|Figure 1. A mountain climber above the clouds.|
You have been training for this day for a long time. Climbing local hills in your home town as you build up your endurance to a point where you feel ready to take on the challenge of a large mountain. Now imagine you’ve spent two days climbing Mt. Whitney and you can see the summit. You are at 14,000 feet above sea level and yet the last couple yards seem impossible. All of a sudden you feel as though you had one too many shots at the bar. You’re seeing double and gasping for air, as if someone is sitting on your chest. You feel dizzy and have to sit down after taking a step. Aside from the shifting daily temperature extremes and decreased humidity, why is this happening to you?
The answer is simple; it’s the air pressure at high altitude. As you ascend up into the mountains the pressure of the atmosphere decreases relative to atmospheric pressure at sea level. However, the percentage of oxygen in the air doesn’t really change. Whether you are at sea level, or 500 feet from the summit of Mt. Whitney the air you are breathing contains about 21% oxygen. So why does it feel so hard to catch your breath at higher elevations? Well, as I mentioned before, the decrease in atmospheric pressure means that there are fewer molecules present…basically there is less “air.” Therefore while the ratio of oxygen relative to all of the other gases that make up the air we breathe doesn’t change, each breath we take contains less oxygen, which is why at higher altitudes you might feel as though you are breathing through a straw.
|Figure 2. Oxygen availability compared to relative atmospheric pressure.|
Yet, we see people around the world who live in regions of high elevation and seem to lead relatively normal lives. In fact, Benjamin Levine and James Stray-Gundersen found that acclimatization to moderate altitude when combined with lower altitude training enhances athletic performance in competitive athletes. How is this possible given that we just discussed how breathing is more difficult the higher you are above sea level? Well, given time, your body is able to acclimate to these conditions. Sensory organs called carotid bodies can detect lack of oxygen in your blood and immediately alert the brain to regulate/elevate breathing to restore oxygen levels. Your body also increases red blood cell production, which inadvertently increases your body’s ability to transport oxygen through your bloodstream. This is because more hemoglobin, an oxygen-binding protein found on your red blood cells, is available to bind oxygen. Moreover, your heart is pumping faster to decrease the amount of time it takes oxygenated blood to circulate through your body…so in short, you have more efficient blood and circulation!
However, despite all of this, oxygen deprivation can be very serious and can elicit several additional physiological changes in your body. First of all, your body increases its metabolic rate due to increased energy expenditure required to maintain all of these compensatory functions. Not only will this cause your body to burn more carbohydrates and fat to replace depleted energy levels, but a study by Matthias Tschop and his colleges found that at higher altitudes people also have higher levels of leptin, a hormone that regulates energy homeostasis and appetite. Therefore, at higher altitudes, people also experience a loss of appetite without even being aware of it. This can be detrimental due to the increased energy demands your body has to function at higher elevations.
As mentioned earlier, the decreased humidity and low air pressure often experienced by hikers can quickly lead to dehydration. In combination with increased respiratory function, dehydration can contribute to the onset of respiratory alkalosis. Respiratory alkalosis is a condition affecting the pH of your blood. When your breathing rate elevates, the relative amount of carbon dioxide in your blood stream is depleted as you try to maximize oxygen intake. In the long run, this will inevitably lead to a decreased respiratory rate and you will not be able to continue at the rate needed to maintain adequate oxygen levels. Your breathing will slow down and become more erratic, sometimes stopping for several seconds at a time until carbon dioxide levels return to normal.
Acute Mountain Sickness (AMS) is an altitude-related neurological condition. As with most forms of altitude sickness, AMS can be treated with over the counter drugs, but not cured without acclimatization or descent to lower elevations. Symptoms include anything from headaches to loss of coordination and trouble thinking.
|Figure 3. What is high altitude?|
Meanwhile, High Altitude Pulmonary Edema, HAPE, is a condition characterized by fluid leakage from your blood vessels into your lungs! When oxygen levels are low, your blood vessels often constrict causing your blood pressure to elevate, forcing fluid out of spaces in vessel walls. Your chest will become tight making it harder to breath and the fluid buildup in your lungs will cause you to wheeze/cough persistently. If that wasn’t bad enough other side effects include confusion, amnesia, and possibly even loss of consciousness, so it is very important to be aware of your symptoms!
