Aeromedix Flight Blog

Survival at 25,000 feet

I have a friend who was flying at FL210 and rolled the empty copilot seat back which crimped the oxygen tubing to his mask. The ATC tapes of his subsequent radio transmission were beyond ugly. Fortunately, he got this oxygen supply straightened out before a disaster occurred. He was a lucky one.

No one would deny that the top of Mount Everest is a hostile environment for humans. There is no
difference in an aircraft cabin when we fly over the top of the mountains and even higher. By getting
their faster and with an engine or two, our bodies may not be ready without preparation, support, and a
dose of caution.

The most significant issue at the flight levels in an unpressurized aircraft is oxygen. The most significant issue in pressurized aircraft is oxygen as well! Since we now have many production turbocharged aircraft capable and perform better at the flight levels, more and more pilots are setting their altimeter to 29.92. A thorough understanding of the physiology of this harsh environment at these flight levels is important for pilots to understand.

First, let us go over a little bit of physics. At sea level, one quart of air has a specific number of oxygen molecules. Think of a bag of potato chips in your aircraft. If you take a bag up in an aircraft, the amount of atmospheric pressure on the air inside the bag decreases, the space between the air molecules increases, and the bag expands. The expanding pressure may actually burst the bag. Thus, a quart container of air holds fewer air molecules per volume because the air molecules are spilling out of the container.

This means that when we breathe at higher altitudes, each quart of air we take in has fewer molecules
of air which contains about 19% oxygen molecules and 80% nitrogen molecules (the other 1% is minor
stuff). The percentages of each type of gas stay basically the same at all altitudes but each breath (a
fairly constant volume) contains less oxygen molecules and thus, there is less oxygen to be absorbed
into our blood stream.

Normal human oxygen saturation of the blood at sea level is between 95-99%. As we go up, this
saturation drops. It generally becomes problematic about five percentage points below our home
altitude saturation and more significant ten percentage points below the baseline. Where we live
makes a difference because people who live at higher altitudes have more red blood cells per unit
volume to compensate for lower saturations and will be acclimatized and tolerate higher altitudes over
sea level inhabitants.

For healthy people, the altitude where the ten percentage point drop in saturation is reached is in the
eight to ten thousand foot range. If there is underlying lung, heart, or circulatory problems, the blood
oxygen saturation may be much more critical at lower altitudes. Even without medical issues, higher
age causes less resilience to increasing altitude as well. An example might be the person who has
unknown partial coronary artery blockage who has increased heart oxygenation compromise as they increase their altitude with the corresponding decrease in oxygen saturation.

Oxygen supplementation with a mask or cannula increases the number of molecules per inhaled breath
which increases saturation. When we wear a nasal cannula, as we inhale through our nose or mouth
(mouth breathing causes a Venturi effect which still sucks oxygen in through the nose), we may raise
the oxygen percentage in each breath up to 24 or 26%. With a well fitting non rebreather oxygen
mask (the kind with a reservoir bag to collect oxygen from the source between breaths), the oxygen
content in the breath may reach 40%. This is important to understand since it is the basis for the FAA’s
limitation on nasal cannula for use to FL180 or below and the requirement for a mask for altitudes
above FL180. 24 to 26% with a cannula is just not a high enough percentage to keep adequate
saturation above FL180. For higher oxygen percentages needed above FL300 in un-pressurized aircraft,
pressure oxygen systems like those used in the military are necessary.

Here is a real world example of how one could get into a problem: We are flying our Cirrus at FL250
enjoying a 100 knot tailwind cruising from Santa Monica to Houston. We are on a mask with an oxygen
flow rate of 2.5 liters per minute (the FAA recommended flow rate is 1 LPM/10,000 feet) and our
saturation is 92%. Not bad for a flatlander. Our dog decided to change their napping position and
unbeknownst to us, his paw pulls the tubing out from oxygen tank regulator.

How long before we notice something is wrong? Wrong question Bucko! How long do we remain able
to perform flight duties is the correct question and is called “Time of Useful Consciousness.” (See Chart
1 below). You will not be dead in seconds but you will not respond to those F-16s buzzing your windscreen. If you are lucky and do not progress to death, you may wake up when the plane runs out of gas and descends to “thicker” air. You might even get it together enough to pull that Cirrus’ parachute. Another concern with oxygen masks is removing them to drink or eat at altitude. It does not take long with the oxygen mask off to become just distracted enough wiping that mayonnaise off your cheek to forget to put the oxygen mask back on before “goofiness” sets in!

