Tibet: Altitude Sickness Primer

I've experienced altitude before and, coincidentally, it was during my previous trip to China 9 years ago. I was on Emei Shan which is just over 3000 m at its peak (or roughly 10,000 ft.). I had no idea at the time what altitude I was at but I definitely noticed a marked decrease in stamina. After bounding up one flight of stairs I'd be huffing and puffing madly from the exertion.

Tibet was going to be a completely different kettle of fish because the altitude is above a safety zone. Up to 3,000 m you are quite safe with shortness of breath on exertion being the only symptom you might feel. Not coincidentally, 3,000 m is also what the cabin of commercial flights is pressurized to. Over 3,000 m you are in altitude sickness territory and you have to be a lot more careful with acclimatisation otherwise you can get very, very ill. With Tibetan valleys at 4,000 m and passes over 5,000 m, preparation is key.

Altitude sickness is one of those conditions where we know what happens and we know the causes but we don't exactly know how it happens. The low oxygen pressure in the atmosphere will result in hypoxia and low oxygen levels in your body. This is not good. Your body can adapt but most of the processes will take time. First, you will unconsciously start to hyperventilate in an attempt to get more O2 and this is why you get breathless so easily with exertion. Next, your red blood cells will build up more 2,3 DPG which is a compound that encourages more O2 offloading in the tissues. Finally, your bone marrow will kick in and produce more red blood cells in an attempt to increase the O2 carrying capacity of your blood.

Acclimatisation is very individualized so there are no hard and fast rules. In general, you should try to spend at least one night below 3,000 m. Starting at 3,000 m, you should not try to ascend more than 300-500 m per night. In addition, mountain climbers have developed a simple rule: "climb high, sleep low" which emphasizes the importance of your sleeping altitude. You climb high during the day so you can monitor yourself for symptoms and expose yourself to lower oxygen levels. You then sleep at night at a lower, safer altitude. There is one cardinal rule: if you get altitude sickness you must not climb any higher!

Altitude sickness is diagnosed when, after a recent ascent, one gets a headache and one of the following: nausea and vomitting, loss of appetite, fatigue or weakness, dizziness or light headedness, and insomnia. Since these symptoms are also common to other conditions and may be somewhat vague, the rule-of-thumb is that one experiencing these symptoms has altitude sickness until proven otherwise.

The symptoms are thought to be the result of cerebral edema as a reaction to cerebral hypoxia. In its most severe forms, altitude sickness can be quite lethal within a matter or hours or a few days. (Your brain swells but your skull does not).

High altitude pulmonary edema is also a known condition that develops at altitude although it is unrelated to altitude sickness per se. Your pulmonary vessels will physiologically vasoconstrict when exposed to hypoxia. In some people, the response is so vigourous that the pressures in your pulmonary circulation skyrocket resulting in pulmonary edema with fluid leaking into your alveoli. With fluid in the alveoli, your lungs are even less efficient at extracting oxygen from the air and a vicious cycle develops. Pulmonary edema can also be quite lethal and like cerebral edema it's considered to be a medical emergency.

There is only one treatment or cure for altitude sickness -- immediate descent to a lower altitude. There are a number of medications that can temporize the situation or hasten acclimatisation but they do not cure. Medevac equipment includes a special bag that attempts to raise the ambient pressure of the patient and supplemental oxygen (which is a luxury in high, remote places and is reserved for the sickest patients).

There is no way to predict who will or will not get altitude sickness. In addition, if a person has successfully ascended to altitude before it is no guarantee s/he will not develop altitude sickness in the future on an identical itinerary.

After reading a few websites I made the following preparations:

1. Overnight in Xining (2275 m). I would have liked to stay for 2 nights but lacked the time.
   
   
2. Take the train into Lhasa. Flying into Lhasa was ruled out. Even though the train is pressurized, it's only to 3,000 m and it will count as a day of acclimatization.
   
   
3. Itinerary. Fortunately, all the Tibetan tour companies start their tours with 4 days in Lhasa to help their sea-level customers acclimate.
   
   
4. Drugs: Acetazolamide to speed up acclimatization, and Dexamethasone (steroid) for emergencies.

Acetazolamide is a drug which inhibits carbonic anhydrase in your kidneys and forces you to excrete more bicarbonate. As you hyperventilate at altitude you blow off a lot of CO2 and this increases the pH of your blood. This high pH is thought to cause the breathing and sleep difficulties at night. Acetazolamide has the effect of lowering your blood pH to counteract the effect of hyperventilation. It's ability to speed up acclimatization isn't really understood. Another important effect of the drug is its diuretic effect on the kidneys. One needs to be careful and drink plenty of fluids while on this drug because you're already at moderate risk for dehydration by hyperventilating in an arid environment. Personal experience: after acetazolamide and several cups of tea I was running for the bathroom in less than an hour!

Dexamethasone is a steroid that treats inflammation and reduces edema (cerebral and pulmonary). Not a cure but rather a drug to buy you enough time to get off the mountain.

I'll be making occasional comments in my Tibet posts because, unfortunately, I did get symptoms on my trip!