RESUMO
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.
