Recently I did a trek in the Himalayan region and one of the group members was affected by what we believed was AMS at around 4600m. We had to evacuate the person from there and get to around 4100m before she stabilized.

On review of the situation we found the following:

  • She did not have any headache or nausea. She just fainted. Which pointed towards lack of oxygen supply to her brain (subsequently she fainted around 5-6 times while we were bringing her down).
  • The person concerned was on Diamox (should have helped her acclimatize, but I realize that this is not a guarantee).
  • She was well hydrated (she was drinking around 4L of water per day).
  • She did exert herself a bit more than required before AMS hit her (she was breathless as well, which is a symptom of AMS). There were no symptoms of headache or any nausea before that (or even after).
  • Her oxygen reading was around 85% and her HR was registered at 140+ around the time she fainted.
  • We had to administer Dexamethasone steroid to boost her lungs up and we saw an improvement in her condition in about 45min.

So my question is, can overexertion lead to AMS? Would it act as a possible factor in increasing the chances of a person being affected by AMS?

PS: I had a doubt if it was AMS in the first place, but since there's a possibility of lack of oxygen causing her to faint, I'm assuming it was AMS.

Based on one of the answers given by @wills, I'm adding further clarifications:

The person concerned did mention that she witnessed the 'darkness in front of eyes' before fainting, so I'm ruling out weakness as the reason. Also, this continued to happen some 5-6 times in the next 1 hour or so.


2 Answers 2


By 'faint' you mean the person lost consciousness or was it just weakness? I was in a similar position but not that severe. I was in the wrong assumption that I was acclimatizing fine because I was the strongest in the group during that time. I felt great, no headache or nausea or anything at all on the acclimatization mountains up to 4500m.

On the summit attempt I started getting weaker and weaker above 5000m. At 5500m I finally had to return because I was extremely weak plus nausea (no vomitting). My team members had no problems btw. For the details of my story please read this topic.

I think one reason in my example was that my body would have been better acclimatized if I hadn't gone so fast while hiking the preparatory mountains. This doesn't mean you are safe but it helps during acclimatization process if you go very slowly without getting anerob regularly. I can't find a medical paper for now, but at least here are two sources saying the same:

Once on the mountain, a slow walking pace is an essential rule. Keep your heart rate low, reduce your daily rhythm, and let go of your stress. Don’t agitate to go faster as it will only hurt your chances higher up the mountain.


When you first arrive to the high altitude environment, you should only exercise at a level of about half as much as you did in an oxygen-rich environment, and do so at a slower pace. Try not to push yourself to the point of breathlessness for long periods at altitude is the wrong way to go. A slow and steady pace will aid the acclimatization process. It’s better for conditioning, to slow down and continue moving than it is to stop and take breaks. Gradually increase your distances and this is built into our treks and acclimatization program.


Yes, overexertion reduces the ability of the body to acclimate properly and therefore increase the chances of getting AMS.

By the way the Diamox makes this question more difficult so I wasn't referring to this part of the story. I am just generally saying you should keep a slow pace!

  • I believe it was fainting. She did mention that the 'blackness' in front of eyes that takes place while you faint is what she experienced. May 20, 2017 at 6:11
  • I have a question here. Why would a faster pace not allow for acclimatization? Is it because of shallow breathing vs deep breathing? May 20, 2017 at 6:33

Acute Mountain Sickness (AMS) is the name for a group of conditions that arise from the affects of altitude on the body. (source) The time for these conditions to develop varies and are influenced by many factors.

The symptoms your group member experienced are most likely came from an acute lack of oxygen supply to the brain. This would generally be called AMS in the context of high-altitude trekking, but is a more acute condition than the ones usually described in discussion about proper acclimation. The last part of my answer goes into more detail about the other kinds of symptoms.

At higher elevations, less oxygen is available to the body. A range of adaptations to compensate happen over a wide time frame. The most immediate ones have to do with breathing, heart rate, and blood supply. It's easy, even on foot, to ascend much faster than these basic mechanisms can cope with. When the air is thin, breathing hard is not enough to get more oxygen into the blood. There needs to be more blood moving through the lungs, and it needs to stay there longer to become oxygenated. The body needs time to properly adjust.

Exertion increases the oxygen demands of the body. If not properly acclimated, exertion can create an oxygen deficit where it is "used" faster than it is replaced. This leads to severe short term affects like fainting, but luckily can be resolved quickly by descending and rest.

This book goes into detail on the biological mechanisms of the affect of altitude on the body.

Can overexertion lead to AMS?

Yes, at altitude it is easy to overwhelm the body's ability to supply oxygen, which is one of the causes of AMS.

Would it act as a possible factor in increasing the chances of a person being affected by AMS?


Examples of other AMS symptoms are: nausea, weakness, insomnia, and lack of appetite. High Altitude Cerebral Edema (HACE) and High Altitude Pulmonary Edema (HAPE), which are life-threatening, are sometimes included under AMS and sometimes categorized separately.

Proper acclimation can reduce the chance of being affected. Since these conditions are complex and involve multiple anatomical systems, this is not as well understood from a scientific perspective. However, there are vast amounts of experiential evidence that show the best way to do it:

  • Ascend slowly
  • Conserve energy, avoid overexertion
  • Stay hydrated
  • Eat properly
  • Sleep well
  • Hi ararat and welcome to TGO. Great first answer! I think it is important to stress your third paragraph: AMS symptons due to exertion are not as dangerous as those at rest, as there is an easily available "therapy": rest.
    – imsodin
    Nov 10, 2017 at 21:38

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