Key result
Head-down bed rest impairs orthostatic tolerance without increasing stress-induced T-wave alternans amplitude.
Why the study?
Does short- and mid-duration head-down bed rest increase T-wave alternans under stress conditions in healthy male volunteers?
Does short- and mid-duration head-down bed rest increase T-wave alternans under stress conditions in healthy male volunteers?
Prolonged simulated microgravity induces cardiovascular deconditioning and orthostatic intolerance but does not appear to increase ventricular repolarization heterogeneity as measured by T-wave alternans.
Head-down bed rest reduces orthostatic tolerance without raising T-wave alternans; leaves open arrhythmogenic risk in longer exposures or clinical populations.
It is well known that prolonged microgravity leads to cardiovascular deconditioning, inducing significant changes in autonomic control of the cardiovascular system. This may adversely influence cardiac repolarization, and provoke cardiac rhythm disturbances. T-wave alternans (TWA), reflecting temporal and spatial repolarization heterogeneity, could be affected. The aim of this work was to test the hypothesis that 5 d and 21 d head-down (-6°) bed rest (HDBR) increases TWA, thus suggesting a higher underlying electrical instability and related arrhythmogenic risk. Forty-four healthy male volunteers were enrolled in the experiments as part of the European Space Agency's HDBR studies. High-fidelity ECG was recorded during orthostatic tolerance (OT) and aerobic power (AP) tests, before (PRE) and after HDBR (POST). A multilead scheme for TWA amplitude estimation was used, where non-normalized and T-wave amplitude normalized TWA indices were computed. In addition, spectral analysis of heart rate variability during OT was assessed. Both 5 d and 21 d HDBR induced a reduction in orthostatic tolerance time (OTT), as well as a decrease in maximal oxygen uptake and reserve capacity, thus suggesting cardiovascular deconditioning. However, TWA indices were found not to increase. Interestingly, subjects with lower OTT after 5 d HDBR also showed higher TWA during recovery after OT testing, associated with unbalanced sympathovagal response, even before the HDBR. In contrast with previous observations, augmented ventricular heterogeneity related to 5 d and 21 d HDBR was not sufficient to increase TWA under stress conditions.
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Martín-Yebra et al. (2015) studied Healthy volunteers (simulated microgravity) (n=44). Head-down bed rest (-6°) vs. Baseline (pre-bed rest) was evaluated on T-wave alternans (TWA) amplitude and normalized TWA indices during stress conditions. Head-down bed rest for 5 and 21 days significantly reduced orthostatic tolerance time but did not increase T-wave alternans amplitude under stress conditions.
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