Energy and the Human Body Background Material

Pulse Oximeter
As climbers go up to higher altitudes the atmospheric pressure goes down. This makes breathing difficult and it also changes the partial pressure of oxygen in the lungs (specifically in the alveoli, the tiny air-filled sacs in your lungs). The decreased pressure also affects the amount of oxygen carried in the red blood cells. This is called arterial oxygen saturation.

Take a look at the graph to see how the partial pressure of oxygen in the alveoli (PaO2) and the oxygen saturation (SaO2) goes down with altitude.

Graph provided by Wilderness Medical Society

Doctors measure the oxygen saturation because it can tell them whether a person's blood is carrying enough oxygen to all the parts of the body (muscles, brain, etc.). The measurement can also help a doctor predict whether someone is about to get sick. Evidence shows that before someone gets pulmonary edema (an altitude related illness called HAPE where fluid collects in the lungs), his or her oxygen saturation usually drops.

At sea level a normal oxygen saturation is around 99 per cent. At Basecamp (17,500 ft or 6,500 m) a normal reading would be between 80-85 per cent.

Oxygen saturation in arterial blood (the blood flowing in your arteries) can be measured with a small gadget called a pulse oximeter.

It can measure the amount of oxygen in your blood without using a needle to draw blood. A pulse oximeter works by shining both a red and an infrared light through tissue. It works because oxygenated blood is red and de-oxygenated blood is darker, almost blue in appearance.

If you look at the arteries and veins beneath your skin on your wrists you see the blood found in the arteries is a lighter red colour and the blood in the veins a darker, blue colour. Because of this colour difference, the blood in each type of vessel has a different absorption of light at its given wavelength.

As the pulse oximeter shines the red and infrared beams through the skin, it detects the fluctuating signals caused by arterial blood pulsations. Then, using an internal software program, the oximeter figures out the oxygen saturation of the blood by calculating the ratio of the red and infrared signals landing on a receiver. All of that only takes a few seconds, doing nothing more than placing a small electric tube over the climber's finger.

The measurement won't be exactly the same as if a doctor did a more invasive test called a blood gas (where they actually puncture the artery in the wrist) but there is a predictable correlation between the oxygen saturation measured by the pulse oximeter and the blood gas test.

Dr. Virginia Robinson, the Everest 2000 team physician, is planning to take a pulse oximeter readings from Byron Smith and Tim Rippel when they climb higher on the mountain.

For more on the Human Circulatory System, the heart, blood and reactions to altitude see the rest of the Phase 3 Energy & the Human Body Background section.

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