Hypoxia; The silent killer

What is Hypoxia? Anyone who has made it through a Private Pilot course worth its salt can answer. “Hypoxia is a state of oxygen deficiency in the tissues and or blood, sufficient to impair functions of the brain and or other organs” or something to that effect. Next question, can you name the different types of Hypoxia? You may have memorized all 4 of them, but have you ever experienced Hypoxia? Do you know how your body responds to Hypoxia? What are YOUR warning signs? How would you respond to Hypemic Hypoxia (think carbon monoxide) vs Hypoxic Hypoxia (high altitude)? Why is Hypemic Hypoxia more dangerous? How can you tell the difference?  

Where our training falls short: 

Truth is, if you didn’t conduct your initial training in a mountainous area, all of your flying has likely occurred at lower altitudes (below 6000’ MSL). This can result in a huge gap in our practical training and reading dry content in a book doesn’t always drive this home. 

My experiences with Hypoxia: 

My first experience with Hypoxic Hypoxia (the altitude type) was on a flight from NJ to Oshkosh WI for EAA AirVenture back in 2009. I was flying right seat in my friend Frank’s Cirrus SR-22. My job was to run the radios. He brought along an oxygen bottle and briefed me on its use. Having never used oxygen, I thought this was a bit overkill. Heck, the FAA says we don’t need it, so we are good. Right? But after a few hours of flying at just 10’000 feet, I started missing radio calls. I didn’t realize it. I thought everything was just “GREAT”. Frank finally turned to me and asked “Mike, check your oxygen levels, Cleveland center keeps calling us”. I looked at my O2 stats on the meter and saw just 85% (I could write a post on these devices, but just know, what it reads can be much higher than your real O2 levels.) “WOW,” I thought. I turned my oxygen up on the regulator and POOF, the lights came on and I was back to myself. At the time, I was in terrible physical shape. I was overweight and did zero cardio. Now that I’m 30 pounds lighter, I’m comfortable with long flights at altitudes up to 12,000 feet, but I’m still careful.

My second experience with Hypoxic Hypoxia was on a long IFR night flight from Portland Maine back to NJ in a Cessna 172sp. I was flying with my friend, we’ll call him “Bob”. Bob is a happy guy, he is so happy, in my opinion, that he was born to be a photographer “SMILE EVERYONE!” (which is what he has done, quite successfully I might add). We were dodging thunderstorms all over the northeast and ended up being vectored to Albany until finally crossing the front. Just as we turned south, he snapped at me. “This is shit, this just sucks, this is going to take forever!” I responded “Wow Bob, I think you are hypoxic” “NO, I’m not!”. We had been flying at around 7,000 feet for over an hour. I requested a lower altitude and was soon granted a descent to 5,000 feet. A few minutes later, Bob turned to me and said “man, isn’t this great!” I responded, “welcome back.” The scary thing is that he had no recollection of the outburst. Hypoxia can have a HUGE impact on your performance and personality, even at lower altitudes. 

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Video: Crew becomes Hypoxic at altitude:

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What the FAA has to say about Hypoxia: 

FAR 91.211 outlines the basic requirements for oxygen use in pressurized and unpressurized aircraft (we’ll skip pressurized aircraft for this post), but we should understand that these are requirements, or in this case, minimums, and they aren’t necessarily the most conservative options. Let’s look instead at the AIM, keeping in mind my two stories above. But first a note: if you regularly fly at, let’s say 10,000 feet and run marathons for fun, you may be fine holding off on Oxygen use until the FARs kick in. But if you’ve never flown above 6,000’ and enjoy Popeyes Chicken Sandwiches on a weekly basis, I’d recommend an O2 bottle for that trip to Oshkosh in your new Bonanza. Back to business. According to the AIM, Night vision starts to drop off at altitudes as low as 5,000 feet, and functions start to drop off at 10,000 feet during the day. It’s for this reason that they recommend oxygen use above these altitudes. Everyone is different. Best practice, use common sense and err on the side of safety.  

