Aconcagua Altitude Sickness And Acclimatization


Climbing to high altitude always comes with risks. This is certainly true of Aconcagua where less than 50% of climbers reach the summit.

The main reason for this low success rate is Acute Mountain Sickness (AMS) or, as it is more commonly known - altitude sickness.

On top of this, many climbers exhibit the more severe forms of AMS which includes High Altitude Cerebral Edema (HACE) and High Altitude Pulmonary Edema (HAPE).

Therefore, to help you prepare for your Aconcagua climb, we have written this detailed page describing the process of proper acclimatization and also giving you an overview of the altitude sickness symptoms that many climbers experience on their trek. You can also find information on preventative medications such as Diamox.

Please note: We are not medical experts and this information is given as a resource only. The information should not be relied upon as a diagnostic resource or as a patient-doctor relationship. Research surround altitude acclimatization is constantly evolving and we urge ll our readers to get the most up to date information they can. Anyone who is about to climb Aconcagua should seek medical advise first.

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Aconcagua Acclimatization - Introduction

The term acclimatization essentially describes the process in which your body adapts to higher altitude in which there is less oxygen to breathe. Sadly for fast climbers, this process can only be achieved by moving higher very gradually.

Oxygen levels

Acclimatization is most easily understood when you look at how air density and oxygen change at higher altitude.

At ground level air density it at its thickest because of the pressure, however, when you climb the pressure becomes less, the air density lessens and so the oxygen molecules spread out and become far less concentrated. This means that as humans, we struggle to breathe because the oxygen is not packed tightly together. The molecules are spread out like a very thin layer of butter over too much bread. Therefore, we get less oxygen per breath.

For example, at about 3,600 meters (12,000 feet), barometric pressure is around 480 mmHg. With less air density, oxygen molecules are more widely dispersed in any given column of air and hence less oxygen is available per breath.

Blood oxygen saturation

To counter the altitude, our bodies naturally adapt by breathing deeper and faster to increase the oygen levels in our blood - much like when we're running. Because we breathe faster and deeper (even when resting), our blood oxygen saturation increases.

The chart below demonstrates the blood oxygen saturation levels as one ascends higher in altitude. Notice that at 7,000 meters (Aconcagua summit height), oxygen saturation levels have decreased to roughly 70%!

Altitude Zones

The general altitude sickness line is 2,400 meters. However, some people will experience symptoms below this altitude whilst many will be able to go much higher with no negative effects.

Predicting if you will suffer from AMS is exceptionally difficult. All research suggests that age, fitness, gender, and health bear no correlation to your chances of experiencing AMS.

It is know though that climbing too fast will increase your chances, as will over exerting yourself and not keeping well hydrated. Therefore, to give yourself the best chance of reaching the summit of Aconcagua, you should be planning a slow route whilst ensuring you drink plenty of fluids and rest often, even if you feel well.

One common term climbers refer to is their acclimatization line.

The term is used to describe someones acclimatization point, i.e where your body can go to without feeling the effects of altitude.

Everyone's acclimatization line will be different. This often makes it slightly difficult when climbing with a group and why many people are left behind. As soon as you hit your acclimatization line you need to stop and allow your body to acclimatize to the new altitude. If you stop immediately, your body will be able to adjust without having to descend.

If you continue to ascend past your acclimatization line then your body won't be able to cope and the only to ease your symptoms will be to rapidly descend.

How your body adapts and acclimatizes

Luckily, our bodies are intelligent and learn to adapt to their new surroundings. Given enough time, your body will be able to adapt to very high altitudes. Among the most notable are:

1. You begin to breathe deeply and more frequently

2. Higher rate of breathing increases Red blood cell count. This allows oxygen to be carried through the blood more easily

3. The pressure in your pulmonary capillaries greatly increases which forces your blood into areas of your lungs that are dormant when breathing at sea level

4. An enyme is produced thast allows oxygen to be released from hemoglobin to the blood tissue

Acute Mountain Sickness (AMS)

AMS is extremely common at high altitudes, especially at altitudes above 3,ooo meters where at least 75% of individuals experience some form of mild altitude sickness.

Experiencing AMS is very much dependent on how fast you ascend, how high you ascend and how susceptible you are to the sickness.

Most people will experience some form of altitude sickness, usually within 12-24 hours of arrival. If the symptoms are mild then symptoms will usually clear after 24 hours if you stay at the same altitude.

