In the second of our four-part series on acclimatisation, Jamie Macdonald (high altitude physiologist at Bangor University) and Calum Muskett (professional climber and mountaineering instructor), discuss the acclimatisation process and explore how we can best plan our trips to the high mountains in order to reap the rewards of successful acclimatisation.

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The high altitude native

The Altiplano, or Andean Plateau, situated in west-central South America, is the second most extensive area of high plateau in the world behind the Tibetan Plateau. The Aymaran people have lived in this part of the Andes for over 800 years, preceding both Columbus and the Incan empire. With such a length of habitation in this high-altitude region, which dates back as far as 5,000 years according to some estimates, the Aymaran highlanders have undergone an evolutionary modification which has helped them to physiologically adapt and thrive amongst the mountains of the Andes.

Aymaran highlanders often live above 4,000m and have adapted to living at such altitudes due to a genetic predisposition to have more haemoglobin (the iron-containing oxygen-carrying part of the blood) and an increased lung volume. This results in better transfer of oxygen from the air to the blood and allows more oxygen to be carried by the blood, countering the hypoxic effects of living at altitude.

In contrast, the natives of the Tibetan plateau have adapted in a very different way to the Aymaran highlanders. Unlike the Aymarans', their haemoglobin concentration is similar to those of lowland populations. However, the Tibetans breathe more rapidly and deeply. The Tibetans also have high levels of a molecule called nitric oxide in their blood as well as broader arteries and capillaries which together allow greater blood flow and oxygen transportation. Although achieved by a different mechanism, again these changes counter the hypoxic effects of living at altitude.

So why is this knowledge useful to the mountaineer? Remember the concept of sense of effort (sometimes called Rating of Perceived Exertion) introduced in our first article? Although the Aymarans' and Tibetans' have evolved to living at altitude in different ways, both groups are able to cope with hypoxia at high altitude by having increased oxygen delivery. Consequently they can perform their daily tasks with a similar sense of effort to that of lowland populations performing similar daily tasks at sea level. Unfortunately, those of us who haven't benefitted from this evolutionary process will never achieve the same level of adaptation to altitude, but we can learn from these changes to our own advantage. Before we delve into the practicalities of how we can best acclimatise for our own alpine objectives, we first need a deeper understanding of physiology to ensure our practical approach will be as effective as possible….

The acclimatisation process for lowlanders

High altitude poses multiple challenges to human survival, but it's the lack of oxygen (hypoxia) that we will focus on here. The percentage of oxygen in the air at 4,000m is essentially the same as at sea level (21%). The main difference at altitude is that air pressure drops the higher we go, reducing the driving force to 'push' oxygen from the air in our lungs into our blood. At 4,000m the air pressure is 37% lower than it is at sea level. This leads to oxygen deprivation in the body (hypoxemia) that is often so severe, if it were present at sea level we would treat it medically and provide supplementary oxygen! Thus, it is no wonder that a decrease in physical performance occurs when at altitude.

The body's initial response to high-altitude is to breathe faster, much like the Tibetans' evolutionary adaptation, and increase pulse rate, even at rest [see box 3]. The body also tries to concentrate its blood, allowing more oxygen to be carried, which is why we pee profusely upon altitude exposure. These acute changes maximise the amount of fresh air in the lungs, increasing the driving force to push oxygen into the blood; increase blood flow around the body; and increase the amount of oxygen that blood can carry - all with the aim of getting sufficient oxygen to cells that desperately need it (particularly in the muscles and the brain). However, this is only the first step of acclimatisation, and the lowlander at altitude will still look very differently physiologically compared to the Tibetans. Oxygen delivery remains compromised. Consequently, during this period we will notice the first symptoms of altitude exposure: increased sense of effort during activity, increased fatigue following activity and, in many cases, mild symptoms of acute mountain sickness (AMS), such as headaches, dizziness and nausea.

As our bodies acclimatise, a more effective response to high altitude occurs. An increased concentration of the hormone erythropoietin (the hormone some athletes use to 'dope') triggers an increase in haemoglobin concentration (the iron-containing oxygen-carrying part of the blood ), much like the Aymarans' evolutionary adaptation. The body's apparatus that senses how much oxygen we have in our blood becomes more sensitive, allowing breathing rate to quicken. Furthermore, there is an increase in the blood vessel network within muscles which allows oxygen to be transferred to where it is needed in the muscle - the mitochondria - where it is used to liberate energy for muscle contraction.

