ORIGINAL RESEARCH
Rate of Ascent and Acute Mountain Sickness at High Altitude Tai-Yi Hsu, MD,*† Yi-Ming Weng, MD,†‡ Yu-Hui Chiu, MD,§¶k Wen-Cheng Li, MD,†** Pang-Yen Chen, MD,§¶ Shih-Hao Wang, MD,k††‡‡ Kuo-Feng Huang, MS,‡‡§§ Wei-Fong Kao, MD,k†† Te-Fa Chiu, MD,†‡ and Jih-Chang Chen, MD†‡
Objective: To examine the effect of ascent rate on the induction of acute mountain sickness (AMS) in young adults during a climb to Jiaming Lake (3350 m) in Taiwan.
Design: Prospective, nonrandomized. Setting: Climb from 2370 to 3350 m. Participants: Young adults (aged 18 to 26 years) (N = 91) chose to participate in either the fast ascent (3 days; n = 43) or slow ascent (4 days; n = 48) group (1 and 2).
Assessment of Risk Factors: Two criteria were used to define AMS. A Lake Louise score $3 and Lake Louise criteria [in the setting of a recent gain in altitude, the presence of headache and at least 1 of gastrointestinal discomfort (anorexia, nausea, or vomiting), fatigue or weakness, dizziness or lightheadedness, or difficulty sleeping]. Main Outcome Measures: Heart rate, blood oxygen saturation (SaO2), and symptoms of AMS were monitored each morning and evening. Results: Baseline characteristics were similar between groups, except for significant differences in history of alcohol consumption (P = 0.009) and climbing experience above 3000 m (P , 0.001). The incidence of AMS was not associated with the rate of ascent. Acute mountain sickness was most prevalent in group 1 on day 2 in the evening and in group 2 on day 3 in the evening. In both groups, AMS correlated with the initial reduction in SaO2. Body mass index (BMI) .24 kg/m2 was identified as a significant risk factor for AMS. Submitted for publication July 10, 2013; accepted January 20, 2014. From the *Department of Emergency Medicine, Chang Gung Memorial Hospital, Chiayi, Taiwan; †Chang Gung University College of Medicine, Taoyuan, Taiwan; ‡Department of Emergency Medicine, Chang Gung Memorial Hospital, Linkou, Taiwan; §Department of Emergency Medicine, Mackay Memorial Hospital, Taipei, Taiwan; ¶Institute of Environmental and Occupational Health Sciences, National Yang-Ming University School of Medicine, Taipei, Taiwan; kDepartment of Emergency, School of Medicine, College of Medicine, Taipei Medical University, Taipei, Taiwan; **Department of Occupation Medicine, Chang-Gung Memorial Hospital, Keelung, Taiwan; ††Department of Emergency Medicine, Taipei Medical University Hospital, Taipei, Taiwan; ‡‡Department of Physical Education, National Taitung University, Taitung, Taiwan; and §§Department of Emergency Medicine, Taiwan Adventist Hospital, Taipei, Taiwan. The authors report no conflicts of interest. Corresponding Author: Shih-Hao Wang, MD, Department of Emergency Medicine, Taipei Medical University Hospital, No.252, Wu-Hsing St, Taipei 11031, Taiwan ([email protected]). Copyright © 2014 Wolters Kluwer Health, Inc. All rights reserved.
Clin J Sport Med Volume 25, Number 2, March 2015
Conclusions: The development of AMS was closely associated with an initial reduction in SaO2. A BMI .24 kg/m2 also contributed to the occurrence of AMS. Clinical Relevance: These findings indicate that factors other than ascent rate should be considered when trying to ameliorate the risk of AMS. Key Words: acute mountain sickness, ascent rate, heart rate, oxygen saturation, body mass index (Clin J Sport Med 2015;25:95–104)
INTRODUCTION Acute mountain sickness (AMS) is a pathophysiological symptom complex that occurs in high-altitude areas with a fall in barometric pressure and a decline in oxygen saturation. Acute mountain sickness is a major reason for high-altitude emergency rescues.1 A variety of adaptive physiological processes are engaged to alleviate the fall in oxygen delivery to tissue. The magnitude and nature of such changes are time dependent, a process known as acclimatization.2 Acute mountain sickness commonly occurs in unacclimatized individuals above an elevation of approximately 2300 m. Symptoms of AMS include headache, gastrointestinal upset, fatigue, dizziness, and sleep disturbance. If left unchecked, AMS may progress to high-altitude cerebral edema. Predicting an individual’s risk of developing AMS is challenging because of the multitude of contributing factors. Simple measurements, including age, sex, body mass index (BMI), ascent rate, sleeping altitude, history of AMS, history of cardiopulmonary disease, oxygen saturation (SaO2), and pulse rate have been evaluated and have demonstrated high variability for predicting AMS.3–7 Preventative measures have been used to reduce the incidence of AMS, including modifications in ascent rate, prolonged acclimatization schedules, and prophylactic medications. These measures are implemented before and during ascent to avoid or reduce AMS and associated symptoms. Findings from several studies suggest that a rapid rate of high-altitude ascent results in a higher incidence of AMS.8–14 The purpose of the current study was to examine the effect of ascent rate on the induction of AMS during a climb to Jiaming Lake (3350 m) in Taiwan. We hypothesized that rapid ascent was more likely to result in AMS than slower www.cjsportmed.com |
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ascent. The current investigation is the first involving young Taiwanese adults.
MATERIALS AND METHODS Study Design and Setting This was a prospective study conducted during a climb to Jiaming Lake in Taiwan from 9 to 13, July 2007. Participants were asked to complete a hike that covered 26 km and reached a maximum altitude of 3350 m. Heart rate (HR), oxygen saturation (SaO2), and signs of AMS were assessed each morning and evening throughout the hike. Signs of AMS included headache, dizziness or lightheadedness, gastrointestinal symptoms, fatigue or weakness, and difficulty sleeping. We used 2 criteria to define AMS. The Lake Louise score (AMS = score $3)15 and Lake Louise criteria [in the setting of a recent gain in altitude, the presence of headache and at least 1 of gastrointestinal discomfort (anorexia, nausea, or vomiting), fatigue/weakness, dizziness/lightheadedness, or difficulty sleeping]. Three trained emergency physicians performed all SaO2 and HR measurements using Nellcor NPB-40 SaO2 Handheld Portable Pulse Oximeters (Covidien/Tyco-Nellcor/Puritan Bennett; Pleasanton, California) when the participants were sitting at rest. Measurements were taken each morning at about 07:00 AM and evening at about 05:00 PM. Participants were not permitted to exercise on arrival at camp. A total of 91 young adults were recruited from college mountaineering clubs in Taiwan. Participants were divided into 2 groups by choice: group 1 = fast ascent (3 days; n = 43) and group 2 = slow ascent (4 days; n = 48). All participants were aware of the differences between groups before they made their choice. Participants hiked together within each group. After an overnight stay nearby, both groups began their hike from the trail head (2370 m). Group 1 climbed to a mountain shelter at an altitude of 3350 m (8.4 km) within the second day; the same
day, group 2 climbed to an altitude of 2850 m (4.28 km). During the third day, group 1 hiked 4.6 km at an altitude between 3200 m and 3400 m to the lakeside at 3310 m; group 2 continued climbing and reached the mountain shelter at 3350 m (4.12 km). On the fourth day, group 1 returned to the mountain shelter after hiking 4.6 km; group 2 hiked a total of 13.32 km, traveling from the mountain shelter to the lakeside and back to the shelter at 2850 m. On the fifth day, both groups returned to the trail head. Figure 1 summarizes the ascent protocol for the groups. At any time during the hike, participants who felt symptoms of AMS were provided with medication and appropriate supportive care. Informed consent was obtained from all participants, and the study was reviewed and approved by the Institutional Review Board at Chang Gung Memorial Hospital. Participants who were professional mountain guides and those who did not answer the questionnaire were not included in the study results. Any participants who were evacuated because of injury were also excluded from the study results.
Statistical Analysis Demographic data and participant characteristics are shown as mean 6 SD or median (interquartile range) for continuous variables and n (percent) for categorical variables. Continuous data were compared between groups by 2sample t test or Mann–Whitney U test. Categorical variables were compared by Pearson x2 test or Fisher exact test. Statistical significance was indicated by P , 0.05. Withingroup comparisons of pulse rate and SaO2 were made using paired t test and Wilcoxon sign-rank test, respectively. An adjusted P , 0.001 was considered significant for withingroup multiple comparisons. The associations between AMS and participants’ characteristics were analyzed using univariate logistic regression and generalized estimating equations model analysis. Variables found to be significantly associated with AMS (P , 0.05) in univariate model analysis were entered into the multivariate model. Statistical analyses were
FIGURE 1. A graphical representation of the ascent protocol for the fast ascent and slow ascent groups.
