Degree of Hypoxia and Physiological Differences Between Fast and Slow Ascents to Very High Altitude

INTRODUCTION Rapid ascent to altitudes of over 5,000m above sea level are associated with dramatic changes in adaptive physiology. The effects of a gradual ascent on symptoms, oximetry and heart rate are described, and compared with the effects of a rapid ascent to the same altitude by a comparable cohort. METHODS A group of 13 (6 females) representing 10 countries from 5 continents, ascended gradually from Lukla (2,300m) to Everest Base Camp (5,300m) in Nepal over an 8-day period, then descended over a further 4 days. All symptoms and medication were recorded, along with pulse oximetry and heart rate (HR) every 500m of ascent. The results were then compared with those obtained at equivalent altitudes using similar methodology from a fast ascent of Mount Kilimanjaro to an equivalent altitude by a comparable cohort over 4 days. RESULTS The gradual ascent group had a median age of 33 years (range 25-66), and all successfully completed the trek. No severe headache, vomiting, orthopnoea nor productive cough occurred, although minor nausea and mild headache was common. Baseline oximetry fell from a median of 96% (93-97%) to a median of 78% (53-86%) at 8 days but recovered to 94% (89-99%) inside 4 days. Corresponding HR rose from a baseline median of 72bpm (57-85) to a median of 103bpm (78-115) at 8 days, then recovered to 80bpm (54-94) after 4 days. Neither age nor gender correlated with outcomes. Individually, HR correlated inversely with oximetry, but there was no group correlation between these two variables. By contrast, a more rapid 4-day ascent from the same starting height, with similar baseline values for HR and oximetry, to the same final altitude was associated with more severe headache, breathlessness, and vomiting. The fast ascent was associated with a significantly more marked reduction in oximetry to a median of 71% (52-76) and an increase in HR to a median of 110bpm (88-140). The fast ascent group also required significantly more medication, rated their experience as less enjoyable, and had a 100% incidence of acute mountain sickness compared to 0% in the slow ascent group. DISCUSSION Oxygen desaturation and tachycardia are inevitable consequences of ascending above 5,000m but the degree to which this occurs can be reduced by slowing ascent times and taking rest days every 1,000m of ascent. This practice is associated with fewer symptoms and greater safety, with less need for either prophylactic or therapeutic medication. Careful consideration should be given to rates of ascent when climbing to altitudes at or above 5,000m.