So now we have some understanding of the acclimatisation process, how can we utilise our knowledge to reduce illness risk and enhance altitude performance?

Periodising acclimatisation

As high-altitude symptoms decrease, successful acclimatisation is said to have occurred. At this point physical and cognitive performance is increased compared to initial exposure but it is important to remember that performance will never be as good as at sea level. The real trick is to understand how long it will take for us to reach this level of acclimatisation for a trip to our intended altitude.

How long does it take to acclimatise?

Have a look at box 4. You can see that different physiological systems take different amounts of time to acclimatise to a new altitude. Ensuring your ascent or summit day occurs when breathing changes have stabilised, and after the body has had the opportunity to increase haemoglobin concentration, is key to maximising your performance. Symptoms of acute mountain sickness will also resolve naturally at the same time. There are too many factors at play to estimate exactly how many days this will take. However, its helpful to know that performance will often be at its lowest, and symptoms will often be at their greatest, 12 to 48 hours after reaching a new altitude. This is why after sleeping at a new altitude you often wake up feeling lethargic and ill; if you want to maximise performance and enjoyment, climbing higher at this point should be avoided if at all possible. Perhaps more difficult to accept is the knowledge that many elite athletes allow two weeks to acclimatise to even moderate altitudes (less than 3000m) before competing.

See text for explanation of terms.

How quickly should I ascend?

The safest and easiest-to-remember way to plan your trek is to use the rule of threes: once above 3000m, climb to sleep no higher than 300m of ascent each day, and take a rest day (i.e. sleep at the same altitude for an additional night) after every third day. However, this rule of thumb was determined using tourists making the trek to Everest base camp, who are not representative of many trekkers and climbers, so the rule of thumb has some caveats. First, some people will still get sick even when using this conservative ascent rate. Second, some people will be able to ascend quicker and not get sick. Third, for some short alpine trips using such a conservative ascent rate may restrict the number of climbs you can complete. The best predictor of whether a person will get altitude illness or not is previous history of altitude illness – if you have been ill before, you are likely to be susceptible to illness every time you climb. Taken together, use the rule of threes until personal experience tells you otherwise.

A grey area remains regarding how high you can climb in a day before returning to a recommended sleeping height. This discussion is important because restrictions on huts and campsites often force summit days that are long and have height gains that are greater than the 300m maximum rule of thumb. There is no scientific data on this aspect but previous observations (such as the written excerpt below) should motivate you to think seriously and manage this unknown risk accordingly. In reality, practical restrictions (e.g. on campsite/water availability) will most likely be your starting point when planning your ascent. Our suggestion would be to think cautiously and creatively in these situations to mitigate risk. For example, can you build in an additional acclimatisation peak or an extra night at a lower camp to aid acclimatisation? Set and stick to a cut off time for returning from your summit push to minimise exposure to risk. Systematically monitoring symptoms throughout the whole itinerary using a self-report system [see box 6] will also aid decision making on summit day about whether to ascend, wait, or descend.

Once acclimatised, how long can we stay acclimatised for?

If remaining at altitude, you will keep your acclimatisation indefinitely. Unless you are very high (more than 5000m) and for a long period of time (more than a few days), you will not deteriorate enough to decrease performance or increase illness risk. However, it is not yet known what happens if you return to sea level. How long you will remain acclimatised? Probably we lose altitude acclimatisation as quickly as we gain it. So reverse the timeline presented in box 4 (above). Or more simply, if planning consequent trips to altitude, try to re-ascend within one week.

Why is it that some people seem particularly prone to altitude sickness?

The causes of altitude illness are complex and poorly understood. See box 2 in our previous article. Hence, we do not know for definite why some people get sick and others do not. Note that being prone to altitude illness does not mean that a person will be unable to acclimatise; nor does it mean that they will perform physical exercise poorly at altitude. It just means that they may take longer to acclimatise.

Does having a cold influence illness susceptibility?

