Cardiovascular autonomic control after short-duration spaceflights
Introduction
Spaceflight induces many changes in the human body [1], [42], [41]. Especially after return to Earth, this causes physiological problems for astronauts. The inability to stand for prolonged periods, with the risk of fainting, and reduced orthostatic tolerance [34] can be considered as some of the most problematic conditions from an operational point of view. The degree of lack of orthostatic tolerance varies a lot from subject to subject. Despite the large amount of studies devoted to solving this problem, or to elucidate the mechanisms behind it, many aspects are still unclear.
In space, most studies agree on diminished vagal baroreflex gain together with an increase in vagal heart rate control [13], [24], which is probably due to an increase in stroke volume (SV) in the first days of spaceflight [31], [33]. At the same time however, muscle sympathetic nerve activity (MSNA) has been shown to be higher in microgravity, as demonstrated with lower body negative pressure (LBNP) experiments, Valsalva manoeuvres, handgrip and cold pressor tests [13], [22], [17], [15]. The increased sympathetic drive was supported by measurements of platelet norepinephrine in space [10].
After return to Earth the human body has to readapt to gravity. At this moment, the most likely cause of decreased orthostatic tolerance after spaceflight is a limitation of compensatory elevation in vascular resistance upon standing [9], [21]. This can be related to a hypoadrenergic responsiveness as supported by lower or equal values of plasma noradrenaline (NA) levels postflight compared to preflight in orthostatic intolerant subjects [40]. At the same time changes in the baroreflex might contribute to reduced orthostatic tolerance [12], [20], [11], [18]. Reduced vagal–cardiac efferent neural outflow in combination with reduced cardiovagal baroreflex gain have been described in relation with reduced orthostatic tolerance [20]. On the contrary, astronauts that are able to complete a stand test show increased levels of plasma NA after spaceflight [21], [40]. The sympathetic dominance after spaceflight might result from the decreased SV and cardiac output (CO) upon standing [31], [39].
Although symptoms of reduced orthostatic tolerance disappear rather quickly after return from space, the autonomic control system may recover more slowly, the duration of which is still unclear. Yet, most studies have performed a follow-up of astronauts and cosmonauts from the first day after landing up to 1 week [13], [19], [22], [9], [29], [35], [16], [27]. However, some studies have shown that cardiovascular control mechanisms were not yet restored in full by that time. Nevertheless, only very few studies have performed post-flight measurements up to between 10 and 18 days.
Using a simple paced-breathing protocol, we recently showed that 25 days of recovery after short-duration spaceflight is sufficient to restore vagal–cardiac outflow to pre-flight conditions [38]. However, the time course of recuperation within this period of 25 days remains uncertain. The scope of the present study was to determine a long-term follow-up of cardiovascular control up to 25 days after spaceflight under control conditions.
We studied heart rate modulation, blood pressure modulation and baroreflex sensitivity (BRS) before, during and after spaceflight using heart rate variability (HRV) and blood pressure variability (BPV) indices in five astronauts who had spent 10–11 days in space [3]. According to the above information, the hypothesis was tested that decreased vagal–cardiac control early post-flight will last more than 2 weeks after return but will be restored completely upon 25 days after landing.
Section snippets
Subjects
This study was performed during three scientific ESA-Soyuz missions to the International Space Station (ISS) (Odissea, Cervantes and Delta: 10–11 day missions). Five male astronauts were studied before, during and after spaceflight. Mean age of the subjects at the time of the pre-flight data collection was 40 (SD 3) years, height 180 (SD 4) cm and weight 76 (SD 10) kg. In space the astronauts had a busy scientific schedule and had no physical exercise programme. Upon return there was also no
Results
The astronauts body weight had decreased from 76 (5) kg before spaceflight kg to 73 (5) kg after return to Earth (p=0.042). Body weight had already fully recovered after 4 days. Respiratory frequency did not differ between the pre-flight measurements (average: 15 breaths/min ∼0.25 Hz; min: 12 breaths/min; max: 18 breaths/min), the in-flight measurements (average: 15 breaths/min ∼ 0.26 Hz; min: 15 breaths/min; max: 17 breaths/min) and the post-flight measurements (see Table 2 for details).
Table 1,
Discussion
In this study a long-term follow-up of five astronauts up to 25 days after their 10-day spaceflight was performed. We found a profound effect of short duration spaceflight on heart rate and HRV after spaceflight in the standing position only. There was a strong decrease in vagal heart rate modulation upon standing early post-fight while blood pressure and blood pressure control were not affected and remained stable at pre-flight levels. Cardiovascular control during spaceflight adapted to a
Conclusion
The importance of this study, in which multiple time points up to 25 days after landing with strict standardisation of measurements between the different space missions were obtained, was to demonstrate that especially the first days after spaceflight the spontaneous vagal autonomic control (without interventions like paced breathing, Valsalva, …) of heart rate is affected, resulting in a sympathetic dominance. After 4 days this control is largely recovered, although the effects on heart rate
Acknowledgements
We thank the astronauts and cosmonauts from the ESA Odissea, Cervantes and Delta flights for their reliable and outstanding efforts both as researchers and as subjects in space. Special acknowledgement must also be made of the efforts of the European and Russian Space Agencies in supporting these missions. A special thanks to the persons at ESTEC, Noordwijk, The Netherlands and Gagarin Cosmonaut Training Center, Star City, Moscow, Russia, who's help before and after the space flights was
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