But, even more serious than HAPE is HACE, High Altitude Cerebral Edema, which manifests if blood vessels in your brain begin to leak fluid into the surrounding tissue. Your brain is your body’s control center; it helps regulate all of your organs, hormones, extremities and biological processes. Therefore, compromising any of these functions can lead to serious complications or death. HACE is similar to HAPE in that people who have it may experience symptoms that alter their motor control or cognitive ability. Needless to say, seeking immediate help is imperative. Descending to lower elevations may help you to restore full mental capacity, however it can take up to several weeks to feel like you have your bearings back.
|Figure 4. Summary of symptoms.|
A study by Fayed and his colleges revealed that HACE could have extensive long-term neurological damage with respect to your overall mental capacity, memory and coordination. In this study Fayed examined MRI brain scans of 12 professional and 23 amateur climbers after they had returned from high altitude expeditions on Mt. Kilimanjaro (Tanzania, 19,341 ft.), Mt. Aconcagua (Argentina, 22,837 ft.), Mt. Blanc (France, 15,781 ft.) or Mt. Everest (Nepal, 29,029 ft.). Of the 12 professionals and one amateur climber that attempted to conquer Mt. Everest, brain damage in all but one professional climber was seen upon their return. Based on these results Fayed concluded that high altitude climbing results in irreversible brain damage. While acclimatization to higher elevations can minimize these consequences, it cannot negate the side effects completely. So, if you are planning on conquering one of these great mountains, just keep in mind that your brain will probably not be in the same condition that it was before you left.
|Figure 5. Several hikers climbing Mt. Everest|
Wow! I know that was a lot to take in, but hopefully you were able to learn just how important it is to be aware of your body and how you’re feeling when you’re up at high elevation. Do not let this discourage you from taking on the challenge, because the sense of accomplishment along with the breath taking views make the physical struggle of the climb all worth it. Just be smart about it. If you experience any of these symptoms, stay put or descend immediately. Altitude sickness is nothing to mess around with. These symptoms can manifest themselves in even the most fit of athletes. Drink plenty of water to avoid dehydration…and most importantly, give your body time to acclimate! Happy hiking!
Basnyat, B. and D.R. Murdoch. High-altitude illness. 2003. The Lancet 361: 1967-1974.
Curtis, R. Outdoor action guide to high altitude: acclimatization and illnesses. Outdoor Action Program, Princeton University, 1995. Web. 05 Feb. 2013.
Fayed, N., P.J. Modrego, and H. Morales. 2006. Evidence of brain damage after high-altitude climbing by means of magnetic resonance imaging. The American Journal of Medicine 119(2): 168.
Hackett, P.H. and R.C. Roach. 2001. High altitude illness. New England Journal of Medicine 345:107-114.
Hackett, P.H. 1999. High altitude cerebral edema and acute mountain sickness: a pathophysiology update. Advances in Experimental Medicine and Biology 474: 23-45.
"High Altitude Sickness in Tibet: What Is High Altitude Sickness?" Tibet Discovery. N.p., n.d. Web. 8 Feb. 2013.
Levine, B.D., and J. Stray-Gundersen. 1997. “Living high-training low”: effect of moderate-altitude acclimatization with low-altitude training on performance. Journal of Applied Physiology 83: 102-112.
"Mountain-climber." Korblaw Offices Bankruptcy Law Blog. N.p., n.d. Web. 08 Feb. 2013.
"Mountain Hiking." Hiking Dude. N.p., n.d. Web. 08 Feb. 2013.
Peacock, A.J. Oxygen at high altitude. 1998. BMJ 317(7165): 1063–1066.
Prabhaker, N.R. 2000. Oxygen sensing by the carotid body chemoreceptors. Journal of Applied Physiology 88(6): 2287-2295.
Schaffer, Grayson. "Everest Deaths in 2012 | Mount Everest | OutsideOnline." Outside Online. Outside Magazine, 12 Sept. 2012. Web. 08 Feb. 2013.
Tschop, M., C.J. Strasburger, G. Hartmann, J. Biollaz, P. Bartsch. Raised leptin concentrations at high altitude associated with loss of appetite. 1998. The Lancet 352: 1119-1120
Virues-Ortega, J., G. Buela-Casal, E. Garrido, and B. Alcazar. 2004. Neuropsychological functioning associated with high-altitude exposure. Neuropsychology Review 14(4): 198-224.
Vilas, G.L. On the mountains: high altitude sickness in Nepal. Journal of Young Investigators. 2006. Web. 08 Feb. 2013.http://www.jyi.org/issue/on-the-mountains-high-altitude-sickness-in-nepal/