Results of multiple logistic regression for predictors of AMS as the dependent variable are presented in Table 3. The odds that a climber from this sample would experience AMS were significantly reduced with increases in age and the number of hours spent above 3000 m in the 2 wk preceding the ascent. The odds of experiencing AMS were also significantly lower for females in this sample than for males. The odds of AMS were significantly increased, however, for climbers who reported having had AMS previously, as well as for climbers who reported taking an analgesic during the ascent. No significant interactions were observed.
Model fit statistics (χ 2 = 84.95; P < 0.0001) indicates that the predictive value of the final model was significantly better than the null model (overall classification rate = 64.4%). No indications of collinearity were observed among the main effects in the final model (variance inflation range = 1.01-1.03).
Variables with odds ratio (OR) estimates less than 1 are interpreted as protective factors for AMS. As climber age increased, the odds of AMS decreased significantly (adjusted OR for 10-yr age increments = 0.78; 95% confidence interval [CI] = 0.69-0.86; P < 0.001). The odds of an older climber experiencing AMS are estimated to be 0.78 to 1 compared with those of a climber 10 yr younger. The mean age of climbers without AMS was 40 yr (SD = 14.35) compared with a mean age of 35 yr (SD = 13.42) among the climbers with AMS. The slight curve shown in Figure 1A indicates that the protective effect of age is greatest between ages 12 and 32 yr, where the line is steepest. Modest decrements in the effect are observed from that point, where the curve begins to level off. Similarly, as the number of hours spent above 3000 m increased, a significant decrease in the odds of AMS was observed (adjusted OR for 24-h increments = 0.71; 95% CI = 0.57-0.84; P < 0.001). The odds of AMS for a climber who had spent a total of 24 h above 3000 m over the 2 wk preceding the current ascent are estimated to be 0.71 to 1 compared with a climber who had not spent any time above 3000 m. The mean number of hours spent above 3000 m among climbers with AMS was 3.8 h (SD = 15.15) compared with 11.93 h (SD = 36.98) among climbers who did not experience AMS. As shown in Figure 1B, the negative relationship between hours above 3000 m and the OR for presence of AMS is linear, indicating a sustained protective effect.
Results of multiple logistic regression for predictors of a successful summit bid as the dependent variable are presented in Table 4. Increases in the average number of hours per week spent training, rate of ascent (m·h−1), and altitude attained before the current ascent were all associated with significantly increased odds for summit success among climbers in this sample. Increased age was associated with a significant decrease in the odds that a climber would have reached the summit.
Model fit statistics (χ 2 = 30.93; P < 0.0001) indicates that the predictive value of the final model was significantly better than the null model (overall classification rate = 87.7%). No indications of collinearity were observed among the main effects in the final model (variance inflation range = 1.00-1.04).
As climber age increased, the odds of reaching the summit decreased significantly (adjusted OR for 10-yr age increments = 0.75; 95% confidence interval [CI] = 0.64-0.88; P < 0.001). The mean age of climbers who reached the summit was 37 yr (SD = 13.08) compared with a mean age of 40 yr (SD = 15.18) among climbers who did not summit. The odds of reaching the summit were significantly greater for climbers who had spent more time training for the current ascent (adjusted OR for 5-h increments = 1.24; 95% CI = 1.01-1.58; P = 0.05). The OR for a climber who spent an average of 5 h·wk−1 training versus a climber who did not train was estimated to be 1.24 to 1. Figure 2A shows the curvilinear nature of the relationship between the adjusted OR for summit success and the average number of hours spent training. This indicates that increases in the odds of a successful summit bid become greater the more time spent training. Similarly, increases in the rate of ascent were also associated with greater odds for summit success (adjusted OR based on 50 m·h−1 increments = 1.13; 95% CI 1.00-1.27; P = 0.04). Figure 2B shows a nearly linear relationship between the adjusted OR for summit success and the rate of ascent, however, indicating a constant increase in the odds for reaching the summit as rate of ascent increases. Having attained higher altitude before the current Mt. Whitney ascent was also associated with greater odds for summit success (adjusted OR based on 500 m increments = 1.26; 95% CI = 1.11-1.44; P < 0.001). The slope of the curve shown in Figure 2C gets steeper as altitude increases, indicating that the effect of previous altitude experience becomes more profound at higher altitudes.
