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The best solution to recognizing the gradual onset of hypoxia is to wear a pulse oximeter anytime the cabin altitude is above 5,000 feet.
the physiologist who described it in the early 1900s. It is also known to medi- cal students as the “oxygen/hemoglobin dissociation curve,” the graph for which they must thoroughly understand and memorize. The graph is not linear, but has an exponential downward curve, meaning that the problem accelerates or becomes worse and worse very quickly. This results in even less oxygen being supplied to needy tissues like the heart and brain at a rate faster than the alti- tude is changing. So now, even though the airplane’s pressurization system is operating as it was designed to with a perfectly legal cabin altitude between 10,000 and 12,000 feet, the pilot’s body has much less oxygen in the bloodstream. And because of the oxygen/hemoglobin dissociation curve, what oxygen there is in the bloodstream is not as available to the needy tissues as it was even a couple of thousand feet lower. And this is just the beginning of the problem.
There is another phenomenon that also decreases potential oxygen to essen- tial organs like the heart and brain, and that is a gradual narrowing of the inside of the blood vessels or so-called “arterio- sclerosis.” This happens to everyone as they age but occurs earlier and faster in people with a history of high cholesterol, high blood pressure, poor exercise habits, diabetes, and nicotine use. So, whether the result of a perfectly normal aging process or common health issues, the size of the internal diameter of blood vessels is smaller, which further limits the amount of already poorly oxygen-
ated blood available to key organs. And again, the issue continues to
compound itself.
The heart’s timing mech- anism known as the sinoatrial node (or SA node) is a key re- ceiver of oxygenated blood and is very sensitive to any decrease in oxygen levels. As the amount of available oxygen drops, the SA node becomes very unhappy about this and can frequently start to mis- fire, which is similar to upsetting the timing mechanism on a piston engine. All of a sudden, the efficiency of the heart as a pump is drastically reduced, and with it even a further decrease in oxygenated blood to both the heart it- self (which in turns makes the misfiring even more pronounced) and also to the brain. The latter usually produces a loss of consciousness from which spontane- ous recovery is improbable, and death
usually follows.
In summary, when a pressurized piston or turboprop aircraft is in the high 20 flight levels and operating just as it was designed (cabin altitude of 10,000– 12,000 feet), the pilot’s body is only being supplied with half the oxygen available at sea level. This, in turn, triggers the Bohr effect, further decreasing the amount of oxygen available to the brain and heart, which if the pilot is of mature age, are already compromised due to the narrowing of blood vessels. Finally, the hearts timing mechanism becomes irritated causing it to misfire, which can rapidly lead to loss of consciousness and death. Depending upon who else is on board, a fatal crash of confusing cause usually follows.
Given this physiologic reality, is it really safe for pilots with grey hair and some common health issues such as elevated cholesterol, high blood pres- sure, and possible arterial narrowing, to operate pressurized aircraft at their highest legal altitudes with cabin altitudes? The answer is probably not. But, if the pilot is willing, some steps can
be taken to lower the physiologic risk to a more acceptable level, and it involves the use of supplemental oxygen.
Supplemental oxygen is something that needs to be used before hypoxia is present because its effect on the brain is very insidious and makes such recogni- tion of what is occurring, and the logical solutions that would follow, nearly impos- sible. The best solution to recognizing the gradual onset of hypoxia is to wear a pulse oximeter anytime the cabin al- titude is above 5,000 feet and watch the numbers on the dial. These are simple to use and available from various aviation supply stores for a very nominal amount. Their application should ideally be on the “FL180 checklist” along with switching altimeters to 29.92. It is also a good idea to brief any passenger sitting in the right seat to keep an eye on the oximeter read- ings. If the blood oxygen level as shown on the oximetry starts to drop into the low 90s, then the pilot should automati- cally go on supplemental oxygen even if the aircraft is at an altitude below FL180. In the relatively low flight level altitudes piston aircraft fly, pilots do not require a huge oxygen flow to fix the problem of hypoxia; usually, just a 2-liter flow of oxygen via nasal prongs will restore the partial pressure of that gas to that of an airline cabin.
Some pilots, however, are hesitant to use supplemental oxygen even if the pulse oxim- eter shows it is indicated because they feel it should be saved for an “emergency.” Part of this is due to poor
December 2018
TWIN & TURBINE • 27