Combating Chronic Hypoxia in the Pilot

"Editor's Note: Wow! What a well-done article on such an important safety topic. JATO encourages all renter pilots to buy and maintain their own oxygen equipment, and to use oxygen per the recommendations in this article.

Please contact the JATO office at ops@jatoaviation.com or (650) 654-5286 for a copy of the list of O2 equipment for our rental fleet. This list includes part numbers, where to order and general pricing."

Joseph Romson M.D. PhD, CFI

The typical general aviation pilot is often well into his or her flying career before the effects of hypoxia (i.e.- low blood oxygen levels) due to altitude are taken into serious consideration. As part of private pilot training, the student pilot is taught the general effects of increasing altitude on pilot physiology. Typically, this information centers on the dramatic effects of extreme altitudes and the related rapid loss of consciousness (or time of useful consciousness), when oxygen levels plummet.  It would be unusual in a new pilot's experience to be subjected to these extreme altitudes, unless they were flying a high performance aircraft or experience hypoxia in an altitude chamber. While the dangers of extreme altitude are important because of the nature of the hostile surrounding atmosphere and the subsequent rapid loss of consciousness should an in-flight problem arise, the majority of the time is spent at substantially lower altitudes where chronic, insidious hypoxia may be a greater, unrecognized threat. The purpose of this article, therefore, is to raise the pilots’ awareness of the dangers of chronic, low-level hypoxia that may be experienced at substantially lower than expected altitudes. In addition, we will explore effective strategies to combat hypoxia in this commonplace situation.

A good place to start this discussion would be a brief overview of the required use of oxygen, as stipulated by the FAA regulations:

 (CFR 91.211 paraphrased: supplemental oxygen is required for crew between 12,500 ft and 14,000 ft, if that altitude is maintained for more than 30 min; and continuous use of supplemental oxygen above 14,000 ft. Each passenger is to be provided with supplemental oxygen for all flight above 15,000 ft.)

At first appreciation, these requirements would appear to be reasonable and adequate to prevent hypoxia. After all, we know that climbers of Mount Everest have been able to face the extreme, hostile conditions of the mountain and climb to 29,029 feet without supplemental oxygen! Therefore, wouldn't it seem reasonable that a pilot could fly below the Flight Levels (below 18,000 feet) and perform well, without the use of supplemental oxygen? As always, real answers to this question can be found by referring to the research literature and understanding what researchers have found in well-conducted studies.

Without delving too deeply into the details of published studies on hypoxia at moderate altitudes, a nice summary of this body of research literature can be found in work by Petrassi et al., in a recent edition of Aviation Space and Environmental Medicine (“Hypoxic Hypoxia at Moderate Altitudes: Review of the State of the Science,  2012 Oct;83(10):975-84.) These authors carried out a systematic review of the literature regarding hypoxic impairment of mental functions, sensory deficits and other aviation related duties at moderate altitudes (between 8,000 to 15,000 feet). Briefly, they concluded that activities such as learning, reaction time, decision-making and certain types of memory are affected at these altitudes in subjects not receiving supplemental oxygen. Clearly, these are important activities for a pilot involved in flight-critical actions and decision-making.

The authors did note, however, that there were inconsistencies in the results when these quoted studies were considered in toto. These apparent discrepancies in findings between studies were attributed to differences in research techniques and to the subtlety of deficits observed in test subjects at moderate altitudes, compared to dramatic results noted at much higher altitudes. In other words, the authors felt that some of the variation in the results on tested performance, at altitudes between 8000 to 15,000 feet, were likely due to the fact that subject performance was subtly affected by the exposure to moderate altitudes, as opposed to dramatic impact on performance clearly seen at much higher altitudes. Regardless, effects on performance were noted, to varying degrees, in most studies.

To further complicate matters, there is a well-known and normal physiologic response of increasing breathing rate (hyperventilation) in response to altitudes above 8,000 to 10,000 feet. This mild, almost imperceptible, hyperventilation will reduce blood arterial carbon dioxide levels (hypocapnea) that in turn, result in a complex interaction between reduced blood oxygen levels (hypoxia) and brain (cerebral) blood flow. The net result of this complex interaction between blood arterial blood gases and cerebral blood flow is that individuals will have a widely variable response to the effects of altitude regarding performance of complex tasks. In addition, it has been shown that experienced pilots are able to make better speed and accuracy decisions and thus, perhaps, better able to compensate for altitude related performance decrements than less experienced pilots.  Taken altogether, these observations explain why some pilots may appear to be more susceptible to the effects of moderate altitude than others.

On the other hand, measurable decrement in visual acuity in response to altitude-induced hypoxia was more consistently shown in this review of published research studies. Under low light conditions (i.e.-night), visual degradation has been demonstrated at altitudes between 4,000 to 5,000 feet. In addition, under daylight conditions, visual degradation has been shown to occur at 10,000 feet.

