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Effects of simulated microgravity on closed-loop cardiovascular regulation and orthostatic intolerance: analysis by means of system identification

¹Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge 02139; ²Endocrinology-Hypertension Division, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston 02115; ⁴Department of Anesthesia and Critical Care, Massachusetts General Hospital, Boston, Massachusetts 02114; and ³Department of Cardio-Thoracic Surgery, McGill University, Montreal, Quebec, Canada H3G 1A4

Submitted 11 June 2003 ; accepted in final form 17 September 2003

Microgravity-induced orthostatic intolerance (OI) continues to be a primary concern for the human space program. To test the hypothesis 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 noninvasively measured HR, arterial blood pressure, and instantaneous lung volume. The coupling mechanisms between these signals are characterized by 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 days of head-down-tilt bed rest (simulated microgravity). Statistical analyses showed that parasympathetic and sympathetic responsiveness was impaired by bed rest. A lower sympathetic responsiveness and a higher parasympathetic responsiveness measured before bed rest identified individuals at greater risk of OI before and after bed rest. We propose an algorithm to predict OI after bed rest from measures obtained before bed rest.

sympathetic; parasympathetic; syncope; bed rest; autonomic function

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