Oxygen systems can hide secret dangers

 - April 16, 2008, 9:27 AM
For aviators and their passengers, oxygen means life at the high altitudes traversed by modern aircraft. True high-altitude passenger flight wasn’t really practicable until large-cabin pressurization was introduced during the halcyon days of aeronautical development surrounding World War II, most notably aboard the Boeing 307 Stratoliner and Lockheed Constellation transports and Boeing B-29 bomber. Pressurization freed crews from the need to constantly wear oxygen masks by concentrating more air within the cabin volume, effectively lowering the “cabin equivalent” altitude by stuffing more air molecules into it.

Modern commercial aircraft are typically pressurized to the cabin equivalent of 8,000 ft, meaning that someone sitting in an aircraft cruising at 35,000 ft is experiencing the same concentration of this life-giving gas that they would experience at an altitude of 8,000 ft.

All this is routine stuff. The basics of pressurized aircraft date back to the mid-1930s. The technology is a set-and-forget part of every preflight.

However, the world has changed, even if basic pressurization technology has not. Sometimes the systems fail with disastrous results, as in the Oct. 25, 1999, crash of pro golfer Payne Stewart, three fellow passengers and the Learjet 35’s pilot and copilot.

Sometimes passengers are at risk in what seems to be a harmless cabin environment. As the baby-boomer generation gets grayer, it needs increasingly specialized care when it travels. A report from the American Medical Association (AMA) found that more than 30 million Americans suffer from chronic lung diseases, and the most common type–chronic obstructive pulmonary disease (COPD)–afflicts 16.4 million Americans, including 14.5 million with chronic bronchitis and 1.9 million with emphysema.

The prevalence of COPD in America has increased by 60 percent since 1982, resulting in an estimated 800,000 to one million Americans in need of home oxygen.

But you don’t really have to be very sick to experience a strong reaction to changes in altitude. A passenger suffering from even relatively benign pulmonary (heart or lung) conditions at sea level can be sadly surprised by their reaction to an 8,000-ft equivalent cabin. In the words of an AMA report, “Whereas most healthy travelers tolerate the decrease in oxygen in arterial blood during flight, significant hypoxemia (less than normal oxygen in arterial blood) and oxyhemoglobin desaturation (less than normal combination of oxygen and hemoglobin available to transport oxygen in the blood from the lungs to the tissues) can develop in passengers with cardiac and/or respiratory diseases.”

The AMA and other researchers further cite the dangers to passengers who show little or no symptoms of respiratory distress on the ground but will encounter serious problems aloft. For this reason, many more doctors are prescribing supplemental oxygen for their traveling passengers. Just how many even a recent study by the AMA was unable to define, although the data collected for the study from major airlines indicates a well-defined uphill curve throughout the 1990s. Just one major carrier, British Airways, reports transporting 2,500 oxygen-supplemented passengers in 2000.

According to the AMA, airlines surveyed through the mid-1990s indicated they carried as few as 100 and as many as 4,000 oxygen-using patients a year. While the range was broad, the numbers of oxygen-using patients did have one thing in common–a steady increase.

Why can’t passengers, who more than likely administer their own oxygen on the ground, bring their own oxygen on board? Because legally it’s considered a drug, available only by prescription. Moreover, the oxygen canisters themselves are potentially explosive if damaged and intensely flammable. But the rules vary widely from country to country, with Canada, among others, allowing passengers to bring supplemental oxygen on board, reviewing their needs–whether they require the assistance of a nonmedical escort (such as a family member) or a medical escort (such as a registered nurse). Under Canadian flight rules, flight attendants do not accept responsibility for administering oxygen. Every airline requires advance notice of passengers traveling with supplemental oxygen, typically no more than 48 to 72 hr but in some cases up to 30 days.

By U.S. law, Part 121 carriers must provide their own portable oxygen canisters, valves, fittings and gauges for passengers in need of supplemental oxygen. Those passengers are billed an additional $50 to $75 per flight segment for this service.

