We compare scaling properties of the cardiac dynamics during sleep and wake periods for healthy individuals, cosmonauts during orbital flight, and subjects with severe heart disease. For all three groups, we find a greater degree of anticorrelation in the heartbeat fluctuations during sleep compared to wake periods. The sleep-wake difference in the scaling exponents for the three groups is comparable to the difference between healthy and diseased individuals. The observed scaling differences are not accounted for simply by different levels of activity, but appear related to intrinsic changes in the neuroautonomic control of the heartbeat.

Impaired autonomic control represents a cardiovascular risk factor during long-term spaceflight. Little has been reported on blood pressure (BP), heart rate (HR), and heart rate variability (HRV) during and after prolonged spaceflight. We tested the hypothesis that cardiovascular control remains stable during prolonged spaceflight. Electrocardiography, photoplethysmography, and respiratory frequency (RF) were assessed in eight male cosmonauts (age 41-50 yr, body-mass index of 22-28 kg/m2) during long-term missions (flight lengths of 162-196 days). Recordings were made 60 and 30 days before the flight, every 4 wk during flight, and on days 3 and 6 postflight during spontaneous and controlled respiration. Orthostatic testing was performed pre- and postflight. RF and BP decreased during spaceflight (P < 0.05). Mean HR and HRV in the low- and high-frequency bands did not change during spaceflight. However, the individual responses were different and correlated with preflight values. Pulse-wave transit time decreased during spaceflight (P < 0.05). HRV reached during controlled respiration (6 breaths/min) decreased in six and increased in one cosmonaut during flight. The most pronounced changes in HR, BP, and HRV occurred after landing. The decreases in BP and RF combined with stable HR and HRV during flight suggest functional adaptation rather than pathological changes. Pulse-wave transit time shortening in our study is surprising and may reflect cardiac output redistribution in space. The decrease in HRV during controlled respiration (6 breaths/min) indicates reduced parasympathetic reserve, which may contribute to postflight disturbances.

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The article presents the main provisions of the methodology for the analysis of heart rate variability (HRV), which is now actively and widely implemented in many fields of medicine and applied physiology. This methodology was first developed in space medicine, where, already during the first manned spaceflights, there was a need in operative assessment of the person's reactions and abilities to maintain high performance and good level of health under different stress conditions.The HRV analysis methodology is based on the measurement of a consecutive series of cardiac cycle durations, for which electrocardiography, rheocardiography, ballistic cardiography, etc., can be used. The resulting numerical series are subjected to mathematical analysis using statistical, spectral and other methods. The results are interpreted as medical and physiological criteria of the functional state of the organism.Based on the mathematical model, a probabilistic approach to the prediction of pathological conditions was proposed. Indicators of the stress degree of regulatory systems and their functional reserve, which are calculated from the HRV analysis data, are used in the mathematical model of the functional states. In order to obtain the decision rules for the recognition of identified classes of functional states the stepwise discriminant analysis has been applied.Equations of the discriminant function were obtained. This article examines in detail this new probabilistic approach to the HRV analysis and provides examples of its use for assessing the functional state of cosmonauts at various stages of long space flights.

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Balistocardiography was recorded in 3-D on a free floating astronaut in space as well as on healthy volunteers participating to a dry immersion study in a terrestrial laboratory. We demonstrate a new technique suitable for the analysis of 3-D BCG. The spatial curve of the displacement vector is analyzed instead of the three components of acceleration. The technique presented is invariant from the axis of representation and provides important novel physiological information.

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