Clinician's Corner: What Do We Know About Safe Ascent Rates at High Altitude?

Introduction

Individuals ascending to high elevation are at risk for developing one or more of three forms of acute altitude illness, acute mountain sickness (AMS), high altitude cerebral edema (HACE), and high altitude pulmonary edema (HAPE). While pharmacologic options are available for reducing the likelihood of these disorders, perhaps the best means of preventing acute altitude illness is to undertake an adequately slow ascent to high elevation, as this allows time for multiple acclimatization processes, which facilitate tolerance of the hypoxic environment. Recommendations regarding the proper ascent rate have been put forth in various review articles, consensus statements and documents published by organizations with an interest in mountaineering or mountain medicine. It is interesting that while these guidelines are all generally similar to each other in terms of the recommended rate of ascent and frequency of rest days during which the individual or group sleeps at a given elevation for a second night, the evidence supporting these recommendations is actually somewhat limited. Here we review these issues in greater detail. After describing the guidelines that have been put forth in various forums, we examine the body of evidence regarding ascent rates and susceptibility to altitude illness and then consider a series of unanswered questions regarding this issue. The review will conclude by providing a set of recommendations for how to approach the issue of proper ascent rates in light of these unanswered questions. Issues of pharmacologic prophylaxis, such as when the medications are indicated and the proper choice and dose of medication, have been reviewed extensively elsewhere (Hackett and Roach 2001; Luks et al. 2010; Luks and Swenson 2008) and will not be considered here.

Existing Recommendations Regarding Appropriate Ascent Rates

Table 1 lists selected recommendations regarding appropriate ascent rates that have been published in either the peer-reviewed medical literature or as part of documents put forth by organizations focused on mountaineering or mountain medicine. Each of the recommendations follows the same general pattern. Once an individual ascends above a given elevation (typically between 2500 and 3000 m) it is recommended that they do not ascend by more than a certain number of meters per day (usually between 300 and 500 m/day). In these recommendations, the key issue is the rate at which the individual increases their sleeping elevation rather than the speed with which they are walking or climbing during the day. The recommendations also all stipulate that individuals should take rest days every several days or following certain gains in elevation during which they sleep at the same elevation for at least one extra night.

Evidence Regarding the Rate of Ascent

Despite the widespread use of guidelines that provide generally similar recommendations regarding the appropriate rate of ascent, there is actually little evidence in the literature supporting these particular thresholds. In particular, there are no controlled studies establishing that an ascent rate of 300 m/day, for example, is better than 500 m/day for preventing acute altitude illness.

The majority of data in the literature regarding ascent rates are observational studies which demonstrate that slow ascent prevents altitude illness, but do not demonstrate that one particular threshold for acceptable ascent rate is better than another. The seminal paper that established a clear relationship between speed of ascent and incidence of altitude illness was that of Hackett et al. (1976) in which they retrospectively reviewed the ascent rates of 278 unacclimatized trekkers who had traveled to Pheriche, Nepal (4243 m) from Kathmandu (1300 m). The incidence of AMS was significantly higher among individuals who flew from Kathmandu to the village of Lukla (2800 m) before walking to Pheriche than among the group of individuals who walked the entire way from Kathmandu to Pheriche (60% vs. 42%, p<0.001). The differences were starker when the authors excluded individuals taking pharmacologic prophylaxis. Among this group, 85% of those who flew to Lukla developed AMS compared to 47% among those who trekked the entire distance from Kathmandu (p<0.001). Ascent rate also played a large role in the development of the more severe forms of altitude illness in this study. Among the 7 trekkers who developed HAPE and 5 who developed HACE, all but one case occurred in individuals who had flow to Lukla rather than walking the entire way from Kathmandu.

Subsequent studies have provided similar evidence regarding the role of ascent rate in the development of acute altitude illness. For example, Basnyat et al. (1999) surveyed 550 trekkers in the Everest region of Nepal over a 1-month period and noted that the risk of AMS decreased 18.7% for every additional night the trekkers spent between Lukla (2804 m) and the study site at 4243 m. Of note, the overall incidence rate was only 29.8% in this study, significantly lower than that reported by Hackett et al. (1976), perhaps reflecting increased efforts in the region on the part of the Himalayan Rescue Association at educating trekkers about the risk of acute altitude illness. Most recently, Richalet et al. (2012) conducted a prospective cohort study in which they examined the incidence of severe high altitude illness in 1326 individuals who underwent a hypoxic exercise test prior to a planned sojourn above 4000 m. Among individuals who did not use acetazolamide for prophylaxis, one of the main risk factors for developing severe acute mountain sickness, which was seen in 23.7% of the cohort, was an ascent rate greater than 400 m/day. It should be noted that this was an arbitrarily derived cut-off and other ascent rate thresholds were not assessed as part of their multivariate model.

