document: We start by outlining the motivation, structure and content of the review. It has long been known that cardiovascular signals contain a number of oscillatory components that are not exactly periodic. To put it differently, their periods (frequencies) fluctuate with time. For example, heart rate variability (HRV) has in itself provided a major topic of discussion. We introduce one of the statistical approaches to HRV in Section 3. However, in order to understand the variability of the cardiovascular system, discussion of a single source is insufficient because the cardiovascular system is composed of many different physiological components (subsystems) and it is the effects of their mutual interaction that combine to produce HRV. This is demonstrated in Section 4, revealed by results obtained using the wavelet transform. In Section 5, we discuss the cardio-respiratory interaction in terms of phase synchronization. To set the scene for these later discussions, we summarize the basic principles of phase dynamics in Section 2. For readers who are unfamiliar with the physiological aspects of the research, we provide Appendices A on the cardiovascular system and B on how measurements of cardiovascular signals are conducted. Appendix C provides details of the statistical methods used in the group data analyses.
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