Changes in respiratory dynamics drive RD. Subjects had thoracic and abdominal bands placed to measure expansion and compression, shown with an individual subject in (a). A state space oscillator model was used to characterize instantaneous changes in relative volume (b) and frequency (c), the moving average of which is displayed here. Declines in both relative volume and frequency contribute to the RD, shown by the decline in the MVI (d), the moving average of which is displayed here. Dashed lines correspond to the administration of the fentanyl boluses during the protocol.