Normal acclimatization

Acclimatization is the process of the body adjusting to the decreased availability of oxygen at high altitudes. It is a slow process, taking place over a period of days to weeks.

High altitude is defined as:

  • High Altitude: 1500 - 3500 m (5000 - 11500 ft) 
  • Very High Altitude: 3500 - 5500 m (11500 - 18000 ft) 
  • Extreme Altitude: above 5500 m

Practically speaking, however, we generally don't worry much about elevations below about 2500 m (8000 ft) since altitude illness rarely occurs lower than this. 

Certain normal physiologic changes occur in every person who goes to altitude:

  • Hyperventilation (breathing faster, deeper, or both) 
  • Shortness of breath during exertion 
  • Changed breathing pattern at night 
  • Awakening frequently at night 
  • Increased urination

As one ascends through the atmosphere, barometric pressure decreases (though the air still contains 21% oxygen) and thus every breath contains fewer and fewer molecules of oxygen. One must work harder to obtain oxygen, by breathing faster and deeper. This is particularly noticeable with exertion, such as walking uphill. Being out of breath with exertion is normal, as long as the sensation of shortness of breath resolves rapidly with rest. The increase in breathing is critical. It is therefore important to avoid anything that will decrease breathing, e.g. alcohol and certain drugs. Despite the increased breathing, attaining normal blood levels of oxygen is not possible at high altitude.

Persistent increased breathing results in reduction of carbon dioxide in the blood, a metabolic waste product that is removed by the lungs. The build-up of carbon dioxide in the blood is the key signal to the brain that it is time to breathe, so if it is low, the drive to breathe is blunted (the lack of oxygen is a much weaker signal, and acts as an ultimate safety valve). As long as you are awake it isn't much trouble to consciously breathe, but at night an odd breathing pattern develops due to a back-and-forth balancing act between these two respiratory triggers. Periodic breathing consists of cycles of normal breathing which gradually slows, breath-holding, and a brief recovery period of accelerated breathing. The breath-holding may last up to 10-15 seconds. This is not altitude sickness. It may improve slightly with acclimatization, but does not usually resolve until descent. Periodic breathing can cause a lot of anxiety:

  • In the person who wakes up during the breath-holding phase and knows he has stopped breathing. 
  • In the person who wakes up in the post-breath-holding hyperventilation (recovery) phase and thinks he's short of breath and has High Altitude Pulmonary Edema (HAPE). 
  • In the person who wakes up and realizes his neighbor has stopped breathing.

In the first two cases waiting a few moments will establish a normal breathing pattern. In the final case, the sleeping neighbor will eventually take a breath, though periodic breathing cycles will likely continue until he or she is awake. If periodic breathing symptoms are troublesome, a medication called acetazolamide may be helpful.

Dramatic changes take place in the body's chemistry and fluid balance during acclimatization. The osmotic center, which detects the "concentration" of the blood, gets reset so that the blood is more concentrated. This results in an altitude diuresis as the kidneys excrete more fluid. The reason for this reset is not understood, though it has the effect of increasing the hematocrit (concentration of red blood cells) and perhaps improving the blood's oxygen-carrying ability somewhat; it also counteracts the tendency for edema formation. It is normal at altitude to be urinating more than usual. If you are not, you may be dehydrated, or you may not be acclimatizing well.