Chart 1

Time of useful consciousness (TUC) is defined as the amount of time an individual is able to perform flying duties efficiently in an environment of inadequate oxygen supply. It is the period of time from the interruption of the oxygen supply or exposure to an oxygen-poor environment to the time when useful function is lost, and the individual is no longer capable of taking proper corrective and protective action. It is not the time to total unconsciousness. Smoking drastically reduces oxygen intake efficiency, and can have the effect of reducing tolerance by 3,000-6,000 feet. In addition, the TUC can be reduced by 30 to 50 percent when the decompression is rapid because of the sudden outward flow of oxygen from the body’s cellular tissue.

The proper oxygen system and flow rates are critical at the flight levels. This is not just “I’m getting a headache territory but cemetery property. The only way to truly know if you are getting enough oxygen is to use a pulse oximeter. Since there are excellent American made ones which have come down in price to as little as $100, there is no excuse not to have a pulse oximeter on board. If you really want safety, buy a pulse oximeter with saturation alarms however, these will run you about $600 or more depending on features.Remember even in pressurized aircraft flying high, the cabin differential might still put the cabin at, say, 9,000 feet which means an older pilot, a smoker, or a passenger with lung disease might need additional oxygen supplementation.

Since everyone is different, the oxygen requirements and flow rates the FAA mandates may or may
not be enough. The regulations basically state you must use oxygen if over 12,500 feet if you are there
for more than 30 minutes or anytime over 14,000 feet. These rules stem from the 1950s before pulse
oximetry was available and were based on some postulated physiology but mainly geopolitical
reasons. Oxygen systems were heavy and expensive in the 1950’s and the FAA apparently did not want to force general aviation aircraft in US to be required to use oxygen. Thus, the 12,500/14,000 rule will
allow the crossing of all the mountain ranges in the lower 48 and GA equipment may proceed on their
merry way. However, many people really should use oxygen at lower altitudes. Even going on oxygen
for 30 minutes prior to landing will help clear the senses and help make sure you aren’t on final to that cow pasture next to the airport.

In-Flight Oxygen Emergencies

A plan for emergency descent is a critical requirement for high level flight. In an unpressurized aircraft, emergency descents are not frequently taught despite this training being commonplace for pressurized aircraft. Each aircraft is different and the Pilot Operating Handbook should be consulted. I recommend that although the descent steps should be memorized, a small 3” x 5” card should be clipped to the yoke during all flight above 18K. This way, if you are suffering from hypoxia and/or disoriented from the noise and accompanying dust after an explosive decompression, you can see what you are supposed to do without fumbling for the checklists.

In an unpressurized aircraft, descend first and then check the oxygen supply. You may not have time to
trouble shoot the oxygen system while maintaining altitude so make the descent first. Notify ATC when you can but they become last priority in an oxygen emergency.

For pressurized aircraft, lack of pressurization on ascent (a la Paine Stewart’s tragedy) or a faulty oxygen system may go unnoticed unless you are paying attention to the cabin altimeter, visual flow indicator, and a pulse oximeter. Make at least one of these instruments part of your scan.

There are other important aspects of high altitude flight. Hydration is an important factor. As we
go high, pressurized or not, the amount of vapor in the air decreases dramatically—basically to zero
percent humidity. Since with every breath we take, our lungs humidify the air to almost 100%, at
altitude or in a pressurized cabin, we lose that fluid with every breath. Add to that the fact that the dry ambient air leaches fluid from our skin and dehydration becomes a major factor.

Some might say that dry skin and a mild headache due to dehydration might not be a big problem.
The issue here is not necessarily a simple headache, it is the increased risk of deep vein thrombosis (DVT or blood clots in the major veins) which may result in a pulmonary embolus (PE). A PE is when the clot breaks loose and clogs up your pulmonary arteries. This clog from a PE can be a minor event or precipitate an instantly deadly cardiac arrhythmia.

My rule is that if you do not have to urinate every three hours at altitude, your tank is not full. Your kidneys are a better physiologic monitor of your hydration status than generic suggestions on the number of glasses of water to drink per day so monitor your urinary output. You can always carry portable urinals with you in your aircraft for those times “you just got to go”

Another important factor in aircraft with those big canopies is sun exposure. Sun block is a requirement for high altitude flight in aircraft with minimal shade. Sun causes three types of skin cancer and will prematurely age those pretty pilot faces out there.

