Flight Physiology : Hypoxia
Our story starts in December of 1941.
Following the Japanese bombing of Pearl Harbor on December 7th, the United States declared war on Japan.
Three days later, Germany and Italy declared war on the United States.
A few days after that, a 48 year-old physiologist from the University of Rochester called Wallace O. Fenn assembled a team of biologists with backgrounds in studying snakes, fruit flies and grasshoppers.
Their intention: to win the war!
How to win a war…
The importance of air power was one of the evolving lessons that had come out of WWI and various thinkers within the military establishments believed that the airforce that held the “higher ground” in terms of altitude would carry the day. Most airplanes of the time did not have pressurized cabins and while very little was known about gas exchange and lung physiology. What was known was that there simply was not enough oxygen for pilots to fly much beyond 12 000 feet.
To this day, hypoxia is one of the most important flight stressors to consider. We talk about this at length in the lesson on Dalton’s Law (part of the Flight physiology course). Essentially, the problem is that, because of the relatively low partial pressure of oxygen at altitude, there is no longer a sufficient pressure gradient to drive oxygen into the bloodstream.
There is an equation, known as the alveolar gas equation, that is used to calculate the partial pressure of oxygen in the alveoli. Really, the calculation provides a rough estimate, but it is impractical to directly sample alveolar gas. So, we use practically measurable values and the alveolar gas equation to estimate the gradient of oxygen between the arterial blood and the alveoli.
The alveolar gas equation was published in a 1946 paper called ”A theoretical study of the composition of the alveolar air at altitude” submitted to the American Journal of Physiology by Wallace O. Fenn and his team. And it is true that they had no background in respiratory physiology; their prior fields of research involved grasshopper eggs, rattlesnakes and the speed of fruitfly wings. Nonetheless, their paper became a classic and a cornerstone of modern flight physiology.
The Alveolar Gas Equation
A simplified form of the equation can be written:
PaO2= FiO2 (Patm-pH20) – PaCO2/0.8
Where PaO2 is the partial pressure of oxygen in the alveoli; FiO2 is the “fraction of inspired oxygen; Patm is the atmospheric pressure; pH2O is the saturated vapour pressure of water at body temperature and the prevailing atmospheric pressure and; PaCO2 is the arterial partial pressure of carbon dioxide.
If we solve this equation for different altitudes, we can see that the Sao2 is already critically low at 10 000 feet!
The point is that even the healthiest person is severely affected by hypoxia at altitude unless measures are taken. Our patients are, by definition, not the healthiest person!
To learn more create an account and enroll in the Flight Physiology Course!
What are the effects and symptoms of hypoxia?
Why do fighter pilots actually wear oxygen masks?
How high can you fly without oxygen?
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