Brief Report Initial Orthostatic Hypotension at High Altitude Kate N. Thomas, 1 Keith R. Burgess, 2 Rishi Basnyat, 3,4 Samuel J.E. Lucas, 1 James D. Cotter, 1 Jui-Lin Fan, 1 Karen C. Peebles, 1 Rebekah A.I. Lucas, 1 and Philip N. Ainslie 5 Abstract Thomas, Kate N., Keith R. Burgess, Rishi Basnyat, Samuel J.E. Lucas, James D. Cotter, Jui-Lin Fan, Karen C. Peebles, Rebekah A.I. Lucas, Philip N. Ainslie. Initial orthostatic hypotension at high altitude. High Alt. Med. Biol. 11:163–167, 2010.–-There are several reports on syncope occurring following standing at high altitude (HA), yet description of the detailed physiological responses to standing at HA are lacking. We examined the hypothesis that appropriate physiological adjustments to upright posture would be compromised at HA (5050 m). Ten healthy volunteers stood up rapidly from supine rest, for 3 min, at sea level and at 5050 m. Beat-to-beat mean arterial blood pressure (MAP, Finometer), middle cerebral artery blood velocity (MCAv, Transcranial Doppler), end-tidal PCO 2 and PO 2 , and heart rate (ECG) were recorded continuously. After 14 days at HA, baseline MAP and MCAv were not different to sea level, although HR was elevated. Neither the magnitude of initial (<15 s) responses to standing, nor the time course of initial recovery differed at HA compared with sea level ( p > 0.05). By 3 min of standing, MAP was restored to supine values both at sea level (3  12 mmHg) and HA (4  10 mmHg), although there was more complete recovery of HR at sea level (þ13  10 b  min 1 , p ¼ 0.02 vs. þ 23  10 b  min 1 , p ¼ 0.01). Reduced MCAv at 3min was comparable at sea level and altitude (both 16%). These data indicate that initial cardio- vascular and cerebrovascular responses to standing are unaltered when partially acclimatized to HA. Introduction S tanding up and the associated initial orthostatic hypo- tension poses a common challenge and this represents a critical time in which homeostatic mechanisms must operate appropriately to avoid syncope (i.e., fainting). Orthostatic tolerance appears to be reduced after acute exposure to high altitude (HA, hypobaric chamber at 3660 m; <24 hours) (Blaber et al., 2003) and there have been reports of syncope upon standing at HA in otherwise healthy young individuals (Nicholas et al., 1992; Perrill, 1993; Freitas et al., 1996; Basnyat et al., 2004). It has been established that a common cause of syncope is initial orthostatic hypotension (IOH) (Wieling et al., 2007). However, potential alterations in IOH have yet to be explored following partial acclimatization to HA. Given the critical importance of maintaining adequate balance and stability during both trekking and climbing activities, the lack of knowledge about the key hemodynamic mechanisms that affect orthostatic tolerance at high altitude is somewhat sur- prising. Our objective was to evaluate how healthy lowland- ers responded to the initial orthostatic challenge of standing upright following partial acclimatization to 5050 m, compared with sea level responses. Issues with orthostatic intolerance are fundamentally due to a critical fall in cerebral perfusion, rather than systemic blood pressure per se (Van Lieshout et al., 2003; Brignole et al., 2004; Franco Folino, 2007). Since the ability of the brain to maintain blood flow independent of blood pressure (called cerebral autoregulation) is reduced at HA (Levine et al., 1999; Jansen et al., 2000; Ainslie et al., 2007), along with variable alterations in the baroreflex control of BP (Sagawa et al., 1997; Hansen & Sander, 2003; Hainsworth et al., 2007), we hypothesized that the ability to maintain an upright position would be compromised at HA. Methods Subjects Ten healthy individuals [(7 men; 3 women) aged 32  12 years (mean  SD); body mass index 22.1  1.9 kg/m 2 volunteered for this study, which was approved by the 1 University of Otago, Dunedin, New Zealand. 2 University of Sydney, Sydney, New South Wales, Australia. 3 Nepal International Clinic, Kathmandu, Nepal. 4 Himalayan Rescue Association, Kathmandu, Nepal. 5 University of British Columbia Okanagan, Kelowna, Canada. HIGH ALTITUDE MEDICINE & BIOLOGY Volume 11, Number 2, 2010 ª Mary Ann Liebert, Inc. DOI: 10.1089/ham.2009.1056 163