Although I should not have to say it, quality sunglasses are also a must. Yes, you might be able to see
fine without them, but in addition to the increased eye fatigue factor, bright sunlight contributes to the
formation of cataracts.

Yet another issue in non-pressurized aircraft is corneal hypoxia. There have been a few studies which
show decreased visual acuity in low oxygen environments—especially in people who have had LASIK type surgery. This is due to the reduced oxygen contact with the cornea. There is not much you can do about this factor but some ophthalmologists recommend daily intake of flax seed oil which may increase tear duct production and reduce the hypoxia effect.

Dog and cats will definitely suffer hypoxia with altitudes over 10-12,000 feet msl. Most pilots just let their pets sleep but this may not be the best approach for your pet’s health. There are a few pet-specific solutions that will provide your dog/cat with adequate oxygen when flying at high altitudes. I personally recommend the products from 4Paws Aviation. Although this equipment can run $100 or more, these oxygen hoods are the best way to administer oxygen to pets in aircraft.

Finally, there is still another issue for unpressurized aircraft occupants regarding middle ear pressure equalization. At high altitude, a significant amount of air escapes from the middle ear chamber through the Eustachian tube. On descent, air needs to return to the chamber or the classic ear “squeeze” will occur. This is due to the Eustachian tube being a relative one way valve in some people. This is also complicated by dehydration at altitude which thickens mucous secretions that also block the tube. Gentle Valsalva maneuvers on descent will help alleviate the pressure but must be done at the first sign of pressure and repeated frequently. If the sufferer waits till the pressure is really bad, it is much harder to clear the vacuum due to the increased pressures. High pressure equates to significant pain and possible damage. If simple Valsalva maneuvers do not work, trying the maneuver while simultaneously swallowing may help but this requires some coordination to get right.

High altitude flight is not dangerous but, like many things in piloting aircraft, it must be addressed with respect and preparation. It is not a simple walk in the woods.

Post Script: Viagra and AMS

When Viagra is mentioned anywhere, it creates a lot of interest—especially in the male dominated
aviation community. Viagra has been mentioned as a preventative for acute mountain sickness (AMS) for people who develop AMS at altitude. AMS has been shown to occur in pilots and passengers who fly above 6,000 feet both in absolute and cabin pressure. How often and how significant AMS occurs is up for debate.

AMS is a collection of non specific symptoms which are caused by low barometric pressures and hypoxia. It was thought to be exacerbated by a type of breathing pattern (i.e. long pauses) which was originally described during sleep in mountain climbers. I documented this same breathing in pilots during the wakeful state when we introduced pulse oximetry to general aviation and were testing various types of oximeters.

The symptoms of AMS include nausea, vomiting, anorexia, insomnia, dizziness, fatigue, and/or lassitude.
There are more serious types of AMS called high altitude pulmonary edema (HAPE) and an off-shoot called high altitude cerebral edema (HACE). Since most altitude sickness occurs with overnight exposure
or at least prolonged exposure (defined in a recent study as three to nine hours), this would tend to
eliminate most GA flights from being at risk for AMS. Interestingly, this may indicate that jet lag associated with long airline flights may not be time zone related but to AMS.

So how did Viagra get into the mix of this discussion? Since blood pressures in the pulmonary vessels appears to be increased related to altitude and this is thought to be one of the factors in HAPE, Viagra, with its vasodilatation effect could have a beneficial effect—at least theoretically. Since Viagra (and Cialis and Levitra) all affect color vision, their use is not permitted by the FAA. Thus, help with the mile high club cannot be augmented by the use of Viagra for AMS by required crew members.

Be Mindful of the Numbers that Might Save Your Life

The FAA’s Civil Aerospace Medical Institute (CAMI) is the official division that processes for two primary groups: pilot medicals and designates Aviation Medical Examiners. They receive a tremendous amount of data each year about both groups.This February, CAMI released its annual report of this data, the first since 1998, and it provides revealing information about both groups.

In 2010, there were 3651 Aviation Medical Examiners, which include Federal and Military AMEs.The Southern FAA Region has the most with 711examiners followed by the Great Lakes region with 514. The Alaska Region had the fewest with 68.

Most AMEs are family physicians (1790) followed by internal medicine (689), with the rest spread out through other medical specialties. About 82 percent are male and 8 percent are women. I’m not sure why, but the rest are not reported which makes one ponder.