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Ascent Rate and Acute Mountain Sickness
TABLE 1. Demographic and Clinical Characteristics of Participants by Group (N = 91) Variable Age, y Sex Male Female Body weight, kg Body height, m BMI, kg/m2 BMI .24 kg/m2, n (%) Home altitude*, m Medical history†, n (%) Drug use within 30 d of ascent‡, n (%) Acetazolamide Steroids Asthma medication Pain reliever Other Smoker Alcohol consumer Tea consumer Coffee consumer Hypnotics Exerciser Other habit Time since last climb¶, mo Climbs within 3 mo of ascent 0 1 2 3 No. 100-peaks climbedk 0 1 2 History of AMS** Knowledge of AMS Hiking stick use None Left hand Right hand Both right and left hands Either right or left hand
Group 1 (Fast Ascent) (n = 43)
Group 2 (Slow Ascent) (n = 48)
P
19.86 6 1.54 — 29 (67.4%) 14 (32.6%) 61.73 6 9.64 1.68 6 0.07 21.79 6 2.73 9 (20.9%) 30 (10-70) 13 (30.2%) 6 (14.0%) 0 (0%) 0 (0%) 0 (0%) 3 (7.0%) 4 (9.3%) 2 (4.7%) 20 (46.5%) 37 (86.0%) 20 (46.5%) 0 (0) 36 (83.7%) 0 (0) 3 (0.5-6)
19.83 6 1.31 — 34 (70.8%) 14 (29.2%) 62.01 6 9.63 1.68 6 0.07 22.02 6 2.73 13 (27.1%) 17.5 (10-55) 23 (47.9%) 11 (22.9%) 1 (2.1%) 0 (0%) 0 (0%) 2 (4.2%) 9 (18.8%) 1 (2.1%) 10 (20.8%) 34 (70.8%) 20 (41.7%) 0 (0) 32 (66.7%) 0 (0) 3.5 (0.5-6)
0.928 0.726
19 10 11 3
(44.2%) (23.3%) (25.6%) (7.0%)
25 12 9 2
(52.1%) (25%) (18.8%) (4.2%)
0 37 6 5 38
(0%) (86.0%) (14.0%) (11.6%) (88.4%)
9 38 1 7 34
(18.8%) (79.2%) (2.1%) (14.6%) (70.8%)
9 1 17 9 7
(20.9%) (2.3%) (39.5%) (20.9%) (16.3%)
4 3 24 10 7
(8.3%) (6.3%) (50.0%) (20.8%) (14.6%)
0.891 0.728 0.687 0.494 0.092 0.085 0.273
0.601 0.009§ 0.080 0.642 NA 0.062 NA 0.860 0.770
,0.001*
0.677 0.040 0.446
Data are shown as mean 6 SD or median (interquartile rage) for continuous variables and number (percent) for categorical variables. Continuous variables were compared between groups by 2-sample t test or Manny–Whitney U test, whereas categorical variables were compared between groups by Pearson x2 test or Fisher exact test. *The range of home altitude was 0-160 m in group 1 and 0-240 m in group 2. †Including heart disease, headache, rheumatic disease, asthma, allergic rhinitis, cold within 1 month, and enterogastritis. ‡One participant in group 1 and 1 participant in group 2 had taken both types of drugs (pain reliever and other) within 30 days of the ascent. §P , 0.05 indicates a significant difference between group 1 and group 2. ¶Range = 1-96 months in group 1 and 0-60 months in group 2. kA list of 100 peaks in Taiwan with an altitude typically more than 3000 m. **Lake Louise criteria were used to diagnose AMS, that is, in the setting of a recent gain in altitude, AMS was indicated by the presence of headache with one of the following symptoms: gastrointestinal (anorexia, nausea, or vomiting), fatigue/weakness, dizziness/light-headedness, or difficulty sleeping.
performed using SPSS 15.0 statistical software (SPSS, Inc, Chicago, Illinois). Posteriori analysis revealed that the statistical power for testing the null hypothesis was 5%, assuming a type I error probability of 0.05. Copyright Ó 2014 Wolters Kluwer Health, Inc. All rights reserved.
RESULTS The participants in both groups were of similar age, sex, body weight, height, and BMI (Table 1). There was no difference in medical history between groups. Participants’ www.cjsportmed.com |
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TABLE 2. Summary of AMS Scores for Patients Who Developed AMS During the Ascent No. (Sex/Age)
Day 1 Day 2 Day 3 Day 4 Day 5 At Least Once, Score ‡3 Morning Evening Morning Evening Morning Evening Morning Evening Morning Evening
Group 1 (fast ascent)
#1 (M/20 y) #2 (M/20 y) #3 (M/19 y) #4 (M/19 y) #5 (M/20 y) #6 (M/21 y) #7 (F/19 y) #8 (F/18 y) #9 (M/19 y) #10 (M/20 y) Group 2 (slow ascent) #1 (M/22 y) #2 (M/19 y) #3 (M/19 y) #4 (F/20 y) #5 (M/19 y) #6 (M/19 y) #7 (M/20 y) #8 (M/19 y) #9 (F/22 y)
Yes Yes Yes No No No Yes Yes No No Yes Yes No Yes Yes Yes Yes Yes No
2 0 0 0 0 0 0 2 0 0 0 0 0 0 0‡ 0 0 0 0
2 0 0 1 0 2 0 2 0 1 1 0 0 0 0‡ 0 0 0 0
0 0 0 0 0 0 0 1 0 0 0 0 0 1 0‡ 0 0 0 0
2* 2 3 2 2 2 5 3 2 2 0 0 1 0 0‡ 0 0 0 0
4* 3† 2† 1 2 1 0 0 0 1 0 0 1 1 0 0 0 0 0
0 0 0 2 0 0 0 0 0 0 5*† 8*† 2 3 3 3 3 3 2
0 0 0 0 0 0 0 0 0 0 2 0 1 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
AMS was identified as subjects with Lake Louise score $3. *Acetaminophen. †Acetazolamide (Diamox). ‡Drugs for common cold. M, male; F, female; y, years old.
knowledge of AMS and personal history were also similar between groups. Reported history of alcohol consumption was significantly higher in group 1 compared with group 2 (P = 0.009). Significantly more participants in group 1 reported experience of climbing above 3000 m in the previous 3 months compared with participants in group 2 (P , 0.001). The incidence of AMS was not associated with the rate of ascent and was similar between groups (Lake Louise AMS criteria: group 1, n = 10; group 2, n = 9; Lake Louise score $3: group 1, n = 5; group 2, n = 7). The highest incidence of AMS in group 1 (n = 5) occurred during the first half of hike (day 1-3), whereas the highest incidence of AMS in group 2 (n = 7) occurred during the second half of the hike (day 3-5). The highest incidence of AMS in group 1 (7%; 3/43) occurred on the
evening of day 2, whereas, the highest incidence of AMS in group 2 (12.5%; 7/48) occurred on the evening of day 3. Individual participant Lake Louise scores for participants who developed AMS and the incidence of AMS (as determined by Lake Louise criteria) at each assessment point are summarized by group in Tables 2 and 3. Table 2 also highlights the medications taken by participants who developed AMS. There was a significant difference in BMI between participants who were AMS positive and AMS negative (P = 0.001), with a higher proportion of participants with a BMI .24 kg/m2 being AMS positive compared with those who had a BMI #24 kg/m2 (Table 4). The development of AMS was associated with an initial decrease in SaO2 (Figure 2). Mean SaO2 was significantly different between groups 1 and 2 on the evening of day 2
TABLE 3. Summary of the Incidence of AMS From Day 1 to Day 5 Day 1 Group 1 (fast ascent) (n = 43) Group 2 (slow ascent) (n = 48)
Day 2
Day 3
Day 4
Day 5
Morning
Evening
Morning
Evening
Morning
Evening
Morning
Evening
Morning
Evening
0 0
1 0
0 0
8* 0
5* 0
0 9†
0 1†
0 0
0 1
0 0
Data are summarized as the number of participants with AMS. Lake Louise criteria were used to diagnose AMS, that is, in the setting of a recent gain in altitude, AMS was indicated by the presence of headache with one of the following symptoms: gastrointestinal (anorexia, nausea, or vomiting), fatigue/weakness, dizziness/light-headedness, or difficulty sleeping. *Four participants experienced repeated AMS in the evening of day 2 and in the morning of day 3. A total 10 participants in this group experienced AMS. †One participant experienced repeated AMS in the evening of day 3 and in the morning of day 4. A total of 9 participants in this group experienced AMS.
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Ascent Rate and Acute Mountain Sickness
TABLE 4. Characteristics of Participant Stratified by AMS Status (N = 91) AMS Negative AMS Positive (n = 72) (n = 19) Group 1 2 Sex Male Female BMI #24 kg/m2 .24 kg/m2 Blood type A B AB O History of AMS Yes No Mountain climbs within 3 mo of ascent 0 1 2 3 100-peaks* climbed within 3 mo of ascent 0 1 2
P 0.616
33 (76.7%) 39 (81.3%)
10 (23.3%) 9 (18.7%)
48 (76.2%) 24 (85.7%)
15 (23.8%) 4 (14.3%)
60 (87.0%) 12 (54.5%)
9 (13.0%) 10 (45.5%)
21 14 4 33
3 4 0 12
0.302
0.001*
0.495 (87.5%) (77.8%) (100%) (73.3%)
(12.5%) (252.2%) (0%) (26.7%) 0.706
9 (75.0%) 63 (79.7%)
3 (25.0%) 16 (20.3%) 0.615
36 16 15 5
(81.8%) (72.7%) (75.0%) (100%)
8 6 5 0
(18.2%) (27.3%) (25.0%) (0%) 0.523
7 (77.8%) 58 (77.3%) 7 (100%)
2 (22.2%) 17 (22.7%) 0 (0%)
Data are summarized number (percent) and were compared between groups by Pearson x2 test or Fisher exact test. *A list of 100 peaks in Taiwan with an altitude typically more than 3000 m.
(88.33 6 3.94 vs 91.90 6 3.32, P , 0.001), the morning of day 3 (89.00 6 3.79 vs 92.10 6 3.20, P , 0.001), and on day 5 (89.60 6 3.82 vs 92.76 6 3.15, P , 0.001). Mean pulse rate was significantly different between groups 1 and 2 (Figure 3) on day 3 (morning: 103.37 6 14.59 vs 96.87 6 15.02, P = 0.043; evening: 100.09 6 14.67 vs 107.26 6 14.17, P = 0.021) and day 4 (morning: 93.44 6 14.54 vs 107.54 6 14.33, P , 0.001; evening: 94.49 6 10.77 vs 104.85 6 12.02, P , 0.001). Participants who developed AMS had significantly lower SaO2 levels than participants who did not develop AMS in the evening of day 3 and in the morning of day 5 (day 3 morning: 88.11 6 5.31 vs 84.37 6 6.73, P , 0.05; day 5 morning: 91.65 6 3.57 vs 89.24 6 4.31, P , 0.05; Figure 4). Univariate regression analysis revealed that BMI .24 kg/m2, a lack of habitual exercise, decreased SaO2, and decreased pulse rate were significantly associated with AMS (all P , 0.05, Table 5). Body mass index .24 kg/m2, exercise, and SaO2 were also found to be significantly associated with AMS on subsequent multivariate analysis (all P , 0.05, Table 5). There were no between-group differences in the use of medications for prophylactic purposes or in response to symptoms associated with AMS at any point during the study. During the study, 20 of 43 participants in group 1 (46.5%) and 21 of 48 participants in group 2 (43.8%) received medications for prophylactic purposes. Table 6 summarizes SaO2 levels by BMI and AMS status. Of the participants who experienced AMS, those with a BMI .24 kg/m2 had significantly lower SaO2 levels in the evening of day 1 compared with those with a BMI #24 kg/m2 (P , 0.05). Table 7 had shown that the mean SaO2 in BMI #24 subjects was significantly higher at day 1, day 3, and day 4 evening as comparing with BMI .24 kg/m2 subjects (all P , 0.05).
