Altitude can be defined as any height above 1500m, as the height increases the altitude can be classified:

• High altitude 2500m to 3500m
• Very high altitude 3500m to 5800m
• Extreme altitude 5800m+

There is a big misconception that at altitude there is less oxygen, however in fact the amount of oxygen remains the same, but the partial pressure of oxygen decreases.

Due to this reduced partial pressure, less oxygen is able to reach the alveoli sacs in your lungs meaning it becomes harder to breathe. To give an example the barometric pressure of oxygen at sea level is 760mmHg making it easy for us to breathe, but as altitude increases the barometric pressure of all the gases in the air decreases proportionally to the increase in altitude . Therefore, at the top of Mount Everest the barometric pressure of oxygen is as low as 253mmHg.

As expected, the decrease in oxygen pressure causes various physiological changes:

• Increase in ventilation – This means your breathing rate will increase in an attempt to increase the amount of oxygen in your blood and the amount being transported to working muscles.
• Increased Heart Rate – This is the body's attempt to transport the reduced amount of oxygen around the body at a faster rate.
• Increase Blood Pressure – This increases the ability of the oxygen to diffuse into the working muscle cells.
• Change in substrate utilisation – This means your body increases its dependency on carbohydrate to produce energy.
• Increase in exertion – You will begin to feel tired from less strenuous activities that were easier to complete at sea level.

One way to check how much oxygen you have in your blood, also known as oxygen saturation, is to use a pulse oximeter. This small device can be placed on your finger and using a inferred light can detect how much oxygen is within your red blood cells, from this an oxygen saturation can be calculated.

It is important to keep track of your oxygen saturation at altitude as if the levels drop too low you can become ill. Normal oxygen saturation levels at sea level should fall between 98% to 100%, however,at altitude they can decrease to 70%,  if your oxygen saturation drops below this level you should seek medical help.

References:

Barry, P.W. and Pollard, A.J. (2003) “Altitude illness”, British Medical Journal, 326(7395), pp.915-919.

Bezruchka, S. (2005) Altitude illness: Prevention & Treatment, Seattle: Mountaineers Books.

Luks, A.M., Auerbach, P.S., Freer, L., Grissom, C.K., Keyes, L.E., McIntosh, S.E., Rodway, G.W., Schoene, R.B., Zafren, K. and Hackett, P.H. (2019) “Wilderness Medical Society Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness: 2019 Update”, Wilderness & environmental medicine, 30(4), pp.3-18.