Only 48% of examiners are pilots, which is disappointing. I have always said that an AME who is a pilot will be far more interested in spending the extra time with a pilot who has a medical problem to get them certified than one who is doing it to garner more patients. Of the 1747 pilot-examining doctors, 39 are airline transport pilots, 167 are commercial with instrument ratings, and 84 are commercial without instrument ratings. Private pilot AMEs are instrument rated 148 times and 1155 are private pilots who do not hold instrument ratings. There are two light sport rated examiners and 152 students pilots.

Fifty nine percent of AMEs are Senior examiners which means they can perform first class medicals and almost 100% of international examiners examine airline pilots – probably the only reason for their designation overseas.

There is one very disturbing issue with AMEs – The average age of all aviation medical examiners is 58.9 years old. Male examiners are 59.1 years old while female AMEs are 52.0. What is even more concerning is 64.5% of all examiners are 50 years old or older. To put this in perspective, only 137 AMEs of all ages have been designated annually in the last three years of available statistics. Unless this number changes dramatically, there will be a national shortage of aviation medical examiners in the next decade.

THE STATISTICS FOR PILOTS ALSO RAISES CONCERNS

In the year 2000, there were Just over 450,000 medicals processed by the FAA. In 2010, there were only about 395,000. The largest single drop was from 2008 to 2009 from 425.000 to 397,000 but this was probably reflective of the new regulations extending the duration of certificates for pilots under 40. Third class medicals became good for five years for less than 40 years of age while first class medicals were extended from six months to one year.

The FAA Southern Region leads in pilot medical exams with about 120,000 of which 41,689 were first class and 52,000 were third. The Western-Pacific region had the next most third class exams with just under 45,000.

At the end of 2010, there were a total of 598,146 medicals, which were still current, with 118,000 first class, 90,000 second class, and 389,000 third class. This includes medicals which reverted to a lower class such as a second class medical, which turns into a third class after one year. This also does NOT mean there are almost 600,000 active pilots out there since many first class pilots may have two exams and sometimes three exams; e.g., January, June, and December, in one calendar year plus air traffic controllers and others who use the FAA system as their standard, such as some race car organizations. Only seven percent of the medicals were issued to women.

Fortunately, the 20-24 year old and 25-29 year old age groups lead with the most third class medicals
at 50,418 and 51,083 respectively, which may show student starts, since most student pilot and medical certificates are third class. However, we will not know if these pilots stay active until their third class medicals are up for renewal in 2015. The next age group for third class renewals was the 65 years plus with just under 43,000. The 50-54 year age group led for first class medicals with 18K and 15K second class medicals were issued.

The average age across all ages and classes was 42.6 years. However, of the 10,617 first class medicals
issued to women, 72.8% were in the 16-39 age brackets. Just over 64% of all female medicals were issued to the same age bracket reflecting an influx of female pilots.

Male pilots averaged 70.5 inches in height across all medical classes and females averaged 65.4 inches, which is within the Bell Curve for all Americans being about a half inch higher than the U.S. mean for respective sexes.

HERE IS WHERE THINGS START GETTING UGLY

The average weight of pilots was 193.0 for men and 147.4 for women. There were 360 pilots who weight in more than 250 pounds and 94 who weight more than 2BO pounds. Forget that 170 average FAA weight for the guys.

For those of you who read the last paragraph and called “foul-there is not height versus weight,” here is the bad news: Body mass index (BMI) is a technical classification to judge the level of obesity in a group or population. Less than 18.5 BMI is underweight, normal is 18.5 to 24.9. Overweight is 25 to 29.9 and clinically obese is 30 or over.

Male pilots overall average a BMI of 27.3 with 27.5 for second and third class while first class pilots run26.8. The large number of first class medicals on those 39 or younger may have helped the airline pilot’s numbers. Female pilots averaged 24.2. The women can congratulate themselves on beating the “pants” off the men in this area.

THE HARD NUMBERS ARE EVEN MORE STRIKING

There are 258.351 male pilots across all medicals who are overweight and 126,536 who medicals show them as clinically obese. The female side is no better with 8.9% overweight women and 4,040 clinically obese female pilots. This means that 46% of mate pilots are overweight. and 23% are frankly, obese. The numbers are 23% and 10% respectively for women.

For pilots wanting to keep their medicals, the special issuance stats are interesting. Pilots who are clinically obese made up a whopping 32% of the special issuances during 2010. When adding in the overweight pilots to the obese ones, the number is 78.6% of all special issuances, male and female, are defined statistically as overweight or obese.