Most of the evidence in the literature regarding ascent rate focuses on the risk of developing AMS. Additional evidence can be derived from other reports to support the claim that the rate of ascent also affects the incidence of HAPE although no systematic studies aside from the study by Richalet et al. (2012)—which included HAPE in their definition of severe high altitude illness—have examined this question. Bartsch, for example, (Bartsch et al. 2002; Bartsch et al. 2003) has reported that the incidence of HAPE among climbers ascending to 4559 m is only 0.6% when the ascent is made over 3–4 days compared with 6% in climbers with no history of HAPE and 62% in those with a history of HAPE when the ascent is made within about 24 hours. Similarly, Singh and Roy (1969) reported an incidence of HAPE of 15.5% in Indian soldiers flown to 5500 m in elevation compared to only 2.3% in soldiers transported by road. Systematic data comparing the incidence of HACE between different ascent rates is not available, likely as a result of the low overall incidence of the disorder, which makes systematic analysis difficult.

While there is considerable observational data in the literature supporting the notion that “slower is better” during ascent to high altitude, it is noteworthy that only a single study has sought to examine this issue in a prospective, randomized fashion. During an ascent of Muztagh Ata (7546 m), Bloch et al. (2009) randomized 34 healthy mountaineers to ascend to the summit over either 15 or 19 days. Those individuals assigned to the slow-ascent group had lower AMS symptom scores on the summit and at three intermediate camps (5533 m, 6265 m, 6865 m), and ascended without AMS for significantly more days than climbers in the fast ascent group. Although weather-related issues forced significant deviation from the pre-defined protocol such that the ascent rate measured over the length of the entire expedition from the base elevation of 3750 m to the summit elevation of 7546 m was only slightly faster in the fast ascent group than the slow ascent group (191 m/day vs. 201 m/day), ascent rates measured for different segments of the climb were significantly different between the two groups. The ascent rate from 3750 m to Camp 2 (6265 m) were faster in the fast ascent group (211 m/day vs. 158 m/day) as was the rate used for the final push from base camp (4497 m) to the summit (7546 m; 762 m/day vs. 508 m/day). The authors did account for the protocol deviation in their multiple regression analysis and found no significant effect of this issue on the main study outcomes.

Aside from this study, only a single other study has examined the effect of ascent rate in a prospective manner. Purkayastha et al. (1995) compared the incidence of AMS in Indian soldiers traveling to 3500 m and noted a higher incidence of AMS in individuals flown to this elevation compared to those who traveled by truck (84% vs. 51%). Group assignments were not made in randomized fashion although attempts were made to match the two groups based on age, height, and weight. The study provides further evidence that faster ascents are, in fact, more problematic than slower ascents but provides little guidance regarding the appropriate ascent rates with travel to high altitude.

Unanswered Questions Regarding Ascent Rates at High Altitude

As indicated above, the literature makes clear that faster ascents are more likely to cause acute altitude illness compared to slower ascents but does not provide much evidence to support the particular ascent rates recommended in the guidelines and review articles cited in Table 1. In addition to this issue, there are several outstanding questions regarding ascent rates that are not adequately addressed in the current literature.

Should The Guidelines Be Applied Equally to All High Altitude Travelers?

The available guidelines and recommendations in the literature generally imply that the recommended ascent rates apply to all individuals ascending to high altitude. There is reason to believe, however, that there is room for variability in how these rules are applied, particularly when people have prior experience traveling to high altitude. It is well known, for example, that there is significant inter-individual variability in physiologic responses to high altitude, as demonstrated by the differences in pulmonary vascular responses to acute hypoxia during rest and exercise between normal individuals and those known to be susceptible to HAPE. (Dehnert et al. 2005; Grunig et al. 2000) More importantly, several studies suggest some individuals tolerate relatively fast ascent rates. For example, in a study of climbers on Aconcagua (6962 m), for example, Pesce et al. (2005) noted that low Lake Louise AMS scores were actually associated with faster ascent rates. Similarly, in a study of 283 trekkers in Nepal, Murdoch (1999) noted that half of trekkers ascending at a mean ascent rate of 100–200 m/day developed AMS, while almost half of those ascending at a significantly faster rate of 500–600 m/day remained AMS-free. Rather than countering the argument derived from the studies noted above that fast ascent is a major risk factor for the development of AMS, these two studies, instead, point to the significant inter-individual variability in susceptibility to acute altitude illness; they suggest that those individuals who tolerate hypobaric hypoxia are capable of ascending at relatively fast ascent rates while others require significantly more time in order to avoid problems. The key issue, however, is that before someone can opt to move at these faster rates, they must have prior experience at high altitude that provides an understanding how their body responds to the environment and the types of ascent rates they can tolerate. Individuals on their first trip to high altitude could put themselves at high risk for problems if they move at fast ascent rates in the absence of such knowledge.