We have shown previously that having an upper respiratory tract infection (such as cold or flu) or a gastrointestinal upset (such as diarrhoea or vomiting) substantially increases your chances of developing altitude illness later in your trip. Luckily there are some simple ways to reduce risks of getting ill and prevention of colds, flu, and diarrhoea is thought far superior than trying to cure once sick. So, avoid getting ill in the first place by reducing risk of exposure (be brutal - avoid those that are ill such as young children; ask others to 'catch it, bin it, kill it' when sneezing or wiping their nose; ensure you and others wash hands regularly and thoroughly; use alcohol gel regularly). Its also possible to enhance your immune function to reduce illness susceptibility: get adequate sleep; eat a balanced, healthy diet; avoid psychological stress; allow adequate recovery from training and from long haul travel. See https://binged.it/2Y4Lnil for a helpful infographic on this topic.

Mont Blanc – a classic itinerary

In many circumstances, we try to shortcut the acclimatisation process suggested above due to short holiday periods, lack of knowledge about high altitude, and, quite frankly, impatience at attempting our specific goals. Scientific observations support that many of us attempt our alpine objectives far before our body has successfully acclimatised, but due to the hardy resilience of mountaineers (and a big dose of luck), some mountaineers manage to achieve their goals. So why bother to acclimatise? Calum and Jamie suggest taking the elite sport high-performance approach: maximise every aspect of your preparation, leaving nothing to chance. This is worthy even for the amateur mountaineer because it reduces chances of serious altitude illness; maximise chances of summit success; and perhaps most importantly, increases enjoyment by reducing symptoms such as fatigue.

Mont Blanc is a classic example of a mountain that sees many people not following a timely acclimatisation process. Its ease of access, only one hour's drive away from Geneva airport, and relatively straightforward normal 'Gouter' route, makes it a popular quick-hit to fit in with short holidays and cheaper guiding rates for three-day itineraries. It sees an average of 200 people summiting per day during the busiest summer months but has also seen the highest number of fatalities of any mountain in Europe. This has in part led to the controversial call by the St Gervais mayor to introduce a permit system capping the number of ascents to 214 per day.

Three-day itineraries for Mont Blanc are far less likely to end in a successful ascent than six-day itineraries. Of course, more time and opportunities to get stable weather for summit days will undoubtedly give you a much better chance of reaching the summit, but personal experience of seasoned Guides suggest that lack of acclimatisation is otherwise the most common cause of failure. Meanwhile scientific observations reveal that if a mountaineer has climbed above 4000m in the previous two weeks, their summit ascent time is likely to be a whole hour quicker.

A suggested six-day itinerary for Mont Blanc is presented in box 7. After arrival in Chamonix on day one, an ascent of a peak in the region of 4,000m (such as the Gran Paradiso) is completed in the first half of the trip, followed by a rest day in the valley and then a two- or three-day ascent of Mont Blanc itself. Itineraries such as these are popular with the more reputable guiding agencies but many clients ask whether the additional trip length and cost is really justified. We would argue that it is a resounding "yes" and here's why.

First, such an itinerary is good practice because it balances practicalities on Mont Blanc (e.g. suitable nearby practice peaks; hut position; cost) with the recommended rule of thumb on maximum ascent rate. Thus, physical performance will be maximised on the ultimate objective: Mont Blanc itself. This is very relevant because experience of seasoned Guides and scientific observations suggest slow ascent speed is the second most important cause of failure to summit (the first being inadequate acclimatisation triggering altitude illness). Second, symptoms will be reduced, increasing enjoyment of the climb, and reducing risk of developing more serious altitude illnesses. Remember, it is not at all uncommon for people with signs of serious altitude illness to be helicopter evacuated from the Goûter or Vallot huts. There are also many other more subtle but incredibly important benefits.

The first three days are likely to see you acclimatising to the activity as well as the altitude. Your pace will be adjusted to something sustainable, you'll get used to wearing stiff boots, using crampons, carrying a rucksack, handling the rope efficiently, and coping with early starts. If you hire a guide, you will become more efficient working with your Guide, as your pace will likely adapt to each other, communication will improve, and moving together will become slicker. If those benefits don't persuade you, note that shorter itineraries are likely false economy: Guiding companies with longer itineraries have the higher success rates, happier clients, and more positive reviews. Of course, you would reap even more dividends if you could make your alpine trip even longer…..