There is great diversity regarding the physical characteristics, previous altitude experience, and preparation in the people that attempt the summit of Mt. Whitney, and like our previous summit study (27), the range was wide on nearly all variables included in the study. However, the vast majority (96%) of Mt. Whitney trekkers reside at altitudes below 1500 m.
Predictors of AMS.
Forty-three percent of the sample met the criteria for AMS. This is higher than the 33% incidence rate reported from our previous study of summiteers only (27). It is between the 34% at 3650 m and the 53% at 4559 m reported by Maggiorini et al. (13) despite some methodological differences in the assessment of AMS. Additionally, 57% of the sample reported experiencing at least a mild headache (score of 1 or greater on the LLSS) during the ascent.
Increasing age corresponds to a decrease in the odds of suffering AMS. The notion that the incidence and the severity of high-altitude headache and AMS are greater in younger individuals has been supported by numerous previous studies (8-10,15,20,21,25,27). The direction and the magnitude of the physiological adaptive responses to acute hypoxia appear to be similar between younger and older individuals (12). The reason behind the decreased OR for AMS with increasing age is not fully understood. One hypothesis is that there is a decrease in brain size with advancing age allowing for greater brain swelling (22). However, the pathophysiology of AMS might be more complex than an increase in brain volume, and it is unclear whether such an increase occurs at all. For example, Mórocz et al. (14) observed significant brain swelling (2.8% increase) after 32 h of hypobaric hypoxia corresponding to 4572 m, yet no relationship existed between the magnitude of brain volume increases and the severity of AMS symptoms. Kallenberg et al. (11) reported an increased brain-to-intracranial volume ratio in AMS sufferers, but the increase in brain volume during hypoxia was only about 0.5% of total brain volume. Finally, Fischer et al. (7) found no evidence of cerebral edema with moderate to severe AMS.
Both the present study and our previous work (27) show that the decreased incidence of AMS with increasing age is a curvilinear relationship with the steepest part of the curve occurring at the youngest ages (Fig. 1A). Our Mt. Whitney samples have had wide age ranges with some very young participants that may have contributed to this finding. Further research is needed to determine whether the lower OR for AMS in older climbers is the result of a physiological phenomenon or if older persons simply minimize their symptoms on a questionnaire or interview.
The amount of time spent above 3000 m in the weeks preceding the summit attempt was also a significant protector against developing AMS. This is not surprising as there is a substantial amount of research to support that acclimatization and recent exposures to altitude offer some protection against AMS (2,4,8,10,16,23,26,27,29). In the present study, the amount of time rather than the number of ascents above 3000 m was significant, suggesting that time at high altitude rather than the number of exposures might be a better protector against AMS. Furthermore, there is a linear dose-response relationship; the more time spent above 3000 m, the lower the OR for AMS (Fig. 1B).
Beidleman et al. (4) have suggested that intermittent exposure to altitude increases resting ventilation and resting arterial oxygen saturation, and these increases are associated with a reduction in the severity of AMS. It appears that some of the adaptations that occur with residing at altitude can be obtained with intermittent exposures, and as the present study shows, the longer the duration of these exposures, the better. This is an important finding for anyone venturing to high altitude, especially those who reside at a low altitude, because preacclimatization is a powerful protective strategy. Schneider et al. (23) found that preexposures above 3000 m were as affective as a slow ascent for reducing the risk of AMS.
The third significant predictor of AMS was a self-reported history of altitude illness. Again, research consistently supports this finding that individuals who have had AMS in the past are at significant risk for a subsequent bout (10,16,23,27,29). Pesce et al. (16) noted that this was the most important independent predictor of AMS on ascents of Mt. Aconcagua.
