Understanding elevation and staying safe on high elevation hikes!
In my article "Training for a multi day back packing trip" I describe a mountain mile as being 1.5 times more difficult than a sea level mile (hence the name of the site!) as a general rule of thumb - predominantly because you usually will have a heavier pack, tougher terrain, and the biggest factor - oxygen levels.
The chart here shows the oxygen levels at various elevations. In the lower 48 states of America and here in California, Mt Whitney is the tallest mountain and stands at 14,505 feet. For comparison, Clouds Rest (my favorite mountain) stands at 9,931 feet, and Yosemite Valley stands at 4,000 feet. Respectively (and rounding up) this means that you have 58% oxygen levels at the top of Whitney, 70% for Clouds Rest and 84% in Yosemite Valley.
So at the 10,000 level that most of my back country hikes are at, I have climbs, 30lbs on my back and 70% oxygen levels - again, a mountain mile!
So aside from being tiring, there is also the practicality of acclimatization - essentially giving your body time to adjust to higher elevations and lower oxygen levels. There are a plethora of articles, science journals, forums and opinions out there including this article from the CDC, so I am just going to break down my approach non-scientifically. Why? Because different people doing the exact same hike may experience different effects, and in my case, having done the same hike multiple times, I have experienced altitude sickness in some instances, and not in others. However, since these are life-threatening conditions I strongly recommend your seek out science based research as this is merely an overview.
My approach:
Begin training 10 weeks in advance (see this article).
Begin hydrating 4 days in advance (120-150 oz of water a day).
Drive to elevation 8,000 from sea-level (advised against by many others) and spend one night camping at 8,000.
Take ibuprofen as needed (not exceeding directions)
Avoid exertion for 24 hours at 8,000.
Day one - avoid significant climbs (over 1,000 EG) and long distances (over 10 miles).
Day two - "Climb High, Sleep Low". This means that your peak for the day is higher than you eventual camp site, meaning you are not spending/sleeping at significant gain.
Days three +: Aim for gains no more than 1,000 ft per day.
Types of Altitude Sickness:
Acute Mountain Sickness (AMS) AMS is common at high altitudes. Several factors influence the possibility of developing AMS such as elevation, the rate of ascent, and individual susceptibility. Symptoms usually start within 24 hours, and begin to decrease as the body adjusts, usually within 24 hours, but sometimes up to 72 hours. The symptoms of mild AMS are fatigue, difficulty breathing, headache, nausea, loss of appetite, and dizziness. As long as symptoms remain mild, ascent can continue at a moderate rate if the person feels up to it - which they may not!
In it's moderate form AMS may cause the person may experience some or all of the above symptoms, BUT will also feel that they are having difficulty with normal activities. In this instance descent of 500 feet or more is necessary. Descent will usually be sufficient to stabilize and after 24-72 hours, the person should be able to resume ascending.
In it's severe form, AMS causes shortness of breath at rest, inability to walk, decreasing mental status, and fluid buildup in the lungs. Severe AMS requires immediate descent to lower altitudes (2,000 - 4,000 feet).
High Altitude Pulmonary Edema (HAPE) HAPE is fluid buildup in the lungs that prevents effective oxygen exchange. In severe cases, the level of oxygen in the bloodstream decreases, and this can cause cyanosis, impaired cerebral function, and death. Symptoms include shortness of breath at rest, severe tightness in the chest, sometimes feeling like suffocation, weakness, and a persistent cough that brings up watery fluid. A person may feel confused, or act irrationally. Immediate significant descent is absolutely necessary and medical attention is required.
High Altitude Cerebral Edema (HACE)
HACE is when the brain tissue swells, caused by fluid leakage. It is caused by extended periods of time at high elevation. Symptoms can mimic those of AMS and HAPE, but memory loss, psychosis, hallucinations are common also, and this serious condition can result in coma. Severe instances can lead to death. Immediate descent of 2,000 - 4,000 feet is necessary to avoid death, as is medical attention.