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Oxygen and living at altitude
  1. Robert C Tasker
  1. Robert C Tasker, Department of Paediatrics, University of Cambridge School of Clinical Medicine, Addenbrooke’s Hospital, Hills Road, Cambridge CB2 2QQ, UK; rct31{at}cam.ac.uk

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San Antonio de los Cobres, Puna region in the Andes in northwest Argentina, altitude 3800 m: pulse oximetry oxyhaemoglobin saturation (Spo2) 84% and heart rate 65 beats per minute – both low for me, and certainly much lower than my level 24 h ago at sea level. I feel fine, but I do seem to be breathing at higher lung volume. My Spo2 is at the expected level for acute homeostasis at altitude (fig 1),1 2 and over the next few days it should improve to the level I would have had, had I stayed at 1600 m.3 What surprises me, though, is my travel companion, who has lived at sea level for the last 20 years, yet has a Spo2 of 93%. Could those Quechuan grandparents really have something to do with this response?

Figure 1 Pulse oximetry oxyhaemoglobin saturation (Spo2) and oxygen content in relation to altitude. The theoretical perturbation in the Spo2 curve is derived from the Kelman1 and model atmosphere2 equations and assumes a sea level alveolar arterial oxygen gradient with no homeostatic response. The homeostasis Spo2 curve is based on the degree of hypocapnic alkalosis expected on the first day at 4300 m.3 The adaptation Spo2 curve is the meta-regression curve for mean Spo2 at altitude described by Subhi and colleagues.8 The dot indicated by the arrow shows my Spo2 within 24 h of arriving at 3800 m. The adaptation oxygen content curve combines the exponential regression curve for haemoglobin adaptation in Ecuadorian preschool children living at altitudes ranging from sea level to high altitude15 with the adaptation Spo2 curve in the standard equation for oxygen content.10 The adaptation curves shows that despite a fall in mean Spo2 with altitude, oxygen content at the higher altitudes increases to as much as 15% above sea level values.

Man’s migration across the planet has been mainly along coastal regions, avoiding inhospitable habitats such as mountains, desert and ice.4 However, some 25 000 and 11 000 years ago human populations settled on the Himalayan and Andean plateaus, respectively. Now, 20–30 million people live at altitudes above 3000 m, with more than half of these living in the Andean region of Latin America.5 In relation to human phylogeny, the last time Caucasians, Sherpas and Quechuas shared common ancestors was over one half the age of our species ago; Himalayan and Andean highlanders share common ancestors that date back approximately one third of our species’ history.6 In the ancestral high altitude populations, centuries of …

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