THE MESSAGE IS NOT NEW

If you want to stay alive, and keep your medical as an extra plus, pilots need to lose weight. Is it hard to do? Absolutely! Can one do it? Absolutely! I am now suggesting to patients to see two documentaries, which can be downloaded from Netflix or other video sources. “Fat, Sick. and Almost Dead” is about curing illness and weight loss with a juice diet. If the trucker in this video can do it, pilots can as well. The other movie is “Forks over Knives” where knives are scalpels. It shows how several cities in China have essentially zero cancer rates compared to the rest of the country. It also shows that this difference is probably related to a plant based diet.

I have done a combination of both. Weight has come off without trying, and my BMI is now in the normal
range. What is the most striking is I have not felt hungry at any time and it seems that by essentially sticking to a vegan type diet, I no longer crave things that I used to dream about.

Save your medical. Save you health care dollars. Get off medications. Save your life. Talk to your doctor about it. If he is unreceptive, see someone else.

It is your life and you are responsible, regardless. As I say to pilots all the time, getting your medical is one thing. Staying alive is another.

This article originally appeared in the March/April 2012 edition of PilotMag magazine

I get a lot of questions about the vision requirements for medicals. Basically, the requirements are 20/20 uncorrected or corrected in each eye for first and second class for distant vision. For near vision, the requirement is 20/40 uncorrected or corrected. If you are 50 or over, there is the additional requirement of 20/40 for intermediate (panel distance) uncorrected or correct. Apparently, the FAA thinks that pilots 49 or younger can always see the panel.

For third class, it is 20/40 uncorrected or corrected for far and near vision. Standard distant vision contacts are approved for distant vision for all classes and the only requirement for near or intermediate correction is to have a pair of glasses in the cockpit. There is no requirement to wear them!

Where things get complicated is with bifocal contact lenses, monovision, and other various eye problems and corrective techniques. Since there are so many variables in the vision area, always contact your AME before having surgical correction or spending money on corrective lenses to make sure you will still be legal for flying.

There is also some talk about the color vision requirements. For color blind pilots, it has been no big deal to go out with an examiner and get a green/white/red light gun test and an automatic SODA (statement of demonstrated ability). Since the advent of glass cockpits, the potential for missing information due to color blindness is worrisome. I am sure the FAA will come up with some testing mechanism in the near future for this specific problem.

The requirements can also be found on line in the Aviation Medical Examiner’s Guide (PDF download)

This article originally appeared in the February 2012 edition of Aircraft Owner Online

Hypothyroidism, or failure of the thyroid, is a common and easy to treat problem. Officially, the FAA has had it on its special issuance list for years but because it is such a benign problem, most everyone on both sides of government overlooked this condition. Unfortunately, this common practice is now ended and the FAA wants everyone with hypothyroidism to have a Special Issuance wasting a lot of time and money for pilots, AMEs, and the FAA alike.

The thyroid gland, located in the front of the neck just above the top of the breast bone, regulates the body’s metabolism. The gland is controlled by the hypothalamus and pituitary glands at the base of the brain with a chemical called thyroid stimulating hormone (TSH). For various reasons, the thyroid gland fails and requires one to take a replacement thyroid supplement.

When the gland fails, it almost always fails slowly. Symptoms, which tend to appear gradually, are feeling tired or weak, dry skin, brittle nails, cold intolerance, constipation, and irregular menstrual periods. Diagnosis is by simple blood tests of the thyroid hormone and the TSH.

Once diagnosed, patients are put on a dose of thyroid supplement which can be synthetic or from a natural source. Most patients are put on the synthetic (always use the generics) which is easier to adjust. Once the proper dose is reached by monitoring blood levels which usually only takes a few months, checking the levels once a year is plenty unless the patient has dramatic weight changes.

In this century, a hypothyroid patient’s condition’s chance of affecting flight about as close to zero as you can get. To make pilot’s go through the Special Issuance process for this condition, as opposed to a few questions by the AME to insure the thyroid level is stable, is just plain silly.

If you have hypothyroidism and you are not on a special issuance, go to your AME a couple months before your current medical expires and send in the information for the SI. It is currently taking eight to twelve weeks to get your initial special issuance medical back from OKC. Fortunately, subsequent renewals can be done under the AME Assisted Special Issuance program (AASI) and the medical can be given to you by the AME on the spot but you do have to provide recent medical information.

Originally posted in the Feb 2012 issue of Aircraft Owner Online