FIGURE 2. Summary of SaO2 levels during the ascent as stratified by group. SaO2 was recorded each morning before departure and each evening after arriving at the shelter. Data are presented as mean 6 SD and were compared by Mann– Whitney U test. ***P , 0.001 indicates a significant difference between group 1 and group 2. †Significant difference compared with morning on day 2 in group 1. ‡Significant difference compared with morning on day 3 in group 1. §Significant difference compared with morning on day 2 in group 2. ¶Significant difference compared with morning on day 3 in group 2. Copyright Ó 2014 Wolters Kluwer Health, Inc. All rights reserved.
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FIGURE 3. Summary of pulse rate during the ascent as stratified by group. Pulse rate was recorded each morning before departure and each evening after arriving at the shelter. Data are presented as mean 6 SD and were compared between groups by 2-sample t test and within groups by pair t test. *P , 0.05, ***P , 0.001 indicate a significant difference between group 1 and group 2. †Significant difference compared with morning on day 2 in group 1. ‡Significant difference compared with morning on day 3 in group 1. §Significant difference compared with morning on day 2 in group 2.
DISCUSSION This study was designed to test the hypothesis that rapid ascent at high altitude is more likely to result in AMS than slow ascent in young adults. The occurrence of AMS was not associated with the rate of ascent and was similar between the rapid and slow ascent groups. We found that there was close association between the development of AMS and the initial reduction in SaO2. The development of AMS and the initial reduction in SaO2 correlated with reaching an altitude of 3350 m. Body mass index (.24 kg/m2) was the only significant predictor of AMS in our study population of young adults. Other factors, often suggested to be predictors of AMS such as HR and climbing history were not found to be associated with the development of AMS. To the best of our knowledge, this is the first study to investigate the onset of AMS in a population of young adults. It is believed that rapid ascent to high altitude may result in a higher incidence of AMS.8–14 Schneider et al16
reported that 58% of individuals who rapidly ascended Capanna Margherita (4559 m) developed AMS, whereas, 33% of those who ascended at a slower rate developed AMS. Hackett et al17 also reported an increased incidence of AMS among participants who rapidly ascended Pheriche in the Himalayas (4243 m). The lack of an effect of ascent rate in the current study may be because of the age of the climbers or the maximum altitude attained. Research has demonstrated that 34% of individuals ascending the Swiss Alps (3650 m)18 and 81% ascending Mt. Mauna Kea in Hawaii (4205 m)19 develop AMS. Furthermore, extremely fast ascent (through direct flight) to 3740 m in the Himalayas resulted in 84% of participants developing AMS.9 In the current study, 23.3% (10/43) of participants in the fast ascent group developed AMS, whereas 18.8% (9/48) of participants in the slow ascent group developed AMS. There was no significant between-group difference in the AMS rate.
FIGURE 4. Summary of SaO2 levels during the ascent as stratified by acute mountain sickness (AMS) status (positive, +; or negative, 2). SaO2 was recorded each morning before departure and each evening after arriving at the shelter. Data are presented as mean 6 SD and were compared between AMS+ and AMS2 subjects by 2-sample t test. *P , 0.05 indicates a significant difference between the AMS+ group and AMS2 subjects.
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Ascent Rate and Acute Mountain Sickness
TABLE 5. Summary of Univariate and Multivariate Analyses of Variables Associated With AMS (n = 91) Univariate Variable Group 1 2 Age, y Sex Male Female BMI .24 kg/m2 #24 kg/m2 Blood type A B AB O Home altitude, m Medical history Smoker Alcohol consumer Tea consumer Coffee consumer Exerciser Time since last climb, mo Knowledge of AMS History of AMS Yes No Climbs within 3 mo of ascent 0 1 2 3 100-peaks climbed within 3 mo 0 1 2 Hiking stick use None Left hand Right hand Both right and left hands Either right or left hand SaO2, % Pulse rate, frequency per minute
Multivariate
OR (95% CI)
P
OR (95% CI)
1.313 (0.477-3.616) Reference 0892 (0.600-1.328)
0.598
—
0.574
—
1.875 (0.561-6.268) Reference
0.307
—
5.556 (1.862-16.577) Reference
0.002*
5.231 (1.664-16.439)
Reference 2.000 (0.387-10.337) ND 2.545 (0.641-10.102) 1.001 (0.990-1.011) 0.646 (0.221-1.893) 1.944 (0.167-22.662) 1.243 (0.432-3.571) 1.071 (0.312-3.684) 1.190 (0.432-3.282) 0.268 (0.092-0.784) 0.944 (0.840-1.062) 0.676 (0.208-2.191)
— — — — — — 0.242 (0.076-0.773) — —
1.312 (0.318-5.415) Reference
0.707
—
ND ND ND Reference
NA NA NA
—
ND ND Reference
NA NA
—
0.315 0.923 0.347 0.560 0.013* ,0.001*
— — — — 0.924 (0.875-0.976) 0.983 (0.959-1.008)
Reference (0.319-34.830) (0.246-4.695) (0.056-2.763) (0.007-4.009) (0.889-0.986) (0.159-0.438)
0.005*
— 0.408 NA 0.184 0.873 0.426 0.596 0.687 0.913 0.736 0.016* 0.338 0.514
3.333 1.075 0.392 0.556 0.936 0.264
P
0.017*
0.005* 0.189
Results are summarized as OR with the 95% CI and P value. Univariate analysis was performed using binary logistic regression model analysis for all variables except for SaO2 and pulse rate because of the repeated measurements, for which generalized estimating equations (GEE) model analysis was performed. Variables demonstrating significance in the univariate analysis (P , 0.05) were entered into a multivariate analysis model (GEE model analysis). *Significant association with AMS (P , 0.05). CI, confidence interval; NA, not assessed; ND, not derived; OR, odds ratio.
Manipulating the rate of ascent may be dependent on the availability of adequate shelter. On the trek to Jiaming Lake, shelters were available at altitudes of 2850 m and 3350 m. A high rate of AMS has been documented in trekkers Copyright Ó 2014 Wolters Kluwer Health, Inc. All rights reserved.
climbing mountains such as Mt. Kilimanjaro (75%) despite the lack of extreme conditions (direct flight or rapid transport from sea level).4,9,19 The high rate of AMS may be the result of mountain lodges being separated by 1000 m or more.4 www.cjsportmed.com |
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TABLE 6. Summary of SaO2 Levels by BMI and AMS Status Day 1 Participants with AMS BMI #24 kg/m2 (n = 9) BMI .24 kg/m2 (n = 10) Participants with BMI .24 AMS2 (n = 12) AMS+ (n = 10)
Day 2
Day 3
Morning
Evening
Morning
Evening
Morning
Evening
97.89 6 1.54 97.3 6 1.06
94.33 6 2.12 92.1 6 2.33*
94 6 2.96 91.5 6 3.5
89.44 6 3.47 88 6 4.99
90 6 3.28 89.2 6 5.79
86.22 6 5.76 82.7 6 7.38
98 6 1 97.3 6 1.06
92.92 6 2.61 92.1 6 2.33
93.58 6 1.73 91.5 6 3.5
89.83 6 5.1 88 6 4.99
91.17 6 4.22 89.2 6 5.79
85.58 6 8.01 82.7 6 7.38
Day 4 Participants with AMS BMI #24 kg/m2 (n = 9) BMI .24 kg/m2 (n = 10) Participants with BMI .24 AMS2 (n = 12) AMS+ (n = 10)
Day 5
Morning
Evening
Morning
Evening
90.11 6 6.15 86.8 6 6.91
90.22 6 4.27 87.5 6 7.28
89.29 6 3.82 89.2 6 4.83
94.67 6 2.06 94.4 6 1.07
88.58 6 5.02 86.8 6 6.91
89.36 6 3.59 87.5 6 7.28
91.42 6 3.23 89.2 6 4.83
94.83 6 1.64 94.4 6 1.07
Data are summarized as mean 6 SD. *P , 0.05 indicates significant difference between BMI #24 kg/m2 and BMI .24 kg/m2.