We found that females experienced less AMS. Most previous studies have reported a nonsignificant gender effect for AMS (2,9,15,20,23,27,29); however, the sample sizes of these studies might not have been large enough to detect a significant difference. For example, Basnyat et al. (2) reported an OR of 0.82 for females in a sample of trekkers in the Mt. Everest region, but this effect was reported as not statistically significant. Whereas the smaller Basnyat et al. (2) sample (N = 545; females = 185, males = 360) was not adequate to detect their reported difference, the present Mt. Whitney sample (N = 886; females = 212, males = 674) was sufficient to ascertain the magnitude of this effect (OR = 0.68). Ziaee et al. (29) also reported that proportionally, females in that sample trekking around Mt. Damavand suffered from AMS less than males (58.1% vs. 63.1%). Likewise, Serrano-Dueñas (24) found that the incidence of AMS was statistically significantly lower for experienced female mountaineers compared with their male counterparts. In contrast, other researchers have reported that women are at an increased risk of acquiring AMS (3,8). These equivocal findings of a gender effect for AMS warrant further investigation.
The final variable with a significant relationship to AMS was the ingestion of analgesics, which included aspirin, acetaminophen, or nonsteroidal anti-inflammatory drugs. As in our summit study (27), analgesics use was associated with an increased risk of AMS; however, this finding is likely the result of trekkers with AMS taking analgesics to alleviate their symptoms rather than analgesics "causing" AMS. In our previous study (27), 65% of the sample were taking medication as an aid to reaching the summit or reducing the severity of AMS symptoms. Similarly, 57% of the present sample were taking analgesics with an additional 5% taking acetazolamide and 3% taking a combination of the two. Many were uncertain of the dosage or how much they had consumed.
As mentioned previously, the criteria for AMS in this study were 1) a headache with 2) at least one other symptom and 3) a score of ≥3 on the LLSS. This is consistent with the diagnostic criteria established by the Lake Louise AMS scoring consensus committee (19). However, it could be argued that this is a conservative criterion, and symptoms of "mild" AMS could be confused with symptoms of exhaustive exercise. Thus, we reran the logistical regression using a cutoff score of ≥5, which might be considered "moderate" or "severe" AMS. Of course, there were fewer incidences of AMS with the more rigorous criterion (23.5% as opposed to 42.6%). However, although some of the OR changed slightly, the variables in the predictive model remained unchanged; none of the predictors dropped out of the model, and none of the other measured variables reached significance. Furthermore, the model fit was nearly the same (χ 2 = 76.48; P < 0.0001).
Predictors of summit success.
Eighty-one percent of the sample reported that they had reached the summit. Although summit rates have never been published and there are no official statistics available, we believe that this is an unusually high success rate for this mountain. We attribute the high success rate to ideal and stable weather conditions (barometric pressure did not change by more than 4 mm Hg at the trailhead). During the 12-d data collection, there were no thunderstorms typical of the region that often stop summit attempts. Of the 164 participants who did not reach the summit, 25% reported sickness or symptoms related to AMS as the main reason for turning around, 22% said fatigue, 21% cited partner failure, 10% claimed weather conditions, and 22% gave other reasons (injury, getting lost, etc.).
It is interesting to note that the percentage of those with AMS who reached the summit (43%) and those with AMS who did not reach the summit (41%) was nearly identical. Having AMS is not necessarily associated with not reaching the summit. This is consistent with reports from ascents of Mont Blanc (26) and Mt. Aconcagua (16). Pesce et al. (16) reported that AMS did not prevent climbers from attaining the summit of Mt. Aconcagua, but they were likely to ascend slower and to rely on medication more than those without AMS.
Increasing age reduced the odds of reaching the summit of Mt. Whitney. Similarly, Pesce et al. (16) reported a lower OR for summiting Aconcagua for those older than 40 yr (although not statistically significant), and Tsianos et al. (26) stated that Mont Blanc ascent times slowed as the age of the climber increased. This is not a great surprise as most physiological factors, including those thought to be advantageous to mountaineering such as cardiorespiratory and muscular fitness, decline by about 1% a year after the age of 30 yr (5). Nonetheless, older climbers still regularly reach the summit of Mt. Whitney as evidenced by a 76-yr-old summiting in our previous study (27) and a 74-yr-old reaching the top in the present study.
