How Can We Increase the Oxygen in the Bloodstream?
 
This is the crux of the matter for the general aviation pilot. How can the pilot increase the PP of O2 in her bloodstream, in order to attenuate or eliminate the effects of hypoxia on safety and performance? She can either pressurize the cabin, which increases her Pbaro, or she can breathe supplemental oxygen, which increases her inspired oxygen concentration. In either case, she increases her inspired, alveolar, and blood O2 PP's. Pressurization systems cause air to enter the aircraft cabin slightly more rapidly than it can leave, raising the pressure of the air in the cabin by a certain amount. You are breathing air with the same old 21% O2. However, its pressure is considerably higher than that of the surrounding flight-level atmosphere, though not as high as that at sea level. Most airliner cabins are pressurized to a pressure altitude of about 5000-6000 feet which produces an inspired oxygen PP adequate for a healthy person for the duration of most commercial flights.

How Much Oxygen is Enough?
 
Good question, since this tells us how high we can safely go with and without oxygen. There is no absolute safe level of blood O2 PP or O2 sat. However, some rough guidelines exist regarding hypoxemic tolerance. Judgement and fine motor control begin to deteriorate appreciably at a blood O2 sat less than about 85% in the healthy but unacclimatized pilot.  This corresponds to a blood oxygen PP of about 55-60 mm Hg. Unconsciousness ensues after all but the shortest exposure to a blood O2 saturation of about 50% or less; you will encounter this level of hypoxemia at an altitude of about 23,000 feet as I mentioned earlier. Of course, one's tolerance for hypoxemia is significantly diminished by smoking or by certain heart or lung diseases such as emphysema or congestive heart failure. You are unlikely to qualify for a medical with these problems, but you may well be allowed to board a commercial flight. FAR's require the pilot in a nonpressurized aircraft to don oxygen above 12,500 feet MSL for that part of the flight exceeding 30 minutes. Further, the pilot is required to use oxygen for the entire flight above 14,000 feet. At the latter altitude the pilot's blood O2 sat would be expected to be around 80% without oxygen, clearly below the "safe" minimum level . So physiologically, the altitude regulation makes some sense. For many GA pilots, flying nonpressurized aircraft (the planes I rent usually aren't watertight, much less airtight!), the alternative is to wear a cannula or mask and breathe supplemental oxygen. The conserving cannula systems generally consist of a cylinder of oxygen with a regulator valve and a reservoir which release the oxygen in a controlled manner to decrease waste of the gas. The valve mechanism senses the start of an inhalation and releases oxygen only during the inhalation. The reservoir accumulates a reserve of oxygen so that the air inhaled at the very start of inspiration has a high oxygen concentration. A nonconserving system, by contrast, simply administers the oxygen at a steady, constant flow rate regardless of whether the user is inhaling or exhaling, and has no reservoir to enrich the early-inspiration flow of oxygen. By how much are we able to enrich the oxygen concentration we breathe using such equipment? The average person breathes about 5-6 liters of air per minute. In addition, the rate at which a person inhales is not uniform; it is highest at the start of inspiration and declines to zero as the lungs approach their "full" point. At the start of inspiration airflow rates may be 40-60 liters per minute or more. At this airflow, most of what the pilot inhales when wearing a nonconserving cannula is normal cockpit air entrained, or sucked in around the cannula. If we increase the oxygen flow rate in order to increase the concentration of oxygen inhaled, the resulting gas jet may be uncomfortable; and oxygen is wasted as it pours out uselessly during exhalation (which makes up 2/3 of each breathing cycle), emptying the cylinder far short of the destination. At an oxygen flow rate of 5 liters per minute, a 22 cubic-foot aviation oxygen cylinder will be expend its 700 liters of oxygen in a little over 2 hours. And that's with just the pilot alone breathing from the system. Add a few passengers on oxygen and you are refilling your cylinder much more often than your fuel tanks. With the conserving cannula, on the other hand, the oxygen flow rates can be set quite low and still get the job done. Oxygen accumulates in the reservoir so that with each inspiration one gets a "blast" of high-concentration oxygen; during exhalation no oxygen leaves the system, dramatically decreasing waste. Using that same 22-cubic-foot cylinder a pilot wearing a conserving cannula might get 20 hours of flow at 18,000 feet., for an average oxygen flow of about a half a liter per minute. The tradeoff we accept in using a cannula is that at best we can increase only slightly the oxygen concentration we breathe. For this reason, cannula systems are approved only for altitudes up to 18,000 feet. Above this altitude, the small increase in oxygen concentration from the cannula is inadequate to offset the drop in barometric pressure and produce an inspired O2 PP sufficient to keep the blood adequately saturated with oxygen. For altitudes above 18,000 feet, masks which allow higher concentrations of oxygen must be used. These masks,of course, require higher oxygen flow from the cylinder and consequently use up the available supply much faster. In preparing this article I spoke with Tom Armao of Aerox Systems, a manufacturer of aviation oxygen equipment. He told me that his company's conserving cannulas are designed to increase the inspired oxygen concentration to about 24 percent, compared with the normal 21 percent. This means that at FL180, where Pbaro is about 404 mm , this cannula can increase our inspired oxygen PP from about 84 mm (400 times 0.21) to about 96 mm (400 times 0.24) This increase in inspired O2 PP translates into a jump in blood oxygen PP from a non-supplemented value of around 45 mm to about 55 mm with the cannula on. As mentioned earlier, this puts you just about at the 55-60 mm point (about 85% sat) below which your judgement and motor skills begin to go seriously to pot. You can see that above FL180 the cannula will not be able to provide a sufficient increase in blood oxygen content for you to be able to function adequately for long. A regulation with an actual reason! To illustrate this point, let's consider the hot shot aviator who decides to take her Turbo-Brand-X up to FL250, where Pbaro is only about 282 mm Hg. Up there her blood O2 PP will be only about 20 mm Hg with a saturation in the mid-20's or so without oxygen, assuming she survives long enough to  climb to that altitude. Using her 24% cannula she would improve her blood O2 PP or saturation only marginally, not even enough to enable her to regain consciousness. But using an oxygen mask supplying 50% oxygen, she could improve things quite a bit. With an inspired O2 PP of 141 mm (50% of 282 mm) her blood oxygen PP would be expected in the low-60 mm range, giving her a sat of 90% or better. This is well above our 55mm/85% sat silliness threshold. However, she would have to turn up her oxygen flow rates fairly high, resulting in rapid exhaustion of her oxygen cylinder, to maintain her oxygenation. One can see why a pressurized cockpit makes practical sense for the pilot who regularly flies her aircraft at the flight levels: no nose-hose to wear, no cylinders to refill, and no limit on endurance at altitude other than those imposed by the aircraft's fuel  reserves and the pilot's bladder capacity! I have tried here to give an overview of the physiology of oxygen  etabolism, the dangers of hypoxia, and the rationale for the use of supplemental oxygen at altitude. Now when you head for the flight levels you should better understand just what you are doing, and why, when you put on that cannula or glance at your cabin-pressure gauge. Just remember, OXYGEN, like altitude, IS GOOD!  �1996 by Michael D. Sebastian, M.D.