Thus, it may be impossible for trekkers to ascend gradually after traveling above 3000 m to avoid ascending greater than 300 m per day or to prolong stays at specific altitudes in an effort to decrease the risk of AMS. Trekkers who climb such routes are forced to make rapid ascents and may experience a higher risk of AMS. Thus, the availability of lodging facilities may influence the ascent rate and further affect the incidence of AMS. Clearly, the availability of trail and lodging facilities is an important consideration when planning a hike at altitude. It has been proposed that the best method for preventing altitude illness is to ascend slowly to allow for adequate acclimatization.20 Furthermore, consensus guidelines established by the Wilderness Medical Society state that controlling the rate of ascent in terms of the number of meters gained per day is a highly effective means of preventing acute altitude illness.21 In planning the rate of ascent, the altitude at which someone sleeps is considered more important than the altitude reached during waking hours. Considering the results from the current analysis, where the incidence of AMS
increased between the altitudes of 2370 m and 3350 m, it may be helpful to schedule an additional overnight stay at an altitude of 2850 m. The only significant predictor of AMS in the current study was BMI. A BMI .24 kg/m2 was associated with a significantly greater chance of developing AMS. Ri-Li et al6 reported similar results in obese versus nonobese participants who were compared at a simulated altitude of 3658 m. The investigators found that AMS scores increased significantly more rapidly with time spent at simulated high altitudes for obese compared with nonobese men. After 24 hours in an altitude chamber, 7 obese men (78%) and 4 nonobese men (40%) had AMS scores of 4 or more. Honigman et al22 also reported an increased likelihood of AMS in obese individuals when trekking at altitude. However, other researchers have reported that there is no relationship between obesity and the development of AMS.16,23,24 The cutoff of 24 kg/m2 used in the current study has been previously used in a study examining obesity as a risk factor for AMS.25 The available data do
TABLE 7. Dispersion of SaO2 During the Climbing Period Day 1 SaO2 BMI #24 kg/m2 (n = 69) BMI .24 kg/m2 (n = 22)
Day 2
Day 3
Morning
Evening
Morning
Evening
Morning
Evening
97.49 6 2.2 97.67 6 1.06
94.23 6 1.98 92.55 6 2.46*
93.36 6 2.54 92.64 6 2.82
90.59 6 3.61 89 6 5.01
90.75 6 3.39 90.27 6 4.97
88.31 6 4.71 84.27 6 7.69*
Day 4 SaO2 BMI #24 kg/m2 (n = 69) BMI .24 kg/m2 (n = 22)
Day 5
Morning
Evening
Morning
Evening
90.2 6 3.84 87.77 6 5.87
91.5 6 3.59 88.48 6 5.58*
91.43 6 3.73 90.41 6 4.09
94.84 6 1.77 94.64 6 1.40
Data were represented as mean 6 SD for BMI #24 and BMI .24 kg/m2 subjects, separately. *Mean SaO2 in BMI #24 subjects was significantly higher at day 1, day 3, and day 4 evening as comparing with BMI .24 kg/m2 subjects (all P , 0.05).
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not clearly indicate a cutoff point for obesity/overweight in Asians. It has been proposed by the World Health Organization Expert Technical Consultation for Obesity that a BMI greater than 23 kg/m2 and less than 25 kg/m2 classifies an individual of Asian descent as being overweight or obese.26 An individual with a higher BMI may be less physically fit and require a greater level of exertion (relative to a leaner individual) when ascending. This increased exertion may influence the development of AMS. However, we did not observe subjects occurred with the dyspnea conditions such as high-altitude pulmonary edema during climbing. Our finding that reduced SaO2 was associated with the development of AMS is consistent with previously reported findings. Karinen et al4 recently reported that climbers who maintained rest level oxygen saturation during ascent at altitude and especially during exercise were less likely to develop AMS. Daily evaluation of SaO2 during ascent, both at rest and during exercise, may help to identify climbers at risk for AMS.4 Other studies have reported that SaO2 is a predictor (with as high as 86% predictive value) of AMS susceptibility.7,27 The most commonly examined predictive factors for AMS during high-altitude trekking generally fall into the categories of participant characteristics, lifestyle, climbing history, and climbing plan. Schneider et al16 assessed AMS in climbers arriving at Capanna Margherita (4559 m) and found that the 3 independent determinants of susceptibility to AMS were history of AMS, rate of ascent, and altitude preexposure. Wang et al11 confirmed that a lower incidence of AMS was correlated with high-altitude trekking experience or preexposure, whereas a higher incidence of AMS was correlated with a history of AMS. In the current study, none of these previously reported risk factors were associated with the development of AMS. Incorporating sufficient time for adequate acclimatization to altitude can often be problematic for novice climbers or those on a tight schedule. Climbers may also underestimate the importance of taking extra time during ascent to acclimatize properly, whether it means hiking more slowly or incorporating additional rest days at altitude. If precautions are not taken, the development of AMS may lead to the need for emergency rescue so that appropriate medical care can be administered.1 Five or more days spent above 3000 m in the preceding 2 months is considered to be sufficient altitude preexposure.28 Independent of known susceptibility, both adequate altitude preexposure and slow ascent have been shown to reduce the prevalence of AMS by around 50%.16 An additional factor that should be considered when planning a rapid ascent to high altitude is current guidelines that propose medical prophylaxis for AMS.29 It should be noted, however, that such medication can have adverse effects.29 Our study has several limitations. First, allocation to the fast and slow ascent groups was not randomized, which may have introduced selection bias in that individuals with higher level of fitness may have chosen the faster ascent. Second, although the number of days to reach the summit was different between the groups, the ascent rate during each day was not standardized, that is, the level of physical Copyright Ó 2014 Wolters Kluwer Health, Inc. All rights reserved.
Ascent Rate and Acute Mountain Sickness
exertion may have differed significantly between groups and thus may have been a confounding factor. Third, the effects of medications taken during ascent were not controlled for. Fourth, between-group differences in history of alcohol consumption and climbing experience above 3000 m may have influenced the reported outcomes as these variables have been suggested to affect the development of AMS (however, neither were found to be associated with AMS in our regression analyses). Fifth, the hydration status of the participants may have affected our findings, that is, participants in the fast ascent group may have been less hydrated because of the increased workload intensity. Finally, people with higher BMI may have higher relative exercise intensity during ascent, but we did not perform a good monitor to evaluate exercise intensity.
CONCLUSIONS In this study, the development of AMS was closely associated with an initial reduction in SaO2, which correlated with the time to reach shelter at 3350 m. Among the risk factors identified in previous reports for developing AMS, only a BMI .24 kg/m2 was found to be a significantly predictor of AMS in this study. Future studies should attempt to validate these findings in young adults in other high-altitude settings, while taking participants’ climbing experience, acclimatization, ascent rate, and SaO2 into account.
ACKNOWLEDGMENTS The authors thank Mr. Yu-Chou Wu, Chief of Campus Security Report Center at Ministry of Education, Taiwan, R.O.C., for the time he spent planning the hiking trips. The authors also thank Dr. Ching-Chien Yang and Dr. ChinHung Liu, Department of Pediatrics, Chang Gung Memorial Hospital, for their help in the analysis and collection of the data. REFERENCES 1. Wang SH, Hsu TY, Kuan JT, et al. Medical problems requiring mountain rescues from 1985 to 2007 in Yu-Shan National Park, Taiwan. High Alt Med Biol. 2009;10:77–82. 2. Martin DS, Levett DZ, Grocott MP, et al. Variation in human performance in the hypoxic mountain environment. Exp Physiol. 2010;95: 463–470. 3. O’Connor T, Dubowitz G, Bickler PE. Pulse oximetry in the diagnosis of acute mountain sickness. High Alt Med Biol. 2004;5:341–348. 4. Karinen HM, Peltonen JE, Kähönen M, et al. Prediction of acute mountain sickness by monitoring arterial oxygen saturation during ascent. High Alt Med Biol. 2010;11:325–332. 5. Moraga FA, Pedreros CP, Rodriguez CE. Acute mountain sickness in children and their parents after rapid ascent to 3500 m (Putre Chile). Wilderness Environ Med. 2008;19:287–292. 6. Ri-Li G, Chase PJ, Witkowski S, et al. Obesity: associations with acute mountain sickness. Ann Intern Med. 2003;139:253–257. 7. Burtscher M, Szubski C, Faulhaber M. Prediction of the susceptibility to AMS in simulated altitude. Sleep Breath. 2008;12:103–108. 8. Maloney JP, Broeckel U. Epidemiology, risk factors, and genetics of high-altitude-related pulmonary disease. Clin Chest Med. 2005;26: 395–404.
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9. Murdoch DR. How fast is too fast? Attempts to define a recommended ascent rate to prevent acute mountain sickness. Newsletters Int Soc Mountain Med. 1999;9:3–6. 10. Pradhan S, Yadav S, Neupane P, et al. Acute mountain sickness in children at 4380 meters in the Himalayas. Wilderness Environ Med. 2009;20:359–363. 11. Wang SH, Chen YC, Kao WF, et al. Epidemiology of acute mountain sickness on Jade Mountain, Taiwan: an annual prospective observational study. High Alt Med Biol. 2010;11:43–49. 12. Zafren K, Honigman B. High-altitude medicine. Emerg Med Clin North Am. 1997;15:191–222. 13. Bloch KE, Turk AJ, Maggiorini M, et al. Effect of ascent protocol on acute mountain sickness and success at Muztagh Ata, 7546 m. High Alt Med Biol. 2009;10:25–32. 14. Purkayastha SS, Ray US, Arora BS, et al. Acclimatization at high altitude in gradual and acute induction. J Appl Physiol (1985). 1995; 79:487–492. 15. Hackett P, Oelz O. The Lake Louise consensus on the definition and quantification of altitude illness. In: Hypoxia and Mountain Medicine. Sutton JR, Coates G, Houston CS, eds. Burlington, VT: Queen City Printers; 1992:327–330. 16. Schneider M, Bernasch D, Weymann J, et al. Acute mountain sickness: influence of susceptibility, preexposure, and ascent rate. Med Sci Sports Exerc. 2002;34:1886–1891. 17. Hackett PH, Rennie D, Levine HD. The incidence, importance, and prophylaxis of acute mountain sickness. Lancet. 1976;2:1149–1155. 18. Maggiorini M, Buhler B, Walter M, et al. Prevalence of acute mountain sickness in the Swiss Alps. BMJ. 1990;301:853–855.
19. Gertsch JH, Seto TB, Mor J, et al. Ginkgo biloba for the prevention of severe acute mountain sickness (AMS) starting one day before rapid ascent. High Alt Med Biol. 2002;3:29–37. 20. Barry PW, Pollard AJ. Altitude illness. BMJ. 2003;326:915–919. 21. Luks AM, McIntosh SE, Grissom CK, et al. Wilderness Medical Society consensus guidelines for the prevention and treatment of acute altitude illness. Wilderness Environ Med. 2010;21:146–155. 22. Honigman B, Theis MK, Koziol-McLain J, et al. Acute mountain sickness in a general tourist population at moderate altitudes. Ann Intern Med. 1993;118:587–592. 23. Wagner DR, Fargo JD, Parker D, et al. Variables contributing to acute mountain sickness on the summit of Mt Whitney. Wilderness Environ Med. 2006;17:221–228. 24. Ziaee V, Yunesian M, Ahmadinejad Z, et al. Acute mountain sickness in Iranian trekkers around Mount Damavand (5671 m) in Iran. Wilderness Environ Med. 2003;14:214–219. 25. Hirata K, Masuyama S, Saito A. Obesity as a risk factor for acute mountain sickness. Lancet. 1989;2:1040–1041. 26. James WPT, Chen C, Inoue S. Appropriate Asian body mass indices? Obes Rev. 2002;3:139. 27. Burtscher M, Flatz M, Faulhaber M. Prediction of susceptibility to acute mountain sickness by SaO2 values during short-term exposure to hypoxia. High Alt Med Biol. 2004;5:335–340. 28. Muza SR, Beidleman BA, Fulco CS. Altitude preexposure recommendations for inducing acclimatization. High Alt Med Biol. 2010; 11:87–92. 29. Hackett PH, Roach RC. High-altitude illness. N Engl J Med. 2001;345: 107–114.
