Hypoxia in Aviation
Before becoming a flight doc, I often felt skeptical as the flight attendant showcased what appeared to be a yellow Dixie cup connected to an empty IV bag prior to takeoff on commercial airliners. This darting glance wavered as my attention faded. Then, it was back to a few precious final minutes with the smartphone before all electronic items must be turned off prior to takeoff. After my first experience in the altitude chamber at 25,000 feet without supplemental O2 though, I quickly gained an appreciation for emergency oxygen while flying.
Hypoxia is the inadequate amount or inability to use sufficient oxygen to meet the body’s metabolic needs. Although there are several categories of hypoxia, the main threat to aviators and climbers is hypobaric hypoxia. This form of hypoxia is experienced by one attempting to breath in an oxygen-scarce environment. As you ascend through the Earth’s atmosphere, ambient pressure decreases non-linearly, and with it the partial pressure of oxygen. At 8,000 feet the atmospheric pressure drops to 75% of pressure at sea level and at 18,000 feet it cuts to nearly 50%! The term hyobaric refers to conditions of low pressure.
An altitude of 10,000 feet (3,048 meters) is generally the lowest elevation causing symptoms of hypoxia in a typical person. The highest human residence is a small town near gold mines in the Peruvian Andes called La Rinconada. About 30,000 miners permanently reside here at 5,100 m (16,732 feet). These residents, much like the Sherpas of the Himalaya, have developed genetic changes allowing adaptation to high-altitude living. Mountaineers utilize acclimatization strategies lasting days to weeks in order to promote physiologic changes, which increase the chance for successful summit attempts of altitudes up to nearly 30,000 feet (9144 meters). A fighter pilot flying at this altitude who was to suddenly experience a failure in cabin pressure would respond to the same oxygen levels in a much more deleterious way. The time of useful consciousness (the length of time a pilot is physically able to take corrective action) for the average pilot instantly exposed to a 30,000 ft cabin pressure is 1-2 minutes. This is the reason why pilots and aircrew must train extensively to carry out EP’s (emergency procedures) in the event of a loss of cabin pressure or hypoxic symptoms.
THE RAPID DECOMPRESSION
Although a rapid decompression as described above should be feared by any serious aviator, aerospace engineers have incorporated protections into the design of aircraft and life support equipment in order to prevent this scenario. The first line of defense against hypoxia is cabin pressurization. This technology allows passengers of commercial airliners to experience an altitude equivalent to 8,000 feet although the actual cruising altitude may be nearer to 40,000 feet MSL. This technology dates back to 1931 when a balloon gondola was first pressurized allowing the pilot to reach an altitude > 50,000 feet.
Some people believe the reason fighter pilots where oxygen masks during flight (unlike commercial aviation) is because their cabins are not pressurized. This is inaccurate. Fighter jets do have pressurized cabins, although they allow lower pressure differentials. The prominent difference between these two settings, which leads the military to require their pilots to fly with masks in place, is the cabin size. Cabin size is directly related to the rate of pressure change in the event of decompression. In other words, because a typical cabin of a commercial airliner is much larger, in the event of a compromise in cabin integrity passengers will have significantly more time to don emergency oxygen (the Dixie cup/IV bag alluded to above). For this reason, airline passengers have a much longer time of useful consciousness than a fighter pilot experiencing cabin depressurization at the same altitude. Additionally, military aircraft are likely to take battle damage, which is always a risk for cabin compromise. Lastly, passengers are just that- passengers! They are not critical to aircraft operations. If they pass out, the pilots will still be able to land the plane in the absence of a mentally cognizant cabin full of passengers. No big deal, enjoy the nap frequent fliers.
SYMPTOMS OF HYPOXIA
Unlike the alarming rapid decompression described above, it is possible for a cabin to experience a slow, gradual decompression. This setting can be just as dangerous, but for a very different reason. One of the first and most common symptoms of hypoxia is loss of judgement. When the cabin pressure (and accompanying O2 partial pressure) diminishes gradually, the condition itself may develop without recognition by the aviator. Many mountaineers have lost their lives in this way, making a critical mistake in judgement at a time when the margin of error was literally ice thin, unaware that their oxygen-starved brain was leading them down a path of error and ultimately destruction. Because of the difficulty in self-identifying hypoxia, military aviators are required to experience hypoxia in an altitude chamber for training purposes. This experience allows one to identify their unique symptom presentation and also demonstrate corrective action before the time of useful consciousness (TUC) is reached.
View the short video below to observe the effects of breathing ambient air at 25,000 feet (with no acclimatization) for just a few short minutes. Other common symptoms affecting the hypoxic aviator are increased respiratory rate, headache, euphoria, tremors, drowsiness, loss of color vision, and other visual disturbance. The secondary hyperventilation can cause numbness & tingling of the digits or around the mouth due to an artificial imbalance between O2 and CO2. If you listen closely, you can hear the aircrew member describing his symptoms up until he reaches and surpasses his TUC. At what point is it obvious he has passed his TUC?
4 OF SPADES – USAF AIRCREW HYPOXIA TRAINING
1. Fundamentals of Aerospace Medicine (Ed. 4) by Jeffrey R. Davis (Editor), Robert Johnson (Editor), Jan Stepanek (Editor), Jennifer A. Fogarty (Editor).