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The 91-Percent Solution - Oxygen and Hypoxemia

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The regs require oxygen at certain altitudes, but there’s is a better meter that works for all pilots. The numbers might surprise you.

by John Conroy & Glenn Stewart
Hypoxemia - In-Flight Testing
Rapid blood desaturation past 91 percent was reproducible in altitude chambers and in flight testing.

Imagine yourself, at the end of a 10-hour day and a two-hour flight, descending through 12,000 feet for an instrument approach at night. You have multiple course and frequency changes by ATC, you’re dodging weather patterns in turbulence, the airport is just above minimums, and passengers asking, “Are we there yet?” It’s enough to pucker anyone’s oxygen molecules.

Fast, cross-country, turbocharged singles are tempting more general aviation (GA) pilots into the flight levels. Survival in this oxygen-thin air requires common sense, the ability to detect hypoxemia, and the knowledge of how to handle it. Luckily, all you need to remember is one number.

Into Thin Air
Hypoxemia to an individual pilot is based on the availability of oxygen to the pilot’s brain at any given moment during the flight. Available oxygen to the pilot is based on the pressure altitude, the cardiorespiratory system, and individual oxygen consumption.

The air that we breathe is 21 percent oxygen. However, the amount of oxygen available for your lungs is a factor of partial pressure of oxygen and is measured in torr. As altitude increases, the available oxygen pressure in torr decreases. The percentage of oxygen in the air is always 21 percent no matter how high you go, but it’s 21 percent of a smaller total air pressure. At sea level the partial pressure of oxygen is 21 percent of 760 torr, while at 10,000 feet it is 21 percent of only 199 torr.

Oxygen delivery to the human body starts with the diffusion of oxygen from the air in the lungs into the blood. Young pilots with good lungs have good diffusions; older pilots with older lungs or smokers have poorer diffusion. The next step in the system is cardiac output that drives the red cells loaded with oxygen to the big toe or up to the brain. This too varies from individual to individual. The blood’s hemoglobin molecule unloads the oxygen to feed, say, hungry brain cells, and the deoxygenated blood heads back to the heart and lungs to be reloaded with oxygen.

Consumption at the brain is not constant. High-concentration activities such as reading a map, flying an approach, or even stress because of family concerns consume more oxygen than watching TV. When the brain is not getting enough oxygen, the only options your body has to deliver more oxygen is increasing the cardiac output (your heart beats faster and harder) or unloading more oxygen from the blood. This means that you become tachycardic (high heart rate) and tachypneic (breathing faster).
Tachycardia stresses your heart and breathing faster can lead to hyperventilation with neurologic side effects. You can avoid the problem by mixing some pure oxygen into the air you’re breathing using a mask or canula. This increases the percent of O2 in the air flowing into your lungs, leading to a higher O2 torr, and getting more O2 into your blood. Essentially, breathing supplemental oxygen creates the O2 torr equivalent to a lower altitude.

Forget the Regs
FAA regulations for use of supplemental oxygen date back to the 1930s and were based on the height of the Rocky Mountains and some military test data. FAR 135.157 requires oxygen for commercial flight crews above 10,000 feet MSL in unpressurized aircraft and a two-hour supply for cabin-pressure loss. For GA pilots, FAR 91.211 says oxygen is required above 14,000 feet all the time and above 12,500 feet for visits of over 30 minutes. The AIM recommends supplemental O2 above 10,000 feet during the day and above 5,000 feet during the night.

A graph showing the levels in blodd and 91-percent oxygenation
The shape of the graph is the key. Note how past 91-percent oxygenation, a small change in pressure results in a big drop in blood oxygenation.
With so many personal variables in hemoglobin levels, cardiac and pulmonary function, and responses to stress, this system doesn’t provide reliable protection. Mild hypoxemia — that which increases errors and drops response time — in a pilot is not a specific altitude for all pilots, but rather a specific pressure altitude for an individual pilot. So, when do you need oxygen?

A Common Denominator
The situation inside your body is complex, but the solution is simple. A blood saturation of 91 percent is the critical number. When you reach this point, the oxygen tension of your blood is at 60 torr. This is when we consider the individual hypoxemic in general medicine. Some pilots and writers claim that 88 percent is a better number, but remember that cardiac output is also a factor. A saturation of 91 percent provides sufficient oxygen without extra stress to the heart.

The reason 91 percent is so critical is that the relationship between torr and O2 saturation in your bloods is a sigmoid curve. As torr decreases, O2 saturation decreases slowly from its maximum down to 91 percent. Past this point, even a slight decrease in torr means a precipitous drop in blood saturation. What this means to you is you could be doing fine from sea level to 9000 feet and then suddenly be in a bad situation only 2000 feet higher.

Our studies show that 91 percent can occur as low at 5,000 feet in a 65-year-old physician and can be over 11,000 feet for a 21-year-old college rower. Most adults with no significant underlying medical problems hit 91 percent between 8,000 and 10,000 feet — not the 12,500 feet dictated by the FAA

The pressure altitude where this occurs seems to be reproducible in each pilot over several months and even years, although it does decrease for all pilots as they age. Once you know the critical altitude for yourself, you can use it as a rough gauge even if you don’t have a pulse oximeter. Pilots will desaturate quicker as altitude increases, but resaturation of the blood once O2 is introduced usually happens in two minutes or less up to 20,000 feet.

Incidentally, unpressurized general aviation aircraft are not certified above FL 250 because above 25,000 feet, the pressure outside your lungs is lower than the pressure inside your lungs. If you simply breathe nasal oxygen in an unpressurized atmosphere it will leak back out into the environment. The Air Force flies with positive end-expiratory pressure at FL 250 and above. These systems pump air into the lungs by positive pressure. Don’t go pushing your Columbia 400 past FL250 because you have oxygen and it can still climb. Even at FL250 you must monitor your O2 carefully. Time of useful consciousness without O2 at FL200 is usually 30 minutes, but by FL250 it’s only 3-5 minutes.

The Bottom Line
Any pilot flying above 5,000 feet at night or 8,000 feet during the day should use pulse oximetry to watch their oxygen saturation. When it drops below 91, note the altitude — or, better yet, the pressure altitude — and start the flow of that supplemental oxygen. There are other factors that we did not talk about here: Humidity, carbon dioxide in the blood, carbon monoxide in the cabin, and other items can play a role in how well your brain and muscles use the available O2. These factors matter, but not enough to change the magic number for most pilots most of the time. If you’re flying high, strap a saturation meter on your finger and keep the number in the 90s.

John D. Conroy is an Assistant Professor at Johns Hopkins and Glenn Stewart is a CFI in New Cumberland, Penn.

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