest, a subset of the Human Connectome Project chosen for having high-quality physiological records. We find: 1) ENV, RV, and RVT are all similar, though ENV and RV are more similar to each other than to RVT; 2) respiratory events like deep breaths exhibit characteristic fMRI signal changes, head motions, and image quality abnormalities time-locked to deep breaths evident in the belt traces; 3) all measures can “miss” respiratory events evident in the belt traces; 4) RVT “misses” deep breaths (i.e., yawns and sighs) more than ENV or RV; 5) all respiratory measures change systematically over the course of a 14.4-minute scan, decreasing in mean value. We discuss the implication of these findings for the literature, and ways to move forward in modeling respiratory influences on fMRI scans. Highlights - Examines 3 respiratory measures in resting state fMRI scans of healthy young adults - All respiratory measures “miss” respiratory events, some more than others - Respiration volume per time (RVT) frequently “misses” deep breaths - All respiratory measures decrease systematically over 14.4 minute scans - Systematic decreases are due to decreased breathing depth and rate
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Power et al. (2019) conducted an observational in Healthy. Respiratory measures (ENV, RV, RVT) was evaluated on Comparison of respiratory measures (ENV, RV, RVT) and their ability to capture respiratory events. Respiratory measures (ENV, RV, RVT) can miss respiratory events like deep breaths, with RVT missing them more frequently than ENV or RV, and all measures decrease systematically over 14.4-minute scans.
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