As physical training time increased, the odds of reaching the summit also increased. This is contrary to Pesce et al. (16) who reported that training time was not related to success on Mt. Aconcagua. The differences in the study samples and ascent styles might explain the difference in this result. At 6962 m and the highest point in the Western Hemisphere, Mt. Aconcagua is a major international climbing objective. As such, it can be assumed that most who attempt this peak have probably engaged in at least a moderate amount of preexpedition training, and there was probably little variability in the training status of these climbers. However, there was great variability in the physical training done by those attempting Mt. Whitney. Furthermore, although Mt. Aconcagua is much higher, it is climbed "expedition-style" over many days with multiple camps allowing for shorter distances and less daily altitude gain than Mt. Whitney, which is often attempted in a single day with an ascent of 1869 m and roundtrip distance of 34.4 km. Thus, due to the long distance and large altitude gain, it is not surprising that physical training was a significant predictor of summit success in the present study.
Previous experience at high altitude was also a significant factor in summit success on Mt. Whitney. The higher the person had been in the past, the greater the odds of success at Mt. Whitney. Likewise, Pesce et al. (16) noted that previous exposure to altitudes above 6000 m was the most important predictor of summit success on Mt. Aconcagua. We surmise that previous experience at high altitude might increase one's self-efficacy for reaching a high summit, thereby increasing one's odds for success. This could be an area of future research, as we did not give any psychological inventory to test this hypothesis.
Those who climbed faster were also more successful. This is not to say that running up the mountain is advisable, rather it reflects the characteristics of those who reached the summit with relative ease versus those who did not. In essence, younger individuals who exercised regularly and who were well acclimatized with prior experience at high altitude were able to move faster up the mountain and reach the summit. This is consistent with the published research from studies done at Mont Blanc (26) and Mt. Aconcagua (16).
The data collected were from a self-report questionnaire and interview conducted retrospectively as trekkers completed their descent. We make the assumption that they could accurately recall their ascent data, AMS symptoms, and give truthful answers. Additionally, as stated previously, our data collection occurred over an unusually calm weather period, and this could have affected the results, especially the incidence of summit success. It should be noted that 59.2% of summiteers gained the 1869 m from the Whitney Portal (2550 m) to the summit (4419 m) in 12 h or less. Participants were interviewed immediately upon descent, and it can be difficult to distinguish between symptoms due to AMS and symptoms resulting from exhaustive exercise. In other trekking and mountaineering settings, individuals are typically exposed to high altitudes critical for the development of AMS for a longer period than occurred in this study. Thus, caution should be used when comparing our results to those of studies that have involved prolonged, slower ascents of several days.
Although many people attempt the summit of Mt. Whitney each year, this is the first scientific study to report the success rate and do a regression analysis to determine which variables are predictive of summit success as well as the development of AMS on the mountain. Despite 43% of the sample meeting the criteria for AMS, 81% reached the summit. Good weather was likely a major factor in the high success rate, and having AMS does not appear to be a significant hindrance for reaching the summit. Increasing age, more time above 3000 m in the 2 wk before the summit attempt and being a female all reduced the risk of developing AMS, whereas those who had AMS previously were at an increased risk. These variables remained significant predictors even when a higher AMS cutoff (≥5 on the LLSS) was applied. The gender difference and the time above 3000 m are new and potentially important findings. A large percentage (57%) of climbers were using analgesics, and this coincided with AMS prevalence. Increasing age diminished one's odds for reaching the summit, whereas more hours per week of training, fast rate of ascent, and previous experience at high altitude were all positive predictors of success.
We thank the participants who graciously took the time to answer our questions despite having been on the trail for many hours. Thanks to the Inyo National Forest Service and the Whitney Portal Store (www.whitneyportalstore.com) for their support during this data collection. Page charges were paid by the Health, Physical Education, and Recreation Department of Utah State University. Results of this study do not constitute an endorsement by the ACSM.
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Keywords:©2008The American College of Sports Medicine
HIGH ALTITUDE; MOUNTAINEERING; TREKKING; RISK FACTORS; SUSCEPTIBILITY