When it comes to corporate aircraft, we’re talking about two distinctly different aircraft oxygen systems. The first is that installed as standard equipment on all passenger-carrying transport aircraft and used only in the event of a loss of cabin pressure. The other system provides what’s known as therapeutic levels of oxygen, amounts above the bare minimum required to keep users conscious until a decompressing aircraft can return to an altitude at which the masks are no longer needed. In many of the larger corporate aircraft, hospital-style oxygen outlets are available, either as standard equipment or as options. On most other aircraft, the source for therapeutic oxygen is the portable bottles carried for either emergency use or en route use by passengers with routine need of supplemental oxygen.

It’s important to differentiate between the two major flavors of oxygen. Aircraft oxygen (also popularly known as “dry air”) is oxygen from which nearly all of the moisture has been removed. This drying prevents canister and valve freeze-up, a common problem since the canisters are often stored in unheated areas of the aircraft where extremely low temperatures are common. Therapeutic oxygen is slightly more moist, so it doesn’t dry out a patient’s breathing passages quite as quickly. Moreover, there are devices available to moisten incoming oxygen even more.

By the Book
While the generally accepted rules governing supplemental oxygen use are at times vague, the FARs are not. They require that a transport-category cabin must be pressurized so that it maintains an altitude of no more than 8,000 ft. For certification for operation above 25,000 ft, any probable failure cannot allow the cabin equivalent altitude to exceed 15,000 ft. For any other failure that leads to decompression that is not shown to be extremely improbable, cabin altitude must not exceed 25,000 ft for two minutes and cannot exceed 40,000 ft for any time. The rules also require there be a warning system that alerts the flight crew when cabin pressure altitude exceeds 10,000 ft.

During flight at altitudes above 35,000 ft, the PIC must wear an oxygen mask that is secured and sealed and that either supplies oxygen at all times or automatically provides it when the cabin equivalent pressure exceeds 14,000 ft. Above 41,000 ft, pilots must be on oxygen at all times. The presence of a quick-donning mask in the cockpit does not absolve the crew of the need to wear one at these altitudes. It must be worn.

Put this next rule in your “good to know” file. Research shows that the maximum safe altitude at which an aircraft cabin can decompress and the crew remain conscious, even with the use of oxygen masks, is 48,000 ft. Safely surviving decompression at such high altitude requires a pressure suit, such as those worn by space shuttle astronauts and SR-71 pilots.

Flight crews must have a source of oxygen separate from that intended for the passengers and other crewmembers. If the layout of the aircraft precludes such a design, then there must be a way to reserve a minimum portion of onboard air for the crew, enough for them to get the aircraft down to an altitude such that additional oxygen is not needed. When operating at a cabin altitude of 8,000 ft after a cabin depressurization and emergency descent, the crew must have a two-hour reserve.

Portable oxygen equipment units must be available for each cabin crewmember and located within what’s termed the “immediate reach” of flight crewmembers while they’re at their duty stations.

Flight crewmembers must be able to don their masks in five seconds using just one hand, without disturbing eyeglasses, delaying communications or disrupting needed emergency communications.

Cabin crewmembers must each have readily available a 15-min portable supply of oxygen during flight above FL 250. This oxygen can come either in the form of a portable bottle, or, as is sometimes the case in modern business aircraft, from oxygen outlets and masks distributed throughout the cabin in a way that allows crewmembers to stay connected as they move about the cabin.

As for passengers’ oxygen needs, anyone who has ever flown is at least fleetingly familiar with the yellow O2 masks that drop down from overhead in the “unlikely event of a sudden loss of cabin pressure.” Those masks deploy automatically when the cabin altitude exceeds 15,000 ft and provide oxygen initially at a minimal flow rate of four liters per minute–the bare minimum needed while a descent is started. It is not enough to overcome barometric pressure and supply a long-term, physiologically useful amount of oxygen to the lungs.

Therapeutic Oxygen
Sitting in a pressurized aircraft, cruising along in an 8,000-ft equivalent cabin, an increasing number of passengers are already experiencing hypoxemia (oxygen deficiency on blood, tissues and cells sufficient to cause impairment of body functions). Ever notice how some of your passengers just can’t seem to stay awake? While you might have attributed their dozing throughout a flight to the faith they place in your flying skills, it is far more likely their reaction to the lowered oxygen levels in the cabin.