Beyond Mont Blanc

Moving beyond the alpine environment to the greater ranges, note these short-term altitude hits which we can 'get away with' aren't going to work. In the higher mountains, not only will we be venturing to greater altitudes, but we will also be sleeping at those altitudes and thus exposed to high attitude for a more protracted period, rather than the five or so hours we might spend above 4,000m on an ascent of Mont Blanc. This means performance will be decreased further, symptoms will be more common and more severe, and we are at greater risk of developing more serious altitude illnesses such as cerebral (HACE) or pulmonary (HAPE) oedema. These life-threatening illnesses will also be more difficult to manage (e.g. evacuation and access to medical care will be more difficult).

High altitude: beyond hypoxia

We mentioned earlier that high altitude poses multiple challenges to human survival. Have a look at the photo below and see if you can identify as many environmental and situational factors as possible that challenge human physiology and psychology (spoiler alert – some possible answers are given below!).

So, the factors we had included: increased arduous physical activity (to get there!); difficult terrain underfoot; ultraviolet light exposure (due to increased transmission through the thinner atmosphere and increased reflection from snow and ice); sun, wind and snow, combined with extreme temperature changes from midday heat to midnight cold increasing risk of heat exhaustion and cold injury; insufficient energy and fluid availability leading to nutritional deficit and dehydration; disrupted sleep; increased exposure to pathogens increasing risk of respiratory tract infections and gastro-intestinal upsets; and risk management and group dynamics (the guy was climbing with Calum – enough to stress anyone out!) leading to stress and anxiety.

If we return to our elite sport high-performance approach of maximising every aspect of our preparation, leaving nothing to chance, then these environmental and situational factors require planning and practice to manage them effectively. Chris Pugh, the physiologist tasked with supporting the successful 1953 summit bid on Everest, actually argued that these factors were as least as important as providing bottled oxygen to combat hypoxia in the team's summit success. We would add that they are also often far simpler to "fix". Try scribbling the environmental and situational factor you identified from the photo onto a piece of paper, and working with your climbing partner(s) to think how to mitigate these factors. Some tips:

A final environmental factor often forgotten that can have a significant effect on performance at altitude is changing weather. For those of us accustomed to using altimeters over the past twenty years, we can all remember that first occasion when the altitude reading didn't seem to face up to reality. Barometric altimeters, unlike their digital GPS cousins, use atmospheric pressure measurements to determine the altitude. For this reason, barometric altimeters need to be regularly re-calibrated to mitigate the effects of changing air pressure on readings. Changing weather systems from a sunny high-pressure front to a stormy low, have a significant effect on the air pressure and this is only exacerbated with altitude.

When we go mountaineering, especially in the higher mountains, we try to climb when the weather is nice and stable, meaning we are likely to be in a high-pressure air system. If we see a storm rolling in on the horizon, we know that there is about to be a change in the weather but importantly a low-pressure system is thundering towards us. At high altitude we are already experiencing significantly lower air pressure than at sea level and if a low-pressure system moves in, we can expect to feel like we're at a higher altitude again. The higher you go, the greater variability in altitude readings you will have on a barometric altimeter. In real terms, this can be as much as a 500m difference if the weather is to suddenly change from a clear high-pressure to a stormy low-pressure and if you're already at a high altitude – that can leave you struggling for breath even more (not to mention the effect of weather on your mountaineering activities and morale!) This means that poor weather can further decrease performance, increase altitude symptoms, and increase risk of more serious altitude illnesses.

Now that we are armed with a better understanding of the physiology of acclimatisation, we can plan our trips to take advantage of prime windows where we should be well acclimatised for our set objectives. Of course, successful acclimatisation is only part of the wider picture which will lead to achieving your objectives, but with a progressive and logical acclimatisation plan, you should be in the perfect position to take advantage of good levels of fitness, skill, weather and conditions.

In the next article in the series we will look at whether we can fast track, or even cheat, the acclimatisation process by implementing interesting recent research from unlikely sources.