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1 Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, USA
2 Endocrinology-Hypertension Division, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, USA
3 Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, USA; Endocrinology-Hypertension Division, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, USA; Department of Cardio-Thoracic Surgery, McGill University, Montreal, Quebec, Canada
4 Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, USA; Endocrinology-Hypertension Division, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, USA
5 Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, USA; Department of Anesthesia and Critical Care, Massachusetts General Hospital, Boston, MA, USA
* To whom correspondence should be addressed. E-mail: rjcohen{at}mit.edu.
Microgravity-induced orthostatic intolerance (OI) continues to be a primary concern for the human space program. To test the hypotheses that exposure to simulated microgravity significantly alters autonomic nervous control and thus contributes to increased incidence of OI, we employed the cardiovascular system identification (CSI) technique to evaluate quantitatively parasympathetic and sympathetic regulation of heart rate (HR). The CSI method analyzes second-to-second fluctuations in non-invasively measured heart rate, arterial blood pressure (ABP), and instantaneous lung volume (ILV). The coupling mechanisms between these signals are characterized using a closed-loop model. Parameters reflecting parasympathetic and sympathetic responsiveness with regard to HR regulation can be extracted from the identified coupling mechanisms. We analyzed data collected from 29 human subjects before and after 16-day head-down-tilt bed rest (simulated microgravity). Statistical analyses showed that parasympathetic and sympathetic responsiveness were both impaired by bed rest. A lower sympathetic responsiveness and a higher parasympathetic responsiveness measured before bed rest identified individuals at greater risk of OI both before and after bed rest. We propose an algorithm to predict OI after bed rest from pre-bed rest measures.
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