How Should Travelers Implement The Recommended Ascent Rates?

Another issue that remains unclear from the published recommendations is exactly how a traveler should adhere to the recommended ascent rates. For example, should the traveler ensure that the increase in sleeping elevation is below the recommended threshold every day of the trip or is it more important to simply ensure that ascent rate for the trip as a whole—calculated as the difference between starting and ending sleeping elevation, divided by the duration of the trip—falls below the recommended threshold. One could imagine a situation, for example, where terrain or other logistical factors mandate a gain in sleeping elevation of 700 m at several points during a trip, but the use of rest days before and after such large gains brings the overall ascent rate down to the acceptable range.

Research studies, such as the studies by Richalet et al. (2012) Basnyat et al. (1999) noted above, and a recent article examining whether commercial expeditions are compliant with recommended ascent rates (Shah et al. 2011) calculate ascent rates based on the overall trip ascent profile, as that is the only feasible way to collect the necessary data. There is actually some limited evidence to suggest that the overall ascent rate matters in addition to daily gains in sleeping elevation. In the study by Hackett et al. (1976) noted above, it is interesting to note that the average ascent rate above Lukla was only slightly higher (although statistically significant) in the AMS group than the non-AMS group (393±88 m/day vs. 355±72 m/day). Although the day-to-day gains in elevation above Lukla were likely not much different between the groups who flew or walked to Lukla, the overall ascent rate for the entire trip was significantly lower in the latter group due to the extra time needed to walk to Lukla. It is likely this difference in overall rates, rather than any day-to-day differences in ascent rates above Lukla, that affected the incidence of AMS.

Although the overall rate may be important, the published guidelines do not place any emphasis on this variable and, instead, tend to emphasize only that the ascent rate on any given day of a trip should be below the recommended threshold. While the latter strategy will be useful for decreasing the risk of altitude illness, it may not be feasible in all circumstances. Table 2, for example, presents the altitude of the typical stopping points during a trek to Everest Base Camp in Nepal and a climb on the Machame Route on Mount Kilimanjaro, as well as the altitude difference between those commonly used sleeping locations. Many of the intervals between stopping points fall within the recommended ranges but due to terrain factors (i.e., the location of villages that serve as feasible stopping points) or park regulations mandating that groups or individuals stay in particular areas, larger than recommended gains in elevation are often necessary at other points during the trip.

Individuals may find that it is impossible to adhere to a recommendation to keep the ascent rate below a recommended level every day of the trip but may, instead, use rest days to bring down the overall ascent rate and mitigate the risk of the trip. Future guidelines should recognize this limitation in current recommendations and place emphasis on the overall ascent rate in addition to the daily changes in sleeping elevation.

When Should Rest Days Be Used During the Ascent?

An important means by which individuals slow their rate of ascent is by adding rest days in which they sleep at the same elevation for at least 2 nights, thereby allowing time for their body to acclimatize to the hypobaric hypoxia. For example, on the Everest Base Camp trek, most groups spend at least two nights in Namche Bazaar (3440 m) and, later, Dingboche (4320 m) before moving to higher elevations. As noted in Table 1, the available guidelines specify that rest days should be used at specified time or altitude intervals over the course of the trip.

The optimal spacing between rest days is not clear. Clearly, the more frequently they are inserted in the trip itinerary, the lower the overall ascent rate and the lower the risk of altitude illness. Time, monetary, and other constraints may make very frequent rest days infeasible for some travelers, however, and an alternative approach may be necessary. Prior experience at altitude can help planning in this regard, but the first-time high altitude traveler will need to err on the side of more frequent rest days.