Smokers are especially susceptible to this condition. Hypoxic conditions for a smoker occur at least 2,000 to 3,000 ft below the altitude that affects a nonsmoker. Smokers have just that much more carbon monoxide already bound to their blood hemoglobin. One recent landmark medical study has shown that a considerable number of patients with moderate to severe COPD, but with no sea-level symptoms, developed marked hypoxemia at 8,000 ft.

As noted, an estimated 800,000 Americans rely on home oxygen today. With few exceptions, they require supplemental medical oxygen to travel safely in a high-altitude aircraft.

“But many physicians think a pressurized aircraft cabin is the equivalent of sea level,” said Joan Garrett, president of MedAire, a Phoenix-based aeromedical consultancy, services and training provider. “So someone who does well in, say, San Francisco can be in real trouble once aloft.”

A healthy person at sea level has a bodily oxygen saturation of 97 percent. An oxygen saturation of 93 percent is considered by medical officials to be the lower limit of normal functioning. At an altitude of 10,000 ft, saturation drops to almost 90 percent.; at 15,000 ft it is at 80 percent; and at 25,000 ft it is a mere 55 percent, with unconsciousness not far away.

Some aircraft operators ask if, in an emergency, the standard cabin walkaround oxygen bottle can be used as a stand-in for therapeutic oxygen. The answer is no. The walkaround bottle’s flow rate (four liters per minute) is too low; therapeutic oxygen should be available at a rate of at least six liters per minute. At four liters per minute the flow rate is too low to overcome the effects of Dalton’s Law, which states: “The total pressure of any mixture of gases (with a constant temperature and volume) is the sum of the individual pressures of each gas in the mixture. The partial pressure of each gas is proportional to that gas’ percentage of the total mixture.” Because the percentage of oxygen in the atmosphere remains constant at 21 percent, it is possible to calculate the partial pressure of oxygen in a given volume of air at any altitude using Dalton’s Law.

In short, the 8,000-ft cabin pressure makes it more difficult for the human body to absorb the oxygen it needs. And at a cabin altitude of 25,000 ft or above, the four-liters-per-minute flow rate just isn’t enough. Professionals recommend either a full-up therapeutic oxygen installation or a walkaround bottle capable of providing a 70-psi oxygen flow to a regulator supplying the mask. Another option is the use of the so-called “bubble humidifier,” a device seeing increased acceptance in corporate aviation. Complete with its own regulator and variable flow dial, the bubble humidifier reliably provides high therapeutic flows greater than six liters per minute and carries the added advantage of moistening the usually dry oxygen flow on the way to the patient.

Training and Awareness

In the wake of the Payne Stewart crash, the Flight Safety Foundation convened a working group on the possible dangers of in-flight oxygen systems. After months of research and deliberation, their findings into the issue of oxygen safety procedures centered on several issues.

Inadequate training led the list, especially in the areas of crewmember knowledge of physiological aspects and hazards of high-altitude flight.

Following that was a lack of crewmember familiarity with the preflight and airborne operation of oxygen and pressurization hardware, as well as the capabilities and limitations of that equipment.

Especially important was the need to adequately train contract crewmembers, who might serve aboard several different aircraft types in the course of a month.
In the words of the working group’s finding, “It is possible that crewmembers misunderstand the actual versus their perceptions of system capabilities. Crewmembers may possess inadequate training and knowledge regarding the intent of FAA regulations concerned with onboard oxygen systems. Finally, crews may not be practicing oxygen-related drills with the frequency necessary for proper learning retention…”

The working group further noted, “There is no question that oxygen-related emergencies in flight are increasing in both frequency and severity. MedAire has witnessed calls to its emergency physicians soar to more than 8,500 in 2000, up from just over 500 calls in 1996. This includes in-flight emergencies and pre-travel screening of passengers with pre-existing illnesses. Of these calls, about 10 percent are from the corporate fleet.

“While we may never know for certain the cause of the Payne Stewart incident, it is almost certain that the crew was incapacitated, likely due to hypoxia and probably immediately, since they did not take proper action to remedy the situation…[Their emergency] could have been caused by explosive or rapid decompression, an unfamiliarity with the onboard oxygen equipment, complacency or crew inexperience. This tragedy…reminds us once again that high-altitude flight is not to be taken lightly. Emergency drills and ongoing training need to become the standard.”