Exp Physiol 92.2 pp 427–435 427 Experimental Physiology Autonomic regulation during orthostatic stress in highlanders: comparison with sea-level residents Giosu´ e Gulli 1 , Victoria E. Claydon 1,2 , Marat Slessarev 3 , Guta Zenebe 4 , Amha Gebremedhin 5 , Maria Rivera-Ch 6 , Otto Appenzeller 7 and Roger Hainsworth 1 1 Institute for Cardiovascular Research, University of Leeds, Leeds, UK 2 ICORD, University of British Columbia, Vancouver, Canada 3 Department of Physiology, University of Toronto and Department of Anaesthesia, Toronto General Hospital, Ontario, Canada 4 Department of Neurology, Yehuleshet Higher Clinic, University of Addis Ababa, Ethiopia 5 Department of Medicine, University of Addis Ababa, Ethiopia 6 Department of Physiology, Universidad Peruana Cayetano Heredia, Lima, Peru 7 New Mexico Health Enhancement and Marathon Clinics (NMHEMC) Research Foundation, Albuquerque, NM, USA This report is a comparison of orthostatic tolerance and autonomic function in three groups of high-altitude dwellers: Andeans with and without chronic mountain sickness (CMS) and healthy Ethiopians. Results are compared with those from healthy sea-level residents. The aim was to determine whether different high-altitude populations adapted differently to the prevailing hypobaric hypoxia. Orthostatic tolerance was assessed using a test involving head-up tilt (HUT) and graded lower body suction. This was performed at the subjects’ resident altitude. Blood pressure (Portapres) and R–R interval (ECG) were recorded during the test, and spectral and cross-spectral analyses of heart period and systolic blood pressure time series were performed using data obtained both while supine and during HUT. The transfer function gain in the low- frequency range (LF, ∼0.1 Hz) at the point of maximal coherence was used as a measure of cardiac baroreflex sensitivity (BRS). As previously reported, Peruvians displayed an unusually good orthostatic tolerance, while Ethiopians showed an orthostatic tolerance comparable to that of healthy sea-level residents. There were no significant differences between groups in the supine values of the spectral analysis results. Head-up tilt induced the expected changes in Ethiopians (an increase in the LF components and a decrease in the respiratory components) but not in Andeans. Cross-spectral analysis showed abnormal results from all groups of high-altitude dwellers. These results indicate that Ethiopians, but not Peruvians, behave similarly to sea-level residents in terms of orthostatic tolerance and autonomic responses to orthostatic stress, as assessed from spectral analyses, and this indicates good adaptation to their environment. However, in all the high- altitude groups the results of cross-spectral analysis were atypical, suggesting some degree of impairment in baroreflex function. (Received 21 August 2006; accepted after revision 15 November 2006; first published online 30 November 2006) Corresponding author G. Gulli: Department of Neurological Sciences and Vision, Section of Physiology, University of Verona, Strada le Grazie 8, 37134 Verona, Italy. Email: giosuegulli@yahoo.it Residents in the Peruvian Andes and the highlands of Ethiopia live permanently at high altitudes. It has been suggested that the Ethiopians are the best adapted for life in hypoxic conditions (Beall et al. 2002; Beall, 2003; Appenzeller et al. 2006) showing, for instance, no evidence of chronic mountain sickness (CMS), which may be considered a maladaptation to high-altitude living (Monge & Whittembury, 1976). During the last few years, our group has investigated autonomic function of Andean high-altitude dwellers, both with and without CMS (Claydon et al. 2004, 2005; Norcliffe et al. 2005; Moore et al. 2006; Gamboa et al. 2006; Appenzeller et al. 2006). We have used an orthostatic stress test involving combined head-up tilt and lower body negative pressure, which has been widely used to assess cardiovascular and cerebrovascular control C  2007 The Authors. Journal compilation C  2007 The Physiological Society DOI: 10.1113/expphysiol.2006.035519 ) at UNIV OF WESTERN ONTARIO on October 1, 2013 ep.physoc.org Downloaded from Exp Physiol ( ) at UNIV OF WESTERN ONTARIO on October 1, 2013 ep.physoc.org Downloaded from Exp Physiol ( ) at UNIV OF WESTERN ONTARIO on October 1, 2013 ep.physoc.org Downloaded from Exp Physiol ( ) at UNIV OF WESTERN ONTARIO on October 1, 2013 ep.physoc.org Downloaded from Exp Physiol ( ) at UNIV OF WESTERN ONTARIO on October 1, 2013 ep.physoc.org Downloaded from Exp Physiol ( ) at UNIV OF WESTERN ONTARIO on October 1, 2013 ep.physoc.org Downloaded from Exp Physiol ( ) at UNIV OF WESTERN ONTARIO on October 1, 2013 ep.physoc.org Downloaded from Exp Physiol ( ) at UNIV OF WESTERN ONTARIO on October 1, 2013 ep.physoc.org Downloaded from Exp Physiol ( ) at UNIV OF WESTERN ONTARIO on October 1, 2013 ep.physoc.org Downloaded from Exp Physiol (