Climbers describe three levels of altitude sickness, mild, moderate and severe. We look at each below.

When climbing Aconcagua, it is essential that you tell your fellow climbers or guide as soon as you experience any altitude sickness symptoms.

Mild Symptoms

These include:

  • Fatigue
  • Headaches
  • Loss of appetite
  • Disturbed sleep
  • Shortness of breath
  • Nausea and Dizziness

Even though these are mild symptoms. if you experience any of the above it is critical to tell your climbing partners or guide. Mild symptoms will usually disappear if you stay put for a day, however, if you continue to ascend symptoms will get worse rapidly.

Moderate Symptoms

These include:

  • Throbbing headache that does not go away with medication
  • Very fatigued and weak
  • Breathing difficulty
  • Feeling very sick - vomiting
  • Decreased coordination

The most obvious way to tell that your symptoms are moderate is when your mild symptoms have increased to a level in which the symptoms become debilitating. Most climbers on Aconcagua who have mild symptoms will have bad headaches and be vomiting. However, we have also seen plenty of people loose coordination and speech.

Most people on Aconcagua with these symptoms cannot walk. However, even if you can then don't. To continue to ascend would put your life at risk. Your symptoms would continue to deteriorate and you would need to be evacuated off the mountain on a stretcher. If you tried to continue your condition may result in death.

If you do experience moderate symptoms then you must descend immediately. This is the only cure. You will need to descend at least 1,000 feet and remain there until all symptoms have passed. Only then should you begin to ascend once more.

Severe Symptoms

These include:

  • Inability to walk
  • Loss of mental capacities and hallucination
  • Shortness of breath whilst resting
  • Fluid build up on the lungs

To ascend with severe symptoms is nearly impossible and should never happen. The result would likely lead to death. People who are experiencing severe altitude sickness will be struggling both mentally and physically. Walking will be out of the question and the climber should be carried down the mountain as soon as can be arranged. A person with severe symptoms will have slurred speech, poor coordination and often a splitting headache.

When a climber gets to this stage they will often experience one of two conditions associated with severe AMS. These are High Altitude Pulmonary Edema (HAPE) and High Altitude Cerebral Edema (HACE).

HAPE is when fluid leaks through into the capillary walls of your lungs, where as HACE is when fluid seeps into your brain. Both conditions are uncommon and almost always occur from poor acclimatization. We discuss both below.

High Altitude Cerebral Edema (HACE)

High Altitude Cerebral Edema (HACE) is a life threaning condition and every possible precaution should be taken against it. HACE often follows on from AMS and is the result of fluid seeping into the cranium due to brain tissue swelling.

Anyone who experiences HACE on Aconcagua should descend straight away and seek medical support.

HACE symptoms include the following:

  • Severe headaches which don't go away with medication
  • Loss of consciousness
  • Disorientation
  • Hallucination
  • Memory loss
  • Loss of coordination (i.e. ataxia)
  • Coma

HACE often presents itself during the night. If this is the case, you must descend immediately, even if dark. The condition will only worsen by morning and the longer you leave the disease untreated, the higher the chance of death.

If anyone in your crew has bottled oxygen then administer this to the sufferer. If anyone has the steroid dexamethasone, this should also be administered. However, this is no substitute for descending.

Climbing back down to a lower altitude is the only cure.

High Altitude Pulmonary Edema (HAPE)

HAPE is the result of fluid build-up in the lungs. When this occurs, the fluid build-up prevents free oxygen exchange.

HAPE is a very serious condition and if left untreated will lead to cyanosis, reduced cerebral function and death. Whilst deaths are uncommon on Aconcagua thanks to professional guides, deaths do occur. Because it's a life threatening disease, every possible precaution should be taken to prevent such an occurrence.

Almost always, climbers on Aconcagua experience HAPE after ascending too quickly.

Clear symptoms of HAPE include the following:

  • Very short of breath, even while resting
  • The feeling of suffocation, particularly when asleep
  • Extreme fatigue and weakness
  • Cough that brings up white, frothy fluid
  • Very tight chest
  • Confusion, hallucination and irrational speech and behaviour

If the climber presents the last symptoms mentioned (confusion, slurred speech, irrational behaviour), then it is safe to assume that pulmonary edema has taken effect of the brain from lack of oxygen.

When suffering HAPE it is absolutely vital that you descend immediately as this is the only cure. Depending on who you climb with, a member of your crew or team may have bottled oxygen, if so, administer this straight away.

