Cardiorespiratory Coupling Degree Affects the Efficiency of Cardiopulmonary System

The human body is a complex system composed of many interacting subsystems, and the coupling degree among subsystems determines the physiological function and efficiency. The cardiac and respiratory systems are strongly coupled and modulated by the autonomic nervous system (ANS). In this paper, we explore the relationship between the degree of cardiorespiratory coupling represented by the Heart-Breath Coherence $(H B C)$ metric and its physiological value from the perspective of oxygen uptake per unit of cardiac work. We collected the heart rate and respiratory data of 29 volunteers in multi-scenario experiments (contain arithmetic task, listening to sounds, emotional evocations, smelling odors and watching videos) and calculate their $H B C$ values to obtain different degrees of cardiorespiratory coupling. Then, we calculate the volume of oxygen uptake and cardiac work by a gas exchange model, and its reliability is verified by the significantly positive correlation between oxygen saturation computed by the model and measured in the experiment. Calculation results of experimental data show that: higher cardiorespiratory coupling degree ($H B C$ value) will increase the efficiency of cardiopulmonary system (oxygen intake per unit cardiac work); stressful tasks (low cardiorespiratory coupling degree) require more oxygen and the heart does more work, but the efficiency is lower than relaxation.