Physiological systems are interdependent, interacting dynamically and behaving collectively to produce distinct physiologic states and functions. Thus, nonlinear and dynamical coupling measurement techniques based on time series and complex system methodologies are necessary to distinguish between different physiologic or pathophysiologic states. In recent decades, several studies have reported the development of many coupling measurement techniques such as cross-entropy coupling and cross frequency coupling to study the interaction between multiple systems. However, the studies that investigated cardiorespiratory coupling primarily used electrocardiograms (ECG) and respiratory sensor belts for measuring cardiac and respiratory activity signals. However, both of these devices are cumbersome for ambulatory recording or long-term monitoring. Recent advances in sensing technology have made photo-plethysmography (PPG) available even in mobile phones, opening up a new horizon in measuring cardiorespiratory coupling in a convenient way. However, further research is warranted to study the many different challenges in investigating the cardiorespiratory coupling using only PPG signals. These challenges form the core topics of this Research Topic. Thus, we welcome submissions related to the following sub-topics:

i) Estimation of respiratory effort or activity from PPG signals.
ii) Reliable estimation of respiratory effort in different daily living conditions such as walking, sitting, standing, lying etc.
iii) Novel nonlinear and dynamical methods to study cardiorespiratory coupling.
iv) Investigating effect of data length and noise on different coupling measurement methods.
v) Quality of cardiorespiratory coupling measurement using PPG derived cardiac and respiratory activity compared to standard ECG and Thoracic belt based measurement.
vi) Analysing effect of different PPG channels on measuring cardiorespiratory coupling.
vii) Applications of cardiorespiratory coupling measurement in distinguishing different physiologic or pathophysiologic conditions.
viii) Application of PPG signals in clinical physiological measurements, including blood pressure, vascular assessment, and autonomic function.

Keywords: Coupling measurement, respiratory activity, Photoplethysmogram (PPG), cardiorespiratory, Nonlinear methods

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