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Rate of Ascent and Acute Mountain Sickness at High Altitude Tai-Yi Hsu, MD,*† Yi-Ming Weng, MD,†‡ Yu-Hui Chiu, MD,§¶k Wen-Cheng Li, MD,†** Pang-Yen Chen, MD,§¶ Shih-Hao Wang, MD,k††‡‡ Kuo-Feng Huang, MS,‡‡§§ Wei-Fong Kao, MD,k†† Te-Fa Chiu, MD,†‡ and Jih-Chang Chen, MD†‡
Objective: To examine the effect of ascent rate on the induction of acute mountain sickness (AMS) in young adults during a climb to Jiaming Lake (3350 m) in Taiwan.
Design: Prospective, nonrandomized. Setting: Climb from 2370 to 3350 m. Participants: Young adults (aged 18 to 26 years) (N = 91) chose to participate in either the fast ascent (3 days; n = 43) or slow ascent (4 days; n = 48) group (1 and 2).
Assessment of Risk Factors: Two criteria were used to define AMS. A Lake Louise score $3 and Lake Louise criteria [in the setting of a recent gain in altitude, the presence of headache and at least 1 of gastrointestinal discomfort (anorexia, nausea, or vomiting), fatigue or weakness, dizziness or lightheadedness, or difficulty sleeping]. Main Outcome Measures: Heart rate, blood oxygen saturation (SaO2), and symptoms of AMS were monitored each morning and evening. Results: Baseline characteristics were similar between groups, except for significant differences in history of alcohol consumption (P = 0.009) and climbing experience above 3000 m (P , 0.001). The incidence of AMS was not associated with the rate of ascent. Acute mountain sickness was most prevalent in group 1 on day 2 in the evening and in group 2 on day 3 in the evening. In both groups, AMS correlated with the initial reduction in SaO2. Body mass index (BMI) .24 kg/m2 was identified as a significant risk factor for AMS. Submitted for publication July 10, 2013; accepted January 20, 2014. From the *Department of Emergency Medicine, Chang Gung Memorial Hospital, Chiayi, Taiwan; †Chang Gung University College of Medicine, Taoyuan, Taiwan; ‡Department of Emergency Medicine, Chang Gung Memorial Hospital, Linkou, Taiwan; §Department of Emergency Medicine, Mackay Memorial Hospital, Taipei, Taiwan; ¶Institute of Environmental and Occupational Health Sciences, National Yang-Ming University School of Medicine, Taipei, Taiwan; kDepartment of Emergency, School of Medicine, College of Medicine, Taipei Medical University, Taipei, Taiwan; **Department of Occupation Medicine, Chang-Gung Memorial Hospital, Keelung, Taiwan; ††Department of Emergency Medicine, Taipei Medical University Hospital, Taipei, Taiwan; ‡‡Department of Physical Education, National Taitung University, Taitung, Taiwan; and §§Department of Emergency Medicine, Taiwan Adventist Hospital, Taipei, Taiwan. The authors report no conflicts of interest. Corresponding Author: Shih-Hao Wang, MD, Department of Emergency Medicine, Taipei Medical University Hospital, No.252, Wu-Hsing St, Taipei 11031, Taiwan ([email protected]). Copyright © 2014 Wolters Kluwer Health, Inc. All rights reserved.
Clin J Sport Med Volume 25, Number 2, March 2015
Conclusions: The development of AMS was closely associated with an initial reduction in SaO2. A BMI .24 kg/m2 also contributed to the occurrence of AMS. Clinical Relevance: These findings indicate that factors other than ascent rate should be considered when trying to ameliorate the risk of AMS. Key Words: acute mountain sickness, ascent rate, heart rate, oxygen saturation, body mass index (Clin J Sport Med 2015;25:95–104)
INTRODUCTION Acute mountain sickness (AMS) is a pathophysiological symptom complex that occurs in high-altitude areas with a fall in barometric pressure and a decline in oxygen saturation. Acute mountain sickness is a major reason for high-altitude emergency rescues.1 A variety of adaptive physiological processes are engaged to alleviate the fall in oxygen delivery to tissue. The magnitude and nature of such changes are time dependent, a process known as acclimatization.2 Acute mountain sickness commonly occurs in unacclimatized individuals above an elevation of approximately 2300 m. Symptoms of AMS include headache, gastrointestinal upset, fatigue, dizziness, and sleep disturbance. If left unchecked, AMS may progress to high-altitude cerebral edema. Predicting an individual’s risk of developing AMS is challenging because of the multitude of contributing factors. Simple measurements, including age, sex, body mass index (BMI), ascent rate, sleeping altitude, history of AMS, history of cardiopulmonary disease, oxygen saturation (SaO2), and pulse rate have been evaluated and have demonstrated high variability for predicting AMS.3–7 Preventative measures have been used to reduce the incidence of AMS, including modifications in ascent rate, prolonged acclimatization schedules, and prophylactic medications. These measures are implemented before and during ascent to avoid or reduce AMS and associated symptoms. Findings from several studies suggest that a rapid rate of high-altitude ascent results in a higher incidence of AMS.8–14 The purpose of the current study was to examine the effect of ascent rate on the induction of AMS during a climb to Jiaming Lake (3350 m) in Taiwan. We hypothesized that rapid ascent was more likely to result in AMS than slower www.cjsportmed.com |
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ascent. The current investigation is the first involving young Taiwanese adults.
MATERIALS AND METHODS Study Design and Setting This was a prospective study conducted during a climb to Jiaming Lake in Taiwan from 9 to 13, July 2007. Participants were asked to complete a hike that covered 26 km and reached a maximum altitude of 3350 m. Heart rate (HR), oxygen saturation (SaO2), and signs of AMS were assessed each morning and evening throughout the hike. Signs of AMS included headache, dizziness or lightheadedness, gastrointestinal symptoms, fatigue or weakness, and difficulty sleeping. We used 2 criteria to define AMS. The Lake Louise score (AMS = score $3)15 and Lake Louise criteria [in the setting of a recent gain in altitude, the presence of headache and at least 1 of gastrointestinal discomfort (anorexia, nausea, or vomiting), fatigue/weakness, dizziness/lightheadedness, or difficulty sleeping]. Three trained emergency physicians performed all SaO2 and HR measurements using Nellcor NPB-40 SaO2 Handheld Portable Pulse Oximeters (Covidien/Tyco-Nellcor/Puritan Bennett; Pleasanton, California) when the participants were sitting at rest. Measurements were taken each morning at about 07:00 AM and evening at about 05:00 PM. Participants were not permitted to exercise on arrival at camp. A total of 91 young adults were recruited from college mountaineering clubs in Taiwan. Participants were divided into 2 groups by choice: group 1 = fast ascent (3 days; n = 43) and group 2 = slow ascent (4 days; n = 48). All participants were aware of the differences between groups before they made their choice. Participants hiked together within each group. After an overnight stay nearby, both groups began their hike from the trail head (2370 m). Group 1 climbed to a mountain shelter at an altitude of 3350 m (8.4 km) within the second day; the same
day, group 2 climbed to an altitude of 2850 m (4.28 km). During the third day, group 1 hiked 4.6 km at an altitude between 3200 m and 3400 m to the lakeside at 3310 m; group 2 continued climbing and reached the mountain shelter at 3350 m (4.12 km). On the fourth day, group 1 returned to the mountain shelter after hiking 4.6 km; group 2 hiked a total of 13.32 km, traveling from the mountain shelter to the lakeside and back to the shelter at 2850 m. On the fifth day, both groups returned to the trail head. Figure 1 summarizes the ascent protocol for the groups. At any time during the hike, participants who felt symptoms of AMS were provided with medication and appropriate supportive care. Informed consent was obtained from all participants, and the study was reviewed and approved by the Institutional Review Board at Chang Gung Memorial Hospital. Participants who were professional mountain guides and those who did not answer the questionnaire were not included in the study results. Any participants who were evacuated because of injury were also excluded from the study results.
Statistical Analysis Demographic data and participant characteristics are shown as mean 6 SD or median (interquartile range) for continuous variables and n (percent) for categorical variables. Continuous data were compared between groups by 2sample t test or Mann–Whitney U test. Categorical variables were compared by Pearson x2 test or Fisher exact test. Statistical significance was indicated by P , 0.05. Withingroup comparisons of pulse rate and SaO2 were made using paired t test and Wilcoxon sign-rank test, respectively. An adjusted P , 0.001 was considered significant for withingroup multiple comparisons. The associations between AMS and participants’ characteristics were analyzed using univariate logistic regression and generalized estimating equations model analysis. Variables found to be significantly associated with AMS (P , 0.05) in univariate model analysis were entered into the multivariate model. Statistical analyses were
FIGURE 1. A graphical representation of the ascent protocol for the fast ascent and slow ascent groups.
