Control Criteria of Cardiovascular System Generating 1/f Fluctuations

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In this study, we explored the control criteria of the cardiovascular system generating 1/f fluctuations. Through estimating the baroreflex gain (BRG), state-dependency of the regulatory system was studied. BRG was estimated by the sequence method constructed under an open-loop assumption from blood pressure (BP) to heart rate (HR). As a result, the estimated BRSs were lower in order standing, non rapid eye movement (nonREM) sleep, REM sleep, supine. In addition, under the head-up tilting posing orthostatic stress, it was paradoxically found that BRS becomes reduced as the tilt angle (gravity) increases. As a novel technique estimating BRS based on the closed-loop assumption, a new method was proposed that the sequence method is applied to a time series generated by the identified multivariate autoregressive (MAR) model driven by the innovation process, tentatively called "innovation method". Comparison between the estimates obtained by the sequence and the innovation methods did not s … More how large disparity, which clears the difference in the absolute value of BRS estimated by the open- and closed-loop techniques. In addition, the closed-loop impulse response from BP to BP was found to be accelerated as the orthostatic stress became heavier. Similar analyses were applied to the data of spinal injury patients. As a result, the cardiovascular system showed unstable responses for the patients of upper level injury. In contrast, the patients of lower level injury were found to have similar characteristics to the healthy subjects. Besides, the patients showed the distinct 0.05Hz oscillation, which could provide novel information concerning generation mechanism of Mayer wave. From the analysis of inter-dependency between BP and HR, the cardiovascular system is considered to be unstabilized by the disconnection between them due to the injury. Because the response to the orthostatic stress reflects control of peripheral resistance and behavior of venous return, the control criteria of the cardiovascular system could be explored by modeling them. Additionally, we constructed the cardiovascular model which consists of HR and sympathetic activity as control variables and BP as a variable to be controlled. Simulation of optimal control on the model suggests that the criteria for generating 1/f fluctuation in HR are loose ones allowing deviations to some extent rather than rigid one not allowing fluctuations. Less