Surprised by how susceptible these test subjects appear to be to mild hypoxia at moderate altitudes? Probably shouldn't be, even when compared to your own flight experience(s). These results are not referring to dramatic changes, such as blindness or suddenly becoming so cognitively “stupid" that you couldn't repeat your name. Instead, the review of this research should warn that even the healthiest of pilots are susceptible to subtle changes in performance, cognition and vision occurring at altitudes that we are frequently completely content to fly at, without considering the use of supplemental oxygen. Furthermore, less-than-perfect pilot specimens (e.g.-older pilots, smokers, pilots with mild anemia and other acute or chronic diseases) would be expected to experience greater effects at comparable altitudes or decrement in performance at lower altitudes than would be seen in remarkably fit pilot candidates, like those found in the military.

One obvious solution would be to fly at lower altitudes. However, taking this approach would be negating some real advantages of flying at moderate altitudes. For example, one could enjoy the benefits of contending with less traffic, higher true airspeed's, potentially greater effects of tailwinds and in some cases, avoiding low level weather, simply by flying at altitudes between 8,000 to 18,000 feet.

With the assumption that one wants to take advantage of high(er) altitude flight, let's examine what constitutes best operating practices for such an endeavor. Since there is much variation in an individual pilot's response to altitude, the only way to truly guard against insidious hypoxia is by measuring the pilot’s blood oxygen saturation with a pulse oximeter. These pulse oximeters are relatively inexpensive and widely available from a number of sources, from drug stores to pilot shops.  For today's general aviation pilot, flying above 10,000 feet without pulse oximeter readings is equivalent to flying without current charts. Have a pulse oximeter and use it!

As a general rule, a pilot should strive to maintain their oxygen saturation, as indicated by the pulse oximeter, to be at least 90%, or higher. Do not rely on your subjective sense of wellbeing or perceived performance, as a substitute for obtaining real data regarding blood oxygen saturation from a pulse oximeter! If oxygen saturation levels are lower than 90%, the pilot has one of two choices: a) descend to a lower altitude or b) utilize supplemental oxygen.

Most contemporary high-performance aircraft capable of moderate to high-altitude flight have built-in oxygen systems (assuming the aircraft is an unpressurized model). Another common approach to providing supplemental oxygen to the pilot and passengers is a portable oxygen system. These portable systems can represent a moderate initial investment, but can be an effective means of delivering oxygen in aircraft, not otherwise equipped, for years to come. Prices range from approximately $500-$2000, depending on the sophistication of the system. Some portable systems even have the capability of demand pulse delivery of oxygen when the recipient inhales, therefore reducing the wastage of oxygen seen with constant flow systems.

Regardless of whether the oxygen delivery system is built-in or a portable system, a few key safety items need to be kept in mind:

1. The pilot in command is responsible for assuring that s/he understands the system, it’s condition and it's proper operation. For portable systems, it is absolutely critical that the oxygen bottle be properly secured to prevent movement during flight. A 6 - 8 pound aluminum oxygen bottle can become a lethal missile, if not properly secured during turbulent flight.
2. Recognize that any variety of nasal oxygen cannula can only be used up to 18,000 feet.  Above 18,000 feet, a facemask is required.
3. Absolutely No Smoking (or playing with matches) around oxygen equipment.
4. The FAA has promoted a useful acronym for the pre-flight preparation/evaluation of supplemental oxygen equipment:    P-R-I-C-E

(Copied from FAA Circular :OK-09-439 “Oxygen Equipment- Use in General Aviation”):
Prior to every flight, the pilot should perform the “PRICE” check on the oxygen equipment. The acronym PRICE is a checklist memory-jogger that helps pilots and crewmembers inspect oxygen equipment.

• PRESSURE - ensure that there is enough oxygen pressure and quantity to complete the flight.
• REGULATOR - inspect the oxygen regulator for proper function. If you are using a continuous-flow system, make sure the outlet assembly and plug-in coupling are compatible.
• INDICATOR - most oxygen delivery systems indicate oxygen flow by use of flow indicators. Flow indicators may be located on the regulator or within the oxygen delivery tube. Don the mask (or nasal cannula) and check the flow indicator to assure a steady flow of oxygen.
• CONNECTIONS - ensure that all connections are secured. This includes oxygen lines, plug-in coupling, and the mask (or nasal cannula).
• EMERGENCY - have oxygen equipment in the aircraft ready to use for those emergencies that call for oxygen (hypoxia, decompression sickness, smoke and fumes, and rapid decompressions.) This step should include briefing passengers on the location of oxygen and its proper use.