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Ascent Rate and Acute Mountain Sickness
TABLE 1. Demographic and Clinical Characteristics of Participants by Group (N = 91) Variable Age, y Sex Male Female Body weight, kg Body height, m BMI, kg/m2 BMI .24 kg/m2, n (%) Home altitude*, m Medical history†, n (%) Drug use within 30 d of ascent‡, n (%) Acetazolamide Steroids Asthma medication Pain reliever Other Smoker Alcohol consumer Tea consumer Coffee consumer Hypnotics Exerciser Other habit Time since last climb¶, mo Climbs within 3 mo of ascent 0 1 2 3 No. 100-peaks climbedk 0 1 2 History of AMS** Knowledge of AMS Hiking stick use None Left hand Right hand Both right and left hands Either right or left hand
Group 1 (Fast Ascent) (n = 43)
Group 2 (Slow Ascent) (n = 48)
P
19.86 6 1.54 — 29 (67.4%) 14 (32.6%) 61.73 6 9.64 1.68 6 0.07 21.79 6 2.73 9 (20.9%) 30 (10-70) 13 (30.2%) 6 (14.0%) 0 (0%) 0 (0%) 0 (0%) 3 (7.0%) 4 (9.3%) 2 (4.7%) 20 (46.5%) 37 (86.0%) 20 (46.5%) 0 (0) 36 (83.7%) 0 (0) 3 (0.5-6)
19.83 6 1.31 — 34 (70.8%) 14 (29.2%) 62.01 6 9.63 1.68 6 0.07 22.02 6 2.73 13 (27.1%) 17.5 (10-55) 23 (47.9%) 11 (22.9%) 1 (2.1%) 0 (0%) 0 (0%) 2 (4.2%) 9 (18.8%) 1 (2.1%) 10 (20.8%) 34 (70.8%) 20 (41.7%) 0 (0) 32 (66.7%) 0 (0) 3.5 (0.5-6)
0.928 0.726
19 10 11 3
(44.2%) (23.3%) (25.6%) (7.0%)
25 12 9 2
(52.1%) (25%) (18.8%) (4.2%)
0 37 6 5 38
(0%) (86.0%) (14.0%) (11.6%) (88.4%)
9 38 1 7 34
(18.8%) (79.2%) (2.1%) (14.6%) (70.8%)
9 1 17 9 7
(20.9%) (2.3%) (39.5%) (20.9%) (16.3%)
4 3 24 10 7
(8.3%) (6.3%) (50.0%) (20.8%) (14.6%)
0.891 0.728 0.687 0.494 0.092 0.085 0.273
0.601 0.009§ 0.080 0.642 NA 0.062 NA 0.860 0.770
,0.001*
0.677 0.040 0.446
Data are shown as mean 6 SD or median (interquartile rage) for continuous variables and number (percent) for categorical variables. Continuous variables were compared between groups by 2-sample t test or Manny–Whitney U test, whereas categorical variables were compared between groups by Pearson x2 test or Fisher exact test. *The range of home altitude was 0-160 m in group 1 and 0-240 m in group 2. †Including heart disease, headache, rheumatic disease, asthma, allergic rhinitis, cold within 1 month, and enterogastritis. ‡One participant in group 1 and 1 participant in group 2 had taken both types of drugs (pain reliever and other) within 30 days of the ascent. §P , 0.05 indicates a significant difference between group 1 and group 2. ¶Range = 1-96 months in group 1 and 0-60 months in group 2. kA list of 100 peaks in Taiwan with an altitude typically more than 3000 m. **Lake Louise criteria were used to diagnose AMS, that is, in the setting of a recent gain in altitude, AMS was indicated by the presence of headache with one of the following symptoms: gastrointestinal (anorexia, nausea, or vomiting), fatigue/weakness, dizziness/light-headedness, or difficulty sleeping.
performed using SPSS 15.0 statistical software (SPSS, Inc, Chicago, Illinois). Posteriori analysis revealed that the statistical power for testing the null hypothesis was 5%, assuming a type I error probability of 0.05. Copyright Ó 2014 Wolters Kluwer Health, Inc. All rights reserved.
RESULTS The participants in both groups were of similar age, sex, body weight, height, and BMI (Table 1). There was no difference in medical history between groups. Participants’ www.cjsportmed.com |
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TABLE 2. Summary of AMS Scores for Patients Who Developed AMS During the Ascent No. (Sex/Age)
Day 1 Day 2 Day 3 Day 4 Day 5 At Least Once, Score ‡3 Morning Evening Morning Evening Morning Evening Morning Evening Morning Evening
Group 1 (fast ascent)
#1 (M/20 y) #2 (M/20 y) #3 (M/19 y) #4 (M/19 y) #5 (M/20 y) #6 (M/21 y) #7 (F/19 y) #8 (F/18 y) #9 (M/19 y) #10 (M/20 y) Group 2 (slow ascent) #1 (M/22 y) #2 (M/19 y) #3 (M/19 y) #4 (F/20 y) #5 (M/19 y) #6 (M/19 y) #7 (M/20 y) #8 (M/19 y) #9 (F/22 y)
Yes Yes Yes No No No Yes Yes No No Yes Yes No Yes Yes Yes Yes Yes No
2 0 0 0 0 0 0 2 0 0 0 0 0 0 0‡ 0 0 0 0
2 0 0 1 0 2 0 2 0 1 1 0 0 0 0‡ 0 0 0 0
0 0 0 0 0 0 0 1 0 0 0 0 0 1 0‡ 0 0 0 0
2* 2 3 2 2 2 5 3 2 2 0 0 1 0 0‡ 0 0 0 0
4* 3† 2† 1 2 1 0 0 0 1 0 0 1 1 0 0 0 0 0
0 0 0 2 0 0 0 0 0 0 5*† 8*† 2 3 3 3 3 3 2
0 0 0 0 0 0 0 0 0 0 2 0 1 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
AMS was identified as subjects with Lake Louise score $3. *Acetaminophen. †Acetazolamide (Diamox). ‡Drugs for common cold. M, male; F, female; y, years old.
knowledge of AMS and personal history were also similar between groups. Reported history of alcohol consumption was significantly higher in group 1 compared with group 2 (P = 0.009). Significantly more participants in group 1 reported experience of climbing above 3000 m in the previous 3 months compared with participants in group 2 (P , 0.001). The incidence of AMS was not associated with the rate of ascent and was similar between groups (Lake Louise AMS criteria: group 1, n = 10; group 2, n = 9; Lake Louise score $3: group 1, n = 5; group 2, n = 7). The highest incidence of AMS in group 1 (n = 5) occurred during the first half of hike (day 1-3), whereas the highest incidence of AMS in group 2 (n = 7) occurred during the second half of the hike (day 3-5). The highest incidence of AMS in group 1 (7%; 3/43) occurred on the
evening of day 2, whereas, the highest incidence of AMS in group 2 (12.5%; 7/48) occurred on the evening of day 3. Individual participant Lake Louise scores for participants who developed AMS and the incidence of AMS (as determined by Lake Louise criteria) at each assessment point are summarized by group in Tables 2 and 3. Table 2 also highlights the medications taken by participants who developed AMS. There was a significant difference in BMI between participants who were AMS positive and AMS negative (P = 0.001), with a higher proportion of participants with a BMI .24 kg/m2 being AMS positive compared with those who had a BMI #24 kg/m2 (Table 4). The development of AMS was associated with an initial decrease in SaO2 (Figure 2). Mean SaO2 was significantly different between groups 1 and 2 on the evening of day 2
TABLE 3. Summary of the Incidence of AMS From Day 1 to Day 5 Day 1 Group 1 (fast ascent) (n = 43) Group 2 (slow ascent) (n = 48)
Day 2
Day 3
Day 4
Day 5
Morning
Evening
Morning
Evening
Morning
Evening
Morning
Evening
Morning
Evening
0 0
1 0
0 0
8* 0
5* 0
0 9†
0 1†
0 0
0 1
0 0
Data are summarized as the number of participants with AMS. Lake Louise criteria were used to diagnose AMS, that is, in the setting of a recent gain in altitude, AMS was indicated by the presence of headache with one of the following symptoms: gastrointestinal (anorexia, nausea, or vomiting), fatigue/weakness, dizziness/light-headedness, or difficulty sleeping. *Four participants experienced repeated AMS in the evening of day 2 and in the morning of day 3. A total 10 participants in this group experienced AMS. †One participant experienced repeated AMS in the evening of day 3 and in the morning of day 4. A total of 9 participants in this group experienced AMS.
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Ascent Rate and Acute Mountain Sickness
TABLE 4. Characteristics of Participant Stratified by AMS Status (N = 91) AMS Negative AMS Positive (n = 72) (n = 19) Group 1 2 Sex Male Female BMI #24 kg/m2 .24 kg/m2 Blood type A B AB O History of AMS Yes No Mountain climbs within 3 mo of ascent 0 1 2 3 100-peaks* climbed within 3 mo of ascent 0 1 2
P 0.616
33 (76.7%) 39 (81.3%)
10 (23.3%) 9 (18.7%)
48 (76.2%) 24 (85.7%)
15 (23.8%) 4 (14.3%)
60 (87.0%) 12 (54.5%)
9 (13.0%) 10 (45.5%)
21 14 4 33
3 4 0 12
0.302
0.001*
0.495 (87.5%) (77.8%) (100%) (73.3%)
(12.5%) (252.2%) (0%) (26.7%) 0.706
9 (75.0%) 63 (79.7%)
3 (25.0%) 16 (20.3%) 0.615
36 16 15 5
(81.8%) (72.7%) (75.0%) (100%)
8 6 5 0
(18.2%) (27.3%) (25.0%) (0%) 0.523
7 (77.8%) 58 (77.3%) 7 (100%)
2 (22.2%) 17 (22.7%) 0 (0%)
Data are summarized number (percent) and were compared between groups by Pearson x2 test or Fisher exact test. *A list of 100 peaks in Taiwan with an altitude typically more than 3000 m.
