Altitude and Ascents

article051Rapid ascents and massive elevation gains are often incorporated into adventure races. Rarely will you pass through an area without visiting the peak of the highest mountain, scaling the slope of an impressive volcano or traversing a sky-high ridge.

That some competitors will experience mild symptoms of acute mountain sickness (AMS) is likely. It’s a condition related to altitude and the speed of ascent. Onset is rapid and those susceptible to the condition are not confined to age, gender or fitness categories. Symptoms increase in severity with continued ascent and time spent at altitude. The most effective treatment is immediate descent.

In a race environment repercussions are huge. Consider this scenario. A member of the leading team succumbs to the symptoms of AMS and is unable to continue upwards…

Altitude is defined on the following scale:

High: 2438m – 3658m (8000ft – 12000ft)
Very High: 3658m – 5487m (12 000ft – 18 000ft)
Extremely High: >5500m (>18 000ft)

Basic Physics
Barometric pressure is the amount of force exerted on a surface by the weight of the air molecules above it. As your altitude increases, there is less air above you and consequently the force exerted decreases. Therefore, barometric pressure decreases with increasing altitude.

Air is made up of many different gases. Oxygen molecules comprise about 20.95% (we’ll use 21% for convenience) of the mix, nitrogen accounts for around 79.02% with carbon dioxide clocking in at around 0.03%. There are also miniscule quantities of rare gases – argon, neon, helium, krypton, hydrogen, xenon, and radon, which bear no consideration here.

Dalton’s Law of Partial Pressures explains that we can consider the weight exerted by each gas independently. Added together they would produce the total barometric pressure.

So, now consider a large syringe containing 21 marbles (oxygen molecules). When you squeeze the plunger down, you’re exerting a force (weight of the air above them) on the marbles, squeezing them to the bottom. The 21 marbles are close together and in a smaller, more compressed volume. This is the sea-level image. Now raise the plunger. The same number of marbles are free to roll around in a larger volume. This is the altitude image.

At sea-level the oxygen molecules are concentrated and thus with every breath you inhale a lot of them. As you ascend higher, the weight exerted by the air above decreases and although the same number of oxygen molecules are available, they’re more ‘dispersed’ and not as easy to access. With each breath you inhale less oxygen molecules. Subsequently the amount of oxygen available to the blood and tissues is reduced.

We need oxygen to fuel our cells for various processes, the most important of which is the generation of energy, which keeps every essential biological process ticking over. Oxygen deprivation is comparable to suffocation.

Acclimatisation
The body is able to adjust to being at higher altitudes where there is less oxygen. Physiological adaptations like increased breathing rate (higher breath rate means more oxygen will be inhaled overall) and increased heart rate (pumping blood around faster to increase the oxygen turnover) are important initial adjustments. An increase in the amount of red blood cells (oxygen carrying cells) in your blood is more gradual, taking place over about 10-days. These mechanisms ensure that oxygen carrying potential of the cells is maximised and that oxygen delivery to cells is efficient.

In most races, exposure to high altitude is brief as competitors ascend rapidly and will probably punch a check point before descending even more quickly. As their stay at altitude is brief physiological adaptations are immediate and restricted to an increased heart and respiratory rate.

Altitude induced-illnesses
Going too high, too fast is the major cause of altitude induced-illnesses.

Rapid accent to high altitude often results in a syndrome known as acute mountain sickness (AMS). AMS is common; at elevations over 3048m, 75% of people will have mild symptoms. Onset is dependant on altitude, the rate of ascent and individual susceptibility (not limited to age, gender or fitness) and a headache is the most common symptom. Dehydration does increase the probability of experiencing AMS. Fortunately treatment and symptomatic relief is simple – Go Down!

If the person susceptible to AMS remains at altitude and/or continues to ascend their headache will be accompanied by fatigue, nausea, vomiting, dizziness and irritability. The severity of these symptoms increases with altitude and time spent at altitude.

While severe altitude-induced illnesses are unlikely in adventure races, mild symptoms are possible. And, should a team in the upper ranks be affected, the race outcome could be drastically affected.

Author: Lisa de Speville