Title: A Comparison of Hypergravity Versus Microgravity Influences on Cardiorespiratory Interactions
Abstract: In this article, we investigated the hypothesis that the effects of hypergravity on respiratory sinus arrhythmia (RSA) can mimic the effects observed after spaceflight cardiovascular deconditioning. Artificial gravity along the head-to-feet axis on a short-arm centrifuge induces gravity gradients. This physiological condition of significantly higher g at the feet than at the heart level is specific and likely induces blood sequestration in the lower limbs. After spaceflight, astronauts are in a condition of cardiovascular deconditioning, where blood pooling in the lower part of the body and autonomic adaptation are factors contributing to orthostatic intolerance and changes in heart-rate variability (HRV). ECG and respiration were recorded during imposed and controlled breathing (ICB) protocols, which were repeated at different levels of artificial gravity as well as during supine and standing control conditions, and the changes were analyzed. RSA, usually the high-frequency (HF) (0.15 Hz < HF < 0.4 Hz [10]) component of HRV, is a cardiorespiratory interaction mediated by the vagus nerve. RSA has thus been considered a marker of the parasympathetic modulation of heart rate [1]. However, in the light of recent studies [2], [3], care has to be taken to infer changes in tonic level activity from changes in the phasic parasympathetic modulation, especially when the respiration is not controlled. Previous studies reported decrease in RSA or HF component of HRV after spaceflight [4]– [6].Whether these changescanbe interpretedasresulting from autonomic adaption following exposure to microgravity or only resulting from a decreased blood volemia is still a matter of debate. It is hypothesized that the hypergravity on the European Space Agency (ESA) short-arm human centrifuge (SAHC) can generate blood shift from the thoracic region to the lower part of the body (pooling in the splanchnic region [11], [12] or the legs) that mimic the postspaceflight autonomic response. This hypothesis will be subscribed in this study by the analysis of RSA during ICB experiments, similar to those performed during previous short-duration spaceflights [6], [7]. The slope of RSA amplitude with the duration of the breath cycle (Tresp) has been shown to characterize the gain of RSA [7], which reflects the capacity of the autonomic nervous system (ANS) to respond to challenges. Results from SAHC experiments are compared with the results from ICB experiments performed before, during, and after a short-duration (11 days) spacemission [7]. Methods The ESA SAHC is a short-arm centrifuge (outer radius ¼ 2.82 m) that can generate up to 6 g at the feet of the subject and contains two bed nacelles and two chairs that are radially oriented. The bed nacelles can be rotated from � 20 (head down) to þ50 to generate different gravity gradients along the head-to-foot direction (Gz). The positions of the horizontally oriented beds were chosen so that, when lying supine, the heart was typically situated at 1.05 m from the center of rotation. Rotations were set at approximately 24, 29, and 32 r/min to obtain heart levels at 0.7, 1, and 1.2 Gz, respectively. This yielded values of approximately 0.47, 0.66, and 0.80 Gz at the head and 1.5, 2.2, and 2.6 Gz at the feet. Per Gz level, test time lasted approximately 20 min. Load conditions of 0.7 and 1 Gz were imposed without general stop of the centrifuge. Between 1 and 1.2 Gz however, a pause of 10 min was applied to unload the lower limbs. The fixed order of the g-load conditions was chosen to ensure optimal tolerance from the subjects.
Publication Year: 2009
Publication Date: 2009-01-01
Language: en
Type: article
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