(88.33 6 3.94 vs 91.90 6 3.32, P , 0.001), the morning of day 3 (89.00 6 3.79 vs 92.10 6 3.20, P , 0.001), and on day 5 (89.60 6 3.82 vs 92.76 6 3.15, P , 0.001). Mean pulse rate was significantly different between groups 1 and 2 (Figure 3) on day 3 (morning: 103.37 6 14.59 vs 96.87 6 15.02, P = 0.043; evening: 100.09 6 14.67 vs 107.26 6 14.17, P = 0.021) and day 4 (morning: 93.44 6 14.54 vs 107.54 6 14.33, P , 0.001; evening: 94.49 6 10.77 vs 104.85 6 12.02, P , 0.001). Participants who developed AMS had significantly lower SaO2 levels than participants who did not develop AMS in the evening of day 3 and in the morning of day 5 (day 3 morning: 88.11 6 5.31 vs 84.37 6 6.73, P , 0.05; day 5 morning: 91.65 6 3.57 vs 89.24 6 4.31, P , 0.05; Figure 4). Univariate regression analysis revealed that BMI .24 kg/m2, a lack of habitual exercise, decreased SaO2, and decreased pulse rate were significantly associated with AMS (all P , 0.05, Table 5). Body mass index .24 kg/m2, exercise, and SaO2 were also found to be significantly associated with AMS on subsequent multivariate analysis (all P , 0.05, Table 5). There were no between-group differences in the use of medications for prophylactic purposes or in response to symptoms associated with AMS at any point during the study. During the study, 20 of 43 participants in group 1 (46.5%) and 21 of 48 participants in group 2 (43.8%) received medications for prophylactic purposes. Table 6 summarizes SaO2 levels by BMI and AMS status. Of the participants who experienced AMS, those with a BMI .24 kg/m2 had significantly lower SaO2 levels in the evening of day 1 compared with those with a BMI #24 kg/m2 (P , 0.05). Table 7 had shown that the mean SaO2 in BMI #24 subjects was significantly higher at day 1, day 3, and day 4 evening as comparing with BMI .24 kg/m2 subjects (all P , 0.05).
FIGURE 2. Summary of SaO2 levels during the ascent as stratified by group. SaO2 was recorded each morning before departure and each evening after arriving at the shelter. Data are presented as mean 6 SD and were compared by Mann– Whitney U test. ***P , 0.001 indicates a significant difference between group 1 and group 2. †Significant difference compared with morning on day 2 in group 1. ‡Significant difference compared with morning on day 3 in group 1. §Significant difference compared with morning on day 2 in group 2. ¶Significant difference compared with morning on day 3 in group 2. Copyright Ó 2014 Wolters Kluwer Health, Inc. All rights reserved.
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FIGURE 3. Summary of pulse rate during the ascent as stratified by group. Pulse rate was recorded each morning before departure and each evening after arriving at the shelter. Data are presented as mean 6 SD and were compared between groups by 2-sample t test and within groups by pair t test. *P , 0.05, ***P , 0.001 indicate a significant difference between group 1 and group 2. †Significant difference compared with morning on day 2 in group 1. ‡Significant difference compared with morning on day 3 in group 1. §Significant difference compared with morning on day 2 in group 2.
DISCUSSION This study was designed to test the hypothesis that rapid ascent at high altitude is more likely to result in AMS than slow ascent in young adults. The occurrence of AMS was not associated with the rate of ascent and was similar between the rapid and slow ascent groups. We found that there was close association between the development of AMS and the initial reduction in SaO2. The development of AMS and the initial reduction in SaO2 correlated with reaching an altitude of 3350 m. Body mass index (.24 kg/m2) was the only significant predictor of AMS in our study population of young adults. Other factors, often suggested to be predictors of AMS such as HR and climbing history were not found to be associated with the development of AMS. To the best of our knowledge, this is the first study to investigate the onset of AMS in a population of young adults. It is believed that rapid ascent to high altitude may result in a higher incidence of AMS.8–14 Schneider et al16
reported that 58% of individuals who rapidly ascended Capanna Margherita (4559 m) developed AMS, whereas, 33% of those who ascended at a slower rate developed AMS. Hackett et al17 also reported an increased incidence of AMS among participants who rapidly ascended Pheriche in the Himalayas (4243 m). The lack of an effect of ascent rate in the current study may be because of the age of the climbers or the maximum altitude attained. Research has demonstrated that 34% of individuals ascending the Swiss Alps (3650 m)18 and 81% ascending Mt. Mauna Kea in Hawaii (4205 m)19 develop AMS. Furthermore, extremely fast ascent (through direct flight) to 3740 m in the Himalayas resulted in 84% of participants developing AMS.9 In the current study, 23.3% (10/43) of participants in the fast ascent group developed AMS, whereas 18.8% (9/48) of participants in the slow ascent group developed AMS. There was no significant between-group difference in the AMS rate.
FIGURE 4. Summary of SaO2 levels during the ascent as stratified by acute mountain sickness (AMS) status (positive, +; or negative, 2). SaO2 was recorded each morning before departure and each evening after arriving at the shelter. Data are presented as mean 6 SD and were compared between AMS+ and AMS2 subjects by 2-sample t test. *P , 0.05 indicates a significant difference between the AMS+ group and AMS2 subjects.
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Clin J Sport Med Volume 25, Number 2, March 2015
Ascent Rate and Acute Mountain Sickness
TABLE 5. Summary of Univariate and Multivariate Analyses of Variables Associated With AMS (n = 91) Univariate Variable Group 1 2 Age, y Sex Male Female BMI .24 kg/m2 #24 kg/m2 Blood type A B AB O Home altitude, m Medical history Smoker Alcohol consumer Tea consumer Coffee consumer Exerciser Time since last climb, mo Knowledge of AMS History of AMS Yes No Climbs within 3 mo of ascent 0 1 2 3 100-peaks climbed within 3 mo 0 1 2 Hiking stick use None Left hand Right hand Both right and left hands Either right or left hand SaO2, % Pulse rate, frequency per minute
Multivariate
OR (95% CI)
P
OR (95% CI)
1.313 (0.477-3.616) Reference 0892 (0.600-1.328)
0.598
—
0.574
—
1.875 (0.561-6.268) Reference
0.307
—
5.556 (1.862-16.577) Reference
0.002*
5.231 (1.664-16.439)
Reference 2.000 (0.387-10.337) ND 2.545 (0.641-10.102) 1.001 (0.990-1.011) 0.646 (0.221-1.893) 1.944 (0.167-22.662) 1.243 (0.432-3.571) 1.071 (0.312-3.684) 1.190 (0.432-3.282) 0.268 (0.092-0.784) 0.944 (0.840-1.062) 0.676 (0.208-2.191)
— — — — — — 0.242 (0.076-0.773) — —
1.312 (0.318-5.415) Reference
0.707
—
ND ND ND Reference
NA NA NA
—
ND ND Reference
NA NA
—
0.315 0.923 0.347 0.560 0.013* ,0.001*
— — — — 0.924 (0.875-0.976) 0.983 (0.959-1.008)
Reference (0.319-34.830) (0.246-4.695) (0.056-2.763) (0.007-4.009) (0.889-0.986) (0.159-0.438)
0.005*
— 0.408 NA 0.184 0.873 0.426 0.596 0.687 0.913 0.736 0.016* 0.338 0.514
3.333 1.075 0.392 0.556 0.936 0.264
P
0.017*
0.005* 0.189
Results are summarized as OR with the 95% CI and P value. Univariate analysis was performed using binary logistic regression model analysis for all variables except for SaO2 and pulse rate because of the repeated measurements, for which generalized estimating equations (GEE) model analysis was performed. Variables demonstrating significance in the univariate analysis (P , 0.05) were entered into a multivariate analysis model (GEE model analysis). *Significant association with AMS (P , 0.05). CI, confidence interval; NA, not assessed; ND, not derived; OR, odds ratio.
Manipulating the rate of ascent may be dependent on the availability of adequate shelter. On the trek to Jiaming Lake, shelters were available at altitudes of 2850 m and 3350 m. A high rate of AMS has been documented in trekkers Copyright Ó 2014 Wolters Kluwer Health, Inc. All rights reserved.
climbing mountains such as Mt. Kilimanjaro (75%) despite the lack of extreme conditions (direct flight or rapid transport from sea level).4,9,19 The high rate of AMS may be the result of mountain lodges being separated by 1000 m or more.4 www.cjsportmed.com |
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TABLE 6. Summary of SaO2 Levels by BMI and AMS Status Day 1 Participants with AMS BMI #24 kg/m2 (n = 9) BMI .24 kg/m2 (n = 10) Participants with BMI .24 AMS2 (n = 12) AMS+ (n = 10)
Day 2
Day 3
Morning
Evening
Morning
Evening
Morning
Evening
97.89 6 1.54 97.3 6 1.06
94.33 6 2.12 92.1 6 2.33*
94 6 2.96 91.5 6 3.5
89.44 6 3.47 88 6 4.99
90 6 3.28 89.2 6 5.79
86.22 6 5.76 82.7 6 7.38
98 6 1 97.3 6 1.06
92.92 6 2.61 92.1 6 2.33
93.58 6 1.73 91.5 6 3.5
89.83 6 5.1 88 6 4.99
91.17 6 4.22 89.2 6 5.79
85.58 6 8.01 82.7 6 7.38
Day 4 Participants with AMS BMI #24 kg/m2 (n = 9) BMI .24 kg/m2 (n = 10) Participants with BMI .24 AMS2 (n = 12) AMS+ (n = 10)
Day 5
Morning
Evening
Morning
Evening
90.11 6 6.15 86.8 6 6.91
90.22 6 4.27 87.5 6 7.28
89.29 6 3.82 89.2 6 4.83
94.67 6 2.06 94.4 6 1.07
88.58 6 5.02 86.8 6 6.91
89.36 6 3.59 87.5 6 7.28
91.42 6 3.23 89.2 6 4.83
94.83 6 1.64 94.4 6 1.07
Data are summarized as mean 6 SD. *P , 0.05 indicates significant difference between BMI #24 kg/m2 and BMI .24 kg/m2.
