Original article Closed and open breathing circuit function in healthy volunteers during exercise at Mount Everest base camp (5300 m) R. C. N. McMorrow, 1,4 J. S. Windsor, 1,5 N. D. Hart, 1,6 P. Richards, 1,7 G. W. Rodway, 1,8 V. Y. Ahuja, 1,9,10 M. J. O’Dwyer, 1,11 M. G. Mythen, 2,12 M. P. W. Grocott 3,13,14 and the Caudwell Xtreme Everest Research Group* 1 Research Fellow, 2 Professor, 3 Director, UCL Centre for Altitude Space and Extreme Environment Medicine, UCL Institute of Child Health, University College London, London, UK 4 Consultant, Department of Anaesthesia, National Maternity Hospital & St Vincent’s University Hospital, Dublin, Ireland 5 Specialist Registrar, Department of Anaesthesia, Barnet and Chase Farm Hospitals NHS Trust, Barnet, UK 6 Senior Lecturer, Centre for Public Health, School of Medicine, Dentistry and Biomedical Sciences, Queen’s University Belfast, N. Ireland 7 General Practitioner, South Essex PCT, Wickford, Essex, UK 8 Assistant Professor, University of Utah College of Nursing and School of Medicine, Salt Lake City, USA 9 Surgeon Lieutenant, Royal Navy, UK 10 General Duties Medical Officer, RMB Chievenor, UK 11 Senior Lecturer, Intensive Care Medicine, Barts and the London Queen Mary’s School of Medicine and Dentistry, University of London, London, UK 12 Smiths Medical Professor of Anaesthesia and Critical Care, UCL ⁄ UCL Hospitals, National Institute of Health Research Comprehensive Biomedical Research Centre, London, UK 13 Professor of Anaesthesia and Critical Care Medicine, Division of Integrative Physiology, University of Southampton, Southampton, UK 14 Consultant Anaesthetist, Department of Critical Care, Southampton University Hospitals NHS Trust, Southampton, UK Summary We present a randomised, controlled, crossover trial of the Caudwell Xtreme Everest (CXE) closed circuit breathing system vs an open circuit and ambient air control in six healthy, hypoxic volunteers at rest and exercise at Everest Base Camp, at 5300 m. Compared with control, arterial oxygen saturations were improved at rest with both circuits. There was no difference in the magnitude of this improvement as both circuits restored median (IQR [range]) saturation from 75%, (69.5–78.9 [68–80]%) to > 99.8% (p = 0.028). During exercise, the CXE closed circuit improved median (IQR [range]) saturation from a baseline of 70.8% (63.8–74.5 [57–76]%) to 98.8% (96.5–100 [95–100]%) vs the open circuit improvement to 87.5%, (84.1–88.6 [82–89]%; p = 0.028). These data demonstrate the inverse relationship between supply and demand with open circuits and suggest that ambulatory closed circuits may offer twin advantages of supplying higher inspired oxygen concentrations and ⁄ or economy of gas use for exercising hypoxic adults. ............................................................................................................................................................... Correspondence to: R. C. N. McMorrow Email: mcmorrow.roger@gmail.com *Members listed in Appendix 1. Accepted: 3 March 2012 Anaesthesia 2012 doi:10.1111/j.1365-2044.2012.07152.x Anaesthesia ª 2012 The Association of Anaesthetists of Great Britain and Ireland 1