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1 Department of Physiology and Pharmacology, Section of Environmental Physiology, Karolinska Institutet, Stockholm, Sweden
* To whom correspondence should be addressed. E-mail: jonas.spaak{at}fyfa.ki.se.
Long term head-down tilt bed rest (HDT) causes cardiovascular deconditioning, attributed to reflex dysfunctions, plasma volume reduction, or cardiac impairments. Our objective with the present study was to evaluate the functional importance and relative contribution of these during rest and exercise, in supine and upright postures. We studied 6 subjects before (BDC), during (day D60, and D113) and after (recovery day R0, R3, and R15) 120 days of -6-degree HDT. We determined cardiac output (CO), stroke volume (SV), mean arterial pressure (MAP) and heart rate (HR) during rest and exercise, in supine and upright postures. CO and SV decreased significantly in all four conditions but the time courses differed for rest and exercise. Upright resting SV was decreased by 24 ± 9% at D60 compared to BDC but had recovered already at R3. Supine exercise SV decreased more slowly: by 5 ± 8% at D60 and by 18 ± 4% at D113, and recovered more slowly after HDT termination. Steady-state MAP showed no changes. HR had increased by 18 ± 4% at D60 and had recovered partially at R3. Our data indicate that long-term HDT causes both a rapid, preload-dependent reduction in SV, most evident during rest in the upright position, and a more slowly developing cardiac dysfunction, most evident during supine exercise. However, the ability to maintain blood pressure and to perform sustained low levels of dynamic exercise is not influenced by HDT.
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