If a person in your group is suffering from HAPE, make sure they don't exert themselves when descending as this will only harm them further. If a stretcher is available this is usually the best option.

After descending to ground level you must seek medical support straight away, even if your symptoms are clearing.

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Lake Louise Altitude Sickness Scorecard

Whilst there are several ways to monitor ones AMS symptoms, the most common system is using the  Lake Louise Altitude Sickness Scorecard (as demonstrated below).

Mild to moderate symptoms is a score anywhere between 3 and 7. A score above 7 indicates severe AMS.

A good Aconcagua operator will use the Lake Louise Scorecard system, along with spot oximeter and pulse readings. This is the most accurate way to tell if a climber is suffering AMS.

If your reading are above 8 on the scorecard or if you have a SO2 reading below 75%, you should turn back and descend off the mountain. The Base Camp at Aconcagua usually has spot oximeter's for you to use, however, it's always advisable to bring your own.

Golden Rules

To give yourself the best chance of summiting Aconcagua, always follow these golden tips:

  • Try pre-acclimatizing before climbing Aconcagua. There are many good trekking mountains in the Andes that will serve as good warm-ups.
  • If you are a novice climber then choose the easiest route (the Normal Route) and take it slowly.
  • Ensure the itinerary has  a good climb high, sleep low profile (check with your operator)
  • Go slowly, rest often and keep hydrated
  • Avoid alcohol, cigarettes, stimulants and caffeine
  • We recommend taking the drug Diamox with you

3 Golden Rules to keep in mind in regards to AMS:

  • If you feel at all unwell, consider yourself to have altitude sickness until proven otherwise
  • Do not ascend higher if you have any symptoms of altitude sickness
  • If symptoms do not clear then descend immediately

Altitude Sickness Medications - Diamox

Acetazolamide, or Diamox, is one of the few drugs that has been proven to help with altitude sickness.

The drug enables your body to breathe faster to increase more oxygen in your blood. it does this by increasing the acidity in your body which your body takes as increased CO2.

Whilst you can breathe fast during the day if you so choose, at night Diamox is particularly effective as it stops your body from breathing too slowly.

It is generally recommended that you begin to take Diamox 24 hours before ascending to altitude. 125 mg twice a day is the recommended dose for climbers.

Although side effects are uncommon, they can include tingling lips, tingling finger tips, blurred vision and a slight alteration in taste.

Diamox should not be taken as a cure to keep ascending. If AMS symptoms present themselves, the only cure is descending. Please note that Diamox is a prescription drug and you'll need to consult your doctor before purchasing.

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About the author 

Andrew Roux

Andrew is one of the senior writers at Mountain IQ. A native of South Africa, Andrew has hiked and climbed all over the world. His favourite destination is Nepal and his most memorable hike was to the summit of Mount Kilimanjaro!

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  1. Hello,
    We live in Bogota, Colombia where the elevation is approximately 8,600 feet. We also climbed Kilimanjaro with no problem. With that in mind do you think we are ready for Aconcagua?

    1. Hi Rick, technically you should cope better with the altitude than people who live at lower altitudes, but there is no guarantee that you won’t be affected by altitude sickness. The golden rules remain – go slowly, no rapid ascent, and descend if you start experiencing moderate to severe symptoms.

  2. Howsit Andrew! Shareez here from Durban. I’ve recently summited Mt Kilimanjaro on an 8 day route. I only experienced mild altitude sickness the night before summit, no vomiting etc. I would love to tackle Aconcagua next. What tips can you offer for the trek, in terms of route, tour operators, training, and altitude sickness? Thanks!

  3. 1/1/2024. I just returned from Aconcagua with HAPE. Very discouraging, didn’t even make it to Mulas. Quick question, everyone says “no diamox”, why do you suggest this? I was told that due to the arid nature of the Andes, Diamox will work against you, not for you. I will be back again, just got to sort out my lungs. Also, FYI, Kilimanjaro and Elbrus I’ve summited successfully. Two months ago, Pikes Peak in CO (and again in 7/23) and Mauna Kea in HI, no problem.

    1. Hi Chris, sorry to hear that you experienced bad altitude sickness on Aconcagua. I’m unfortunately not a doctor so not too sure what is better diamox or no diamox. All I know is I take it when going to high altitude and it works for me. Aconcagua may be a different beast though as it is a longer hiking expedition at altitude so there is more time to acclimatise, and one probably doesn’t need to synthetically drive this process.

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