Analysis on the Synchronization of Electrocardiogram during Sleep

Abstract

Human physiological systems continuously interact for effective body functioning to contribute to physiological homeostasis. Theoretical insights into nonstationary and nonlinear dynamics have been employed to identify the physiological interaction, and have yielded various interaction characteristics. However, the physiological dynamics revealing high degree of complexity transiently change their characteristics in different states and conditions under their own regulatory mechanisms. Thus, understanding the physiological interaction is still challenging. This thesis proposes methods to identify and quantify the interactions of inter- and intra-personal physiological systems during sleep, focusing on the cardiac system. As for the interaction between intra-personal physiological systems, directional coupling in the cardiorespiratory system was evaluated in different physiological states (sleep stages) and conditions, i.e., severity of obstructive sleep apnea (OSA). Directionality analysis was performed with heartbeats acquired from electrocardiogram and abdominal respiratory effort measured from the polysomnographic data of 107 healthy individuals and patients with OSA. Unidirectional coupling from the respiratory system to the cardiac system increases during wakefulness and rapid eye movement sleep, and significantly decreases during light and deep sleep, which is further decreased in deep sleep, approaching bidirectional coupling. In addition, unidirectional coupling from the respiratory system to the cardiac system also significantly increases according to the severity of OSA. The stratification pattern found in both sleep stages and OSA severity is statistically significant (all pairwise multiple comparisons in sleep stages and OSA severity by Tukeys test, p < 0.05). The influence from the respiratory system to the cardiac system gradually decreases from wakefulness and REM sleep to light and deep sleep, and the influence from the cardiac system to the respiratory system is the highest in deep sleep and gradually decreases to wakefulness via light and REM sleep (Tukeys test, p < 0.05). The similar stratification pattern is also observed according to OSA severity (Tukeys test, p < 0.05). The directionality index maintained higher values within the heart and respiratory rates in particular ratios, implying that the increment of directionality index approaching bidirectional coupling is determined by the changes in and/or influence of both systems. Age-dependent characteristics of the directional coupling of the cardiorespiratory system are also found, i.e., the elderly group (age ≥ 50) reveals significantly higher unidirectional coupling from the respiratory system to the cardiac system compared to that in the young group, maintaining a sleep stage-dependent stratification pattern of the cardiorespiratory coupling (independent samples t-test, p < 0.001). These coupling characteristics in different states and conditions are believed to be linked with autonomic nervous modulation. A possibility of coupling between inter-personal physiological systems was investigated for heart rhythms of co-sleeping individuals under an environment where they were in contact with each other. The interaction characteristics were approached with two different aspects, namely, inter-personal heart rhythmic phase synchronization (IHPS) and inter-personal heart rhythmic causal relation (IHCR). Experimental evidence was found that independent heart rhythms appeared in the same relative phase for prolonged periods (IHPS) and their occurrences had bidirectional causal relation (IHCR) in co-sleeping individuals. The degree of IHPS was at least two times higher in co-sleeping individuals than in separately sleeping individuals (Wilcoxon signed-rank test, p < 0.03). IHCR also showed significant increase in both directions of heart rhythms in co-sleeping condition (Wilcoxon signed-rank test, p < 0.03). In addition, IHPS was characterized by an increase of IHCR in both directions compared with those in non-IHPS (independent samples t-test, p < 0.001), indicating that IHPS was determined by fine tuning of rhythmic phases under bidirectional interaction, not by one-way influence from one to another. The standard deviation of R-R intervals in IHPS was significantly lower than that in non-IHPS for co-sleeping individuals (independent samples t-test, p < 0.001), indicating that the phase coupling of heart rhythms in co-sleeping individuals was associated with decreased sympathetic and increased parasympathetic activities. Therefore, this finding implies that IHPS was associated with the regulation of the autonomic nervous system. Under controlled experimental conditions, the finding could be attributed to weak cardiac vibration delivered by one another through bed connection. The approaches in this thesis are useful in understanding how the systemic interaction contributes to efficient body functioning for maintaining physiological homeostasis under internal and external environmental changes. In addition, it is expected that the proposed analytics can provide objective evidence of how one systemic abnormality causes other systemic dysfunctions and/or disorders based on the physiological interaction aspect. The findings imply that intrinsic physiological rhythms can be modulated by the rhythm of an independent external system. Therefore, the research can be extended to identify whether physiological rhythms can be adjusted by an external device, whose frequency and phase can be controlled. Further it can lead to physiological systems in a positive manner. In addition, the approaches are expected to elucidate how sharing behavior or social relationship of individuals is associated with physiological systemic interaction.