Abstract
To the Editor:
Kilimanjaro, at 5896 m, is famous both as 1 of the 7 summits and as an accessible trekking peak that can be climbed with minimal experience or technical skill. It is a popular destination for charity fundraisers from the United Kingdom; each year approximately 30,000 trekkers attempt the climb, 1 with success rates of 61% to 75%, and symptoms of altitude illness developing in as many as 77% of trekkers. 2
It has been suggested that low oxygen saturation may identify climbers who are operating closer to their physiological limits and are therefore less likely to achieve their summit goal. A previous study on Kilimanjaro 3 found no significant difference in resting pulse and oxygen saturation between climbers who were successful and climbers who were unsuccessful in reaching the summit, whereas a study of climbers at Aconcagua Base Camp (4365 m) found that oxygen saturation <75% after a 6-minute walk test (6MWT) could predict with 97.2% sensitivity that a climber would not reach the summit. 4
We wished to see if these results would be replicated on Kilimanjaro, where the 7-day Rongai route allowed the opportunity to repeat this test at an altitude similar to that used on Aconcagua. The subjects were all novice trekkers following the 7-day Rongai route up the mountain, with camps at 2650 m, 3678 m, 4295 m (2 nights), and 4700 m before attempting the summit. This route involves a steep ascent from 4700 m to the crater rim at Gilman's Point and then a further 200 m ascent around the crater rim to the true summit at Uhuru. Although this route involves an additional day of acclimatizing at 4295 m, the incidence of altitude sickness among trekkers is not insignificant, and a significant percentage of trekkers fail to reach the summit.
All 26 members of the party were invited to take part in the study, and 18 agreed to do so. After a night at 4365 m and a morning resting, the volunteers were asked to carry out a 6MWT after having baseline pulse and pulse oximetry carried out with the same Nonin Onyx pulse oximeter (Nonin Medical Inc, Plymouth, MN). This test involved walking as fast as possible between 2 points 25 m apart over a 6-minute period. Pulse rate and pulse oxymetry were then repeated immediately after completing the test. The trekkers were not told their results, and were then followed up as they continued their ascent of the mountain and were stratified according to altitude reached as unsuccessful, successful in reaching Gilman's Point only, or successful in reaching Uhuru.
On returning to the United Kingdom, the groups were compared to see if there were significant differences between trekkers who reached the summit at 5896 m (Uhuru), trekkers who failed to reach the summit but reached the crater rim (Gilman's point at 5681 m), and climbers who failed to reach either. Baseline pulse oximetry, baseline pulse, pulse immediately after 6MWT, pulse oximetry immediately after 6MWT, and percent change in pulse and pulse oximetry were all compared. Finally, the act of participating in the trial was looked at to see whether climbers who chose not to volunteer were actually having symptoms of altitude illness at the time, and therefore were less likely to summit. The results are shown in the Table.
Mean resting pulse and resting oxygen saturation before 6-minute walk test, mean pulse and oxygen saturation immediately after 6-minute walk test, and percentage change in mean pulse and oxygen saturation
Eighty-five percent of the group as a whole reached Gilman's Point or higher, and 62% reached Uhuru. Of the volunteers who took the test, 89% reached Gilman's Point or higher, and 67% reached Uhuru. The data showed no significant difference between successful and unsuccessful trekkers in pulse rate and oximetry either before or after a 6MWT. A wide range of values was recorded for subjects from all 3 groups, and there was considerable overlap between groups, with no single group having significantly different results. One trekker had oxygen saturation of only 68% after the 6MWT and reached the summit at Uhuru, whereas another had a saturation of 78% and failed to reach Gilman's Point. Furthermore, 50% of trekkers not taking part in the trial reached Uhuru and only 25% of trekkers not taking part failed to reach the crater rim at Gilman's point. These results compared well with the results for participants in the trial and suggest there was no significant difference.
These data are extremely limited owing to the small sample sizes, particularly in the cohort who failed to summit, but suggest that there is little difference between these groups and that predicting summit success with pulse oximetry is unlikely to be effective on Kilimanjaro. The results on Aconcagua, although obtained at a similar altitude, reflect climbers with a very different ascent and acclimatization profile as they are more likely to be experienced climbers—and clearly, the results are not applicable to persons climbing Kilimanjaro—but it would seem from this small pilot study that exercise testing is unlikely to have predictive value.
There are many possible reasons for this, but one of the most likely is that, on a climb where symptoms of altitude illness are nearly universal, psychological factors and, in particular, the ability to tolerate mildly unpleasant symptoms of altitude sickness determine whether one will successfully summit. That may be particularly important for teams of charity trekkers who support each other and have strong motivation to succeed. It would be interesting to repeat this study with larger cohorts, and to compare solo and group trekkers.
Footnotes
Acknowledgments
I would like to thank Dr John Simpson for his advice, Ultimate Travel Company for organizing the trip, and the team raising money from Evelina Children's Hospital who volunteered to take part.