Complexity and nonlinearities in cardiorespiratory signals in sleep and sleep apnea

Abstract

Sleep is a complex physiological process that plays a key role in maintaining homeostasis, well-being and overall health. It has an internal structure characterized by sleep stages, which is often affected by either the high demands of the current 24-h society or by different sleep disorders. In fact, these disturbances to the regular sleep structure have been strongly associated with reductions of cognitive and behavioral performance, depression, memory loss, and cardiovascular diseases. With this in mind, multiple studies have analyzed different physiological variables during each sleep stage, and how their dynamics are affected by sleep disorders such as sleep apnea. This chapter discusses the main findings reported in literature with special focus on the dynamics of heart rate and respiration. The first part of this chapter focuses on the description and quantification of the different cardiac and respiratory dynamics during healthy sleep. Linear and nonlinear approaches that have been used to analyze the complexity of the cardiorespiratory system during different sleep stages are presented. For instance, the application of methods such as detrended fluctuation analysis, dynamic warping, correlation dimension, and entropy measures, on cardiac and respiratory signals during sleep are described. In addition, an overview of the cardiorespiratory interactions during sleep is provided, and different algorithms for sleep staging are compared. The second part discusses the effect that sleep apnea has on the cardiorespiratory dynamics and interactions, and special focus is on how this effect has been quantified using linear and nonlinear techniques. The problem of sleep apnea detection is presented together with a comparison between the performances obtained using linear and nonlinear techniques. Finally, a summary of the publicly available datasets that can be used for the analysis of heart rate and/or respiration during sleep and sleep apnea are presented.