Respiratory modulation of cardiovascular rhythms before and after short-duration human spaceflight

Aim: Astronauts commonly return from space with altered short‐term cardiovascular dynamics and blunted baroreflex sensitivity. Although many studies have addressed this issue, post‐flight effects on the dynamic circulatory control remain incompletely understood. It is not clear how long the cardiovascular system needs to recover from spaceflight as most post‐flight investigations only extended between a few days and 2 weeks. Methods: In this study, we examined the effect of short‐duration spaceflight (1–2 weeks) on respiratory‐mediated cardiovascular rhythms in five cosmonauts. Two paced‐breathing protocols at 6 and 12 breaths min^?1 were performed in the standing and supine positions before spaceflight, and after 1 and 25 days upon return. Dynamic baroreflex function was evaluated by transfer function analysis between systolic pressure and the RR intervals. Results: Post‐flight orthostatic blood pressure control was preserved in all cosmonauts. In the standing position after spaceflight there was an increase in heart rate (HR) of approx. 20 beats min^?1 or more. Averaged for all five cosmonauts, respiratory sinus dysrhythmia and transfer gain reduced to 40% the day after landing, and had returned to pre‐flight levels after 25 days. Low‐frequency gain decreased from 6.6 (3.4) [mean (SD)] pre‐flight to 3.9 (1.6) post‐flight and returned to 5.7 (1.3) ms mmHg^?1 after 25 days upon return to Earth. Unlike alterations in the modulation of HR, blood pressure dynamics were not significantly different between pre‐ and post‐flight sessions. Conclusion: Our results indicate that short‐duration spaceflight reduces respiratory modulation of HR and decreases cardiac baroreflex gain without affecting post‐flight arterial blood pressure dynamics. Altered respiratory modulation of human autonomic rhythms does not persist until 25 days upon return to Earth.