Above The Clouds

Dr DINESH ARAB He is chairman, department of medicine, and director, division of cardiology, at Florida Hospital Memorial Medical Center in Daytona Beach in the US.

I had made the last ascent to base camp Everest and had sat down to have a drink of water. I was tired and breathing hard. I took a look at the pulse oximeter, a small clip that measures the oxygen percentage in your haemoglobin, and it read 80 per cent. Normally at sea level my saturation was 99 per cent. The problem was that the number of oxygen molecules I was taking in each breath, at 17,500 feet, was only half that at sea level, contrary to perception that the concentration of oxygen is the same (21 per cent) whether at sea level or at elevation. What does change is the pressure with which the molecules are delivered to our lungs. Think of an ocean of air above us, and we are living at the bottom of the ocean. As you ascend, the pressure exerted decreases, causing the molecules of oxygen to spread out, resulting in less oxygen molecules per breath, thereby causing blood oxygen levels to fall. The haemoglobin responsible for carrying the oxygen is only carrying 80 per cent of its capacity at rest. At exercise it was even worse, and getting to base camp was strenuous exercise. To make matters worse, the less oxygen the haemoglobin carries, the less it wants to give it up to the tissues.