One issue that does not receive sufficient attention in most of the published guidelines is how to handle the situation discussed in the previous section in which terrain and other factors mandate an overly large increase in sleeping elevation on a particular day. A prudent way to mitigate the risk associated with this situation would be to have a rest day before and/or after the large increase in sleeping elevation. This situation is not specifically addressed in the Wilderness Medical Society, Union Internationale des Associations D'Alpinisme (UIAA), or Medex guidelines, as they simply recommend rest days at specified time intervals (e.g., every 2–3 days) and would likely be a useful addition to these recommendations. The Himalayan Rescue Association has recognized this issue in the Everest Region of Nepal and recommends rest days in Namche and Dingboche or Pheriche even though they come at shorter time intervals than recommended in some of the published guidelines (Ken Zafren, Associate Director for the Himalayan Rescue Association, personal communication).

Can Individuals Use Pre-acclimatization Strategies To Facilitate Faster Ascent Rates?

Recently, there has been considerable interest in the notion of pre-acclimatization as a means to decrease the risk of acute altitude illness following ascent to high elevation. The term “pre-acclimatization” refers to strategies in which individuals undertake repeated exposures to hypobaric or normobaric hypoxia in the days and weeks preceding their planned high altitude excursion, referred to as “intermittent hypoxic exposure,” or spend significant time at a moderate altitude prior to the major gain in elevation, a strategy referred to as “staged ascent.” Several studies, reviewed in detail by Muza et al. (2010) suggest that such strategies mitigate some of the physiologic responses to high altitude and may decrease the risk of illness following ascent. Beidleman et al. (2004), for example, showed that 4 hours of daily exposure to the equivalent of 4300 m significantly decreased the incidence of AMS upon subsequent rapid ascent to 4300 m in a hypobaric chamber, while other studies have demonstrated that 6 days of staging at 2200 m decreases the incidence and severity AMS (Beidleman et al. 2009) and blunts pulmonary vascular responses (Baggish et al. 2010) during rapid ascent to terrestrial 4300 m.

While these pre-acclimatization techniques are likely of use in preventing acute altitude illness, they should probably not be used to facilitate faster ascent rates than those recommended in published guidelines, as there is no sense in the present literature as to exactly what pre-acclimatization strategy is most effective and no evidence whether use of such strategies decreases the risk of altitude illness in those who opt for faster than recommended ascents. For example, if individuals opt for an intermittent hypoxic exposure approach, it is unclear how high they need to go, how often they need to go there, and how close to the actual ascent they should undergo the hypoxic exposures in order to derive any subsequent benefit. Most individuals would also likely have trouble implementing the strategies used in the various studies given the lack of access to hypoxic or hypobaric chambers and would instead need to rely on other means of obtaining sufficient exposure, such as repeated trips to higher elevations in local mountains or use of in-home hypoxia systems, the latter of which lack any evidence to support their effectiveness.

Recommendations

At present, there is insufficient evidence in the literature to suggest that any major changes be made recommended ascent rates during travel to elevations above 2500 m. Limiting the ascents to 300–500 m/day increases in sleeping elevation may be slower than necessary for some individuals but likely work for the substantial majority of high altitude travelers. It is really the individuals traveling to high altitude for the first time (i.e., with no prior experience in this environment) who should be particularly careful about adhering to current recommendations when planning their itinerary. On the other hand, those individuals with significant prior experience at high altitude can likely alter their ascent profile from the recommended rates, moving either faster or slower than the published guidelines, depending on how they have fared in the past. When accompanied by first-time high altitude travelers, however, they should adhere to slow ascent rates that follow current recommendations to accommodate these individuals who do not yet know how their body will respond in this environment.

Rest days should be included in any high altitude travel itinerary to facilitate acclimatization. Rather than being used at specified time intervals alone, rest days should also be used prior to and/or following any large gain in elevation that is greater than the recommended increase in daily sleeping elevation due to terrain and other logistical factors. Using rest days in this manner can ensure that, even though a given ascent may exceed the published recommendations for gain in sleeping elevation, the overall ascent rate may remain slow enough to ensure a safe trip. In situations where individuals must ascend at rates higher than those recommended, consideration can be given to pharmacologic prophylaxis with acetazolamide. A more thorough discussion of assessing the risk of a given ascent profile and the decision to use pharmacologic prophylaxis can be found in published guidelines on this topic. (Luks et al. 2010)

There is insufficient evidence to recommend pre-acclimatization strategies as a means to facilitate faster than recommended ascent rates. These strategies likely decrease the risk of altitude illness, but individuals attempting these strategies should adhere to current guidelines until further information is available regarding their effectiveness. Further research on this and other issues regarding ascent rates at high altitude may allow changes in practice in the future, but for the time being, adherence to the current recommendations is the prudent approach for the majority of high altitude travelers.