Thus, it may be impossible for trekkers to ascend gradually after traveling above 3000 m to avoid ascending greater than 300 m per day or to prolong stays at specific altitudes in an effort to decrease the risk of AMS. Trekkers who climb such routes are forced to make rapid ascents and may experience a higher risk of AMS. Thus, the availability of lodging facilities may influence the ascent rate and further affect the incidence of AMS. Clearly, the availability of trail and lodging facilities is an important consideration when planning a hike at altitude. It has been proposed that the best method for preventing altitude illness is to ascend slowly to allow for adequate acclimatization.20 Furthermore, consensus guidelines established by the Wilderness Medical Society state that controlling the rate of ascent in terms of the number of meters gained per day is a highly effective means of preventing acute altitude illness.21 In planning the rate of ascent, the altitude at which someone sleeps is considered more important than the altitude reached during waking hours. Considering the results from the current analysis, where the incidence of AMS
increased between the altitudes of 2370 m and 3350 m, it may be helpful to schedule an additional overnight stay at an altitude of 2850 m. The only significant predictor of AMS in the current study was BMI. A BMI .24 kg/m2 was associated with a significantly greater chance of developing AMS. Ri-Li et al6 reported similar results in obese versus nonobese participants who were compared at a simulated altitude of 3658 m. The investigators found that AMS scores increased significantly more rapidly with time spent at simulated high altitudes for obese compared with nonobese men. After 24 hours in an altitude chamber, 7 obese men (78%) and 4 nonobese men (40%) had AMS scores of 4 or more. Honigman et al22 also reported an increased likelihood of AMS in obese individuals when trekking at altitude. However, other researchers have reported that there is no relationship between obesity and the development of AMS.16,23,24 The cutoff of 24 kg/m2 used in the current study has been previously used in a study examining obesity as a risk factor for AMS.25 The available data do
TABLE 7. Dispersion of SaO2 During the Climbing Period Day 1 SaO2 BMI #24 kg/m2 (n = 69) BMI .24 kg/m2 (n = 22)
Day 2
Day 3
Morning
Evening
Morning
Evening
Morning
Evening
97.49 6 2.2 97.67 6 1.06
94.23 6 1.98 92.55 6 2.46*
93.36 6 2.54 92.64 6 2.82
90.59 6 3.61 89 6 5.01
90.75 6 3.39 90.27 6 4.97
88.31 6 4.71 84.27 6 7.69*
Day 4 SaO2 BMI #24 kg/m2 (n = 69) BMI .24 kg/m2 (n = 22)
Day 5
Morning
Evening
Morning
Evening
90.2 6 3.84 87.77 6 5.87
91.5 6 3.59 88.48 6 5.58*
91.43 6 3.73 90.41 6 4.09
94.84 6 1.77 94.64 6 1.40
Data were represented as mean 6 SD for BMI #24 and BMI .24 kg/m2 subjects, separately. *Mean SaO2 in BMI #24 subjects was significantly higher at day 1, day 3, and day 4 evening as comparing with BMI .24 kg/m2 subjects (all P , 0.05).
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Clin J Sport Med Volume 25, Number 2, March 2015
not clearly indicate a cutoff point for obesity/overweight in Asians. It has been proposed by the World Health Organization Expert Technical Consultation for Obesity that a BMI greater than 23 kg/m2 and less than 25 kg/m2 classifies an individual of Asian descent as being overweight or obese.26 An individual with a higher BMI may be less physically fit and require a greater level of exertion (relative to a leaner individual) when ascending. This increased exertion may influence the development of AMS. However, we did not observe subjects occurred with the dyspnea conditions such as high-altitude pulmonary edema during climbing. Our finding that reduced SaO2 was associated with the development of AMS is consistent with previously reported findings. Karinen et al4 recently reported that climbers who maintained rest level oxygen saturation during ascent at altitude and especially during exercise were less likely to develop AMS. Daily evaluation of SaO2 during ascent, both at rest and during exercise, may help to identify climbers at risk for AMS.4 Other studies have reported that SaO2 is a predictor (with as high as 86% predictive value) of AMS susceptibility.7,27 The most commonly examined predictive factors for AMS during high-altitude trekking generally fall into the categories of participant characteristics, lifestyle, climbing history, and climbing plan. Schneider et al16 assessed AMS in climbers arriving at Capanna Margherita (4559 m) and found that the 3 independent determinants of susceptibility to AMS were history of AMS, rate of ascent, and altitude preexposure. Wang et al11 confirmed that a lower incidence of AMS was correlated with high-altitude trekking experience or preexposure, whereas a higher incidence of AMS was correlated with a history of AMS. In the current study, none of these previously reported risk factors were associated with the development of AMS. Incorporating sufficient time for adequate acclimatization to altitude can often be problematic for novice climbers or those on a tight schedule. Climbers may also underestimate the importance of taking extra time during ascent to acclimatize properly, whether it means hiking more slowly or incorporating additional rest days at altitude. If precautions are not taken, the development of AMS may lead to the need for emergency rescue so that appropriate medical care can be administered.1 Five or more days spent above 3000 m in the preceding 2 months is considered to be sufficient altitude preexposure.28 Independent of known susceptibility, both adequate altitude preexposure and slow ascent have been shown to reduce the prevalence of AMS by around 50%.16 An additional factor that should be considered when planning a rapid ascent to high altitude is current guidelines that propose medical prophylaxis for AMS.29 It should be noted, however, that such medication can have adverse effects.29 Our study has several limitations. First, allocation to the fast and slow ascent groups was not randomized, which may have introduced selection bias in that individuals with higher level of fitness may have chosen the faster ascent. Second, although the number of days to reach the summit was different between the groups, the ascent rate during each day was not standardized, that is, the level of physical Copyright Ó 2014 Wolters Kluwer Health, Inc. All rights reserved.
Ascent Rate and Acute Mountain Sickness
exertion may have differed significantly between groups and thus may have been a confounding factor. Third, the effects of medications taken during ascent were not controlled for. Fourth, between-group differences in history of alcohol consumption and climbing experience above 3000 m may have influenced the reported outcomes as these variables have been suggested to affect the development of AMS (however, neither were found to be associated with AMS in our regression analyses). Fifth, the hydration status of the participants may have affected our findings, that is, participants in the fast ascent group may have been less hydrated because of the increased workload intensity. Finally, people with higher BMI may have higher relative exercise intensity during ascent, but we did not perform a good monitor to evaluate exercise intensity.
CONCLUSIONS In this study, the development of AMS was closely associated with an initial reduction in SaO2, which correlated with the time to reach shelter at 3350 m. Among the risk factors identified in previous reports for developing AMS, only a BMI .24 kg/m2 was found to be a significantly predictor of AMS in this study. Future studies should attempt to validate these findings in young adults in other high-altitude settings, while taking participants’ climbing experience, acclimatization, ascent rate, and SaO2 into account.
ACKNOWLEDGMENTS The authors thank Mr. Yu-Chou Wu, Chief of Campus Security Report Center at Ministry of Education, Taiwan, R.O.C., for the time he spent planning the hiking trips. The authors also thank Dr. Ching-Chien Yang and Dr. ChinHung Liu, Department of Pediatrics, Chang Gung Memorial Hospital, for their help in the analysis and collection of the data. REFERENCES 1. Wang SH, Hsu TY, Kuan JT, et al. Medical problems requiring mountain rescues from 1985 to 2007 in Yu-Shan National Park, Taiwan. High Alt Med Biol. 2009;10:77–82. 2. Martin DS, Levett DZ, Grocott MP, et al. Variation in human performance in the hypoxic mountain environment. Exp Physiol. 2010;95: 463–470. 3. O’Connor T, Dubowitz G, Bickler PE. Pulse oximetry in the diagnosis of acute mountain sickness. High Alt Med Biol. 2004;5:341–348. 4. Karinen HM, Peltonen JE, Kähönen M, et al. Prediction of acute mountain sickness by monitoring arterial oxygen saturation during ascent. High Alt Med Biol. 2010;11:325–332. 5. Moraga FA, Pedreros CP, Rodriguez CE. Acute mountain sickness in children and their parents after rapid ascent to 3500 m (Putre Chile). Wilderness Environ Med. 2008;19:287–292. 6. Ri-Li G, Chase PJ, Witkowski S, et al. Obesity: associations with acute mountain sickness. Ann Intern Med. 2003;139:253–257. 7. Burtscher M, Szubski C, Faulhaber M. Prediction of the susceptibility to AMS in simulated altitude. Sleep Breath. 2008;12:103–108. 8. Maloney JP, Broeckel U. Epidemiology, risk factors, and genetics of high-altitude-related pulmonary disease. Clin Chest Med. 2005;26: 395–404.
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9. Murdoch DR. How fast is too fast? Attempts to define a recommended ascent rate to prevent acute mountain sickness. Newsletters Int Soc Mountain Med. 1999;9:3–6. 10. Pradhan S, Yadav S, Neupane P, et al. Acute mountain sickness in children at 4380 meters in the Himalayas. Wilderness Environ Med. 2009;20:359–363. 11. Wang SH, Chen YC, Kao WF, et al. Epidemiology of acute mountain sickness on Jade Mountain, Taiwan: an annual prospective observational study. High Alt Med Biol. 2010;11:43–49. 12. Zafren K, Honigman B. High-altitude medicine. Emerg Med Clin North Am. 1997;15:191–222. 13. Bloch KE, Turk AJ, Maggiorini M, et al. Effect of ascent protocol on acute mountain sickness and success at Muztagh Ata, 7546 m. High Alt Med Biol. 2009;10:25–32. 14. Purkayastha SS, Ray US, Arora BS, et al. Acclimatization at high altitude in gradual and acute induction. J Appl Physiol (1985). 1995; 79:487–492. 15. Hackett P, Oelz O. The Lake Louise consensus on the definition and quantification of altitude illness. In: Hypoxia and Mountain Medicine. Sutton JR, Coates G, Houston CS, eds. Burlington, VT: Queen City Printers; 1992:327–330. 16. Schneider M, Bernasch D, Weymann J, et al. Acute mountain sickness: influence of susceptibility, preexposure, and ascent rate. Med Sci Sports Exerc. 2002;34:1886–1891. 17. Hackett PH, Rennie D, Levine HD. The incidence, importance, and prophylaxis of acute mountain sickness. Lancet. 1976;2:1149–1155. 18. Maggiorini M, Buhler B, Walter M, et al. Prevalence of acute mountain sickness in the Swiss Alps. BMJ. 1990;301:853–855.
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