Orthostatic reflexes were examined at 375 m and after 60 min of exposure in a hypobaric chamber at 3660 m using a 20-min 70 degrees head-up tilt (HUT) test. Mean arterial blood pressure, R wave-R wave interval (RRI), and mean cerebral blood flow velocity (MFV) were examined with coarse-graining spectral analysis. Of 14 subjects, 7 at 375 m and 12 at 3660 m were presyncopal. Immediately on arrival to high altitude, breathing frequency and MFV increased, and endtidal PCO2, RRI, RRI complexity, and the parasympathetic nervous system indicator decreased. MFV was similar in HUT at both altitudes. The sympathetic nervous system indicator increased with tilt at 3660 m, whereas parasympathetic nervous system indicator decreased with tilt at both altitudes. Multiple regression analysis of supine variables from either 375 or 3660 m and the time to presyncope at 3660 m indicated that, after 1 h of exposure, increased presyncope at altitude was the result of 1). ineffective peripheral vasoconstriction, despite increased cardiac sympathetic nervous system activity with HUT, and 2). insufficient cerebral perfusion owing to cerebral vasoconstriction as the result of hypoxic hyperventilation-induced hypocapnia.

UvA-DARE is a service provided by the library of the University of Amsterdam (http://dare.uva.nl) UvA-DARE (Digital Academic Repository)Circadian blood pressure and systemic haemodynamics during 42 days of 6o headdown tilt Voogel, A.J.; Stok, W.J.; Pretorius, P.J.; van Montfrans, G.A.; Langewouters, G.J.; Karemaker, J.M. Published in:Acta physiologica Scandinavica DOI:10.1046/j. 1365-201X.1997.00203.x Link to publication Citation for published version (APA):Voogel, A. J., Stok, W. J., Pretorius, P. J., van Montfrans, G. A., Langewouters, G. J., & Karemaker, J. M. (1997). Circadian blood pressure and systemic haemodynamics during 42 days of 6o head-down tilt. Acta physiologica Scandinavica, 161, 71-80. DOI: 10.1046/j.1365-201X.1997 General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Download date: 10 May 2018Circadian blood pressure and systemic haemodynamics during 42 days of 6°head-down tilt ABSTRACTHead-down tilted bedrest is a ground-based microgravity simulation model. Since in this position the influence of chief external determinants of circadian blood pressure variation, i.e. activity and posture, are reduced, it may reveal endogenous oscillatory factors. The effects of 42 days of 6°head-down tilt on the circadian profiles of continuous finger blood pressure, heart rate, stroke volume, cardiac output and total peripheral resistance were analysed. In seven healthy volunteers (25±31 years) twelve 22 h Portapres registrations were performed: two in an ambulatory baseline period, eight during 42 days of head-down tilt, and two during recovery. Stroke volume was estimated by a pulse contour method (`Modelflow') from the finger arterial blood pressure tracing. Head-down tilt rapidly reduced circadian BP variation, especially for diastolic blood pressure. No effect of long-term headdown tilt on blood pressure level was observed. The day-night difference in heart rate was essentially unaffected. Cardiac output was maintained through an increase of heart rate and simultaneous decline of stroke volume. Our observations confirm the overriding importance of physical activity and orthostatic load on the diurnal variation of BP. The time-frame of the changes in stroke volume and heart rate during head-down tilt might point to a contribution of other factors besides a r...