After departure, what are appropriate standard operating procedures for assuring safe flight at moderate to high(er) altitudes? Conveniently enough, the P-R-I-C-E acronym can be used to prompt the pilot to monitor the adequacy of his or her blood oxygen level during flight. One significant challenge is to maintain the cognitive wherewithal (a.k.a. "situational awareness") to routinely check for an appropriate level of oxygen saturation and properly functioning oxygen equipment-unless prompted. A convenient technique is to use a timed prompt, such as a GPS unit message or repetitive count-down timer, to remind the pilot to assess the “PRICE” acronym, every 10 to 15 minutes. This technique is far superior to relying on the pilots’ own memory to check his or her blood oxygen saturation without some reminder, clearly a potential problem if the pilot is already cognitively impaired! An extension of this technique would be to use the accepted 5P checklist while on the ground and designate the “Programming" prompt as an appropriate time to “program” the count-down timer or repetitive GPS unit prompt. (As a reminder, the 5P checklist stands for: the Pilot, the Plane, the Plan, the Passengers, the Programming).

During an in-flight assessment, the P-R-I-C-E acronym takes on a slightly different meaning compared to the “pre-flight” evaluation, which is focused on the readiness of the oxygen equipment for flight (noted above). In-flight, the P-R-I-C-E acronym is designed to assure that the oxygen equipment is functioning properly and delivering adequate supplies of supplementary oxygen to pilot and passengers:

• Pressure- (in O2 tank)-adequate supply?
• Regulator-Setting appropriate for altitude?
• Indicators- Pulse oximeter reading and 02 flow indicator
• Connections-Check all unions for interruption of O2 flow
• Emergency Briefing-Altitude appropriate plan, in case of hypoxic emergency

While the versions of PRICE discussed above take a somewhat “equipment-centric” approach, the author has developed another version of the PRICE acronym that has a more “physiologic-centric” emphasis. The advantage of this particular version of PRICE is that it emphasizes a core principle of all discussions regarding pilot hypoxia and the use of supplemental oxygen: Is there adequate oxygen in the pilot’s bloodstream-as indicated by the pulse oximeter?

• Pulse Oximeter reading: Above 90%?
• Rate of flow (of oxygen): Appropriate for altitude?
• Indicator: Is the oxygen actually flowing? (check flow indicator)
• Cognition: Am I thinking/feeling Okay?
• Excess Supply: Do I have enough oxygen remaining to complete the mission?

Thus, if the pilot does nothing else routinely regarding supplementary oxygen assessment during the flight, at least regularly checking blood oxygen saturation with the “P- pulse oximeter” in the PRICE acronym, will go a long way in preventing significant and insidious hypoxia.

Regardless of the prompting system or acronym used, it is vital to regularly assess the adequacy of the crews’ blood oxygen saturation throughout the flight. In addition, some research suggests that there is a progressive decline in complex activity performance the longer the duration of the exposure to mild hypoxic conditions. This means that moderate to long duration flights, at moderate to high(er) altitudes, can have more of a deleterious impact on pilot performance than brief periods at altitude. Therefore, the importance of supplemental oxygen becomes increasingly important on long flights at moderate altitudes. Supplemental oxygen may mitigate decline in performance when it is needed most, like at the end of a long, fatiguing flight when the pilot faces a challenging approach or other difficult landing conditions.

In conclusion, there is ample evidence that flight at moderate altitudes (8000 to 12,000 feet) produces a level of low-level hypoxia that will contribute to a decline in pilot performance, not seen at lower altitudes. Clearly, individuals vary in their response to altitude, thus complicating attempts to make hard and fast recommendations regarding the use of supplemental oxygen during flight. An effective strategy to compensate for the wide individual variation would be to rely heavily on objective pulse oximetry data in the individual and to make more liberal use of supplementary oxygen, below altitudes where oxygen is required by regulation.

What would be the “take-home” recommendations? Recognizing that individuals respond differently to moderate altitude flight, conservative recommendations would be to:

a) Consider using supplemental oxygen for flight above 10,000 feet during the day and 5000 feet at night.
b) Use pulse oximetry measurements on a regular, periodic basis during flight above 8000 to 10,000 feet and strive to maintain blood oxygen saturation above 90%.
c) Be aware of the cumulative effect of altitude, fatigue, dehydration and other flight stressors on longer duration flights at moderate altitudes.

Flight at moderate altitudes can provide some exceptional benefits in speed and utility of the aircraft. Equipment (aircraft and supplemental oxygen systems) generally are less complex than those needed for high altitude flight. In addition, the impact of equipment failure at moderate altitude is generally less severe than a similar failure at extreme altitudes. A key component to safely enjoying flight at moderate altitudes is to acknowledge and respond to the challenges of insidious, low-level hypoxia on pilot performance.

About the Author: Joe spends most of his week delivering liters and liters of oxygen to his patients, as he provides the anesthesia for their cardiac surgery. The remainder of his time is spent passionately combining his love of flying and teaching at JATO Aviation. He welcomes you to come by JATO Aviation and chat about anything from flying to medicine.