Autonomic cardiovascular and respiratory control during prolonged spaceflights aboard the International Space Station.

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.

Changes in basal heart rate in spaceflights up to 438 days.

BACKGROUND: The long-term acclimation of heart rate to microgravity was studied in a cosmonaut who stayed onboard the MIR space station for 438 d. This was the longest mission in the history of manned space exploration. The results are evaluated in the context of findings from three other cosmonauts who lived onboard MIR for a shorter time. HYPOTHESIS: The response of heart rate to the stimulus of microgravity was tested in the course of spaceflights during sleep across sleep stages and during supine waking. It was expected that heart rate would show adaptation effects beyond the first month in space. The size of the adaptation effect would depend on the stage of sleep. METHODS: For the record mission sleep polygraphies were obtained prior to mission on the ground, between the 3rd and the 30th d in space, after 6 mo in space, and toward the end of mission. From each of the sleep polygraphies beat-to-beat intervals of cardiac rhythms were determined and analyzed as the time series of the average beat-to-beat interval. RESULTS: A lengthening of the average beat-to-beat interval by 176 ms was found during the record flight compared with measurements on the ground. This increase in the average beat-to-beat interval corresponds to a reduction of heart rate by about 20%. The lengthening of the average beat-to-beat interval was more pronounced for non-REM sleep than for REM sleep. During the first month, a lengthening by 82 ms was observed. Measurements after 6 mo showed a further lengthening by 94 ms, and at the end of the mission no further change in average beat-to-beat interval was observed. CONCLUSIONS: Testing the response of heart rate to microgravity across distinct and stationary behavioral states appears to be appropriate to investigate the cardiovascular system. The long-term acclimation of heart rate is possibly due to an increased dominance of the parasympathetic control of cardiac rhythms in space.