Time at altitude is what matters: 

In FAR 91.211 they leave some room for time at altitude. For example: From above 12,500 – 14,000 feet you don’t need supplemental oxygen unless you fly up there for more than 30 minutes. Above 14,000 up to 15,000 the crew must be on oxygen. Above 15,000’ you need to provide oxygen to your passengers, but they don’t have to use it. But why the 30 minute rule? This is because the effect of reduced pressure in the lungs takes time. The higher you go, the less pressure there is, so the less time you have before feeling the effects. 

So why even allow 12,500 – 14,000 for 30 minutes? The first example I give to my students is a pilot crossing a mountain range. He or she may only need to fly above 12,500 for 10 minutes, until clear of terrain. The other example I use are skydive pilots. They spend a few minutes at these altitudes before dropping their screaming passengers out the back door. Jumpers away! So what’s so magic about 12,500 feet? Nothing really. In my opinion, if you’re flying at, let’s say, 12,000 for more than 30 minutes, you should have oxygen available. The next question that many DPS’s will ask is, can you just fly to 12,500’ for 29 minutes, and then descend back to, let’s say, 11,000’ and back up to 12,500? Legally, yes, will ATC allow it? Most likely, no. Is that the intent of the rule? No. So again, stay on the side of safety and avoid it. 

Types of Hypoxia from most common to least common: 

Hypoxic Hypoxia (the altitude type) 

As we climb, air is less dense. Simply put, for every cubic foot of air, there is less and less oxygen. You’re breathing, but your lungs can’t absorb the oxygen in the air because there is less of it. Factors can include, but are not limited to, smoking, general fitness, lack of sleep etc. Simply put, turn up your Oxygen and you should be better. Easy…. Unless you didn’t bring Oxygen. Then It’s time to get to a lower altitude! 

Hypemic Hypoxia (Think Carbon Monoxide) 

This is the reduced ability of your blood to carry the oxygen we absorb via the lungs. You can turn that oxygen bottle up, but things remain bad. It’s TIME TO LAND! ASAP! The way I explain this to my students is to imagine a conveyor belt. The empty buckets represent your hemoglobin (the part that carries the oxygen to your organs). Carbon monoxide has an affinity (attraction) to hemoglobin and BLOCKS the receptors before the oxygen can bond with it. Simply put, imagine the carbon monoxide as a blocker that fills the buckets before Oxygen has a chance to be carried. Over time, your oxygen levels drop more and more until you pass out. Then it’s over. Practically put, if you smell exhaust fumes, shut off the heat, open the fresh air vents and get on the ground. I had this happen once when I was a brand new Private Pilot. I had a splitting headache all day. Not fun, but I got on the ground right away. Other causes are anemia, hemorrhage, hemoglobin abnormalities, sulfa drugs and nitrates, but are less common in aviation. 

Stagnant Hypoxia: 

Blood flow is compromised. Most common cause? The example I use is high G maneuvers. Blood is pulled from your brain and you go night night! Other causes are exposure to extreme cold (rapid decompression in a pressurized aircraft) or the failure of your heart to pump blood effectively. In that case, you have bigger problems. 

Histoxic Hypoxia: 

Think TOXIC. Drugs and or Alcohol (this is why the FAA is so strict on what RX drugs you take). My example to my students here is a person goes drinking all night, then goes flying because they feel fine. At 10,000 feet they are drunk again. Why? The cells are impaired and can’t use the oxygen. This is also why you feel BUZZED when drinking. Same thing.   

In closing, the tricky part about hypoxia is that our bodies don’t have an adequate warning system for it. You feel great, then you feel sleepy, then it’s over. There have been many examples of tragic accidents due to this silent killer. It’s best that you understand the risks and know your warning signs. If you’d like to learn your warning signs, in a safe environment, you can attend high altitude training in a hyperbaric chamber. Here’s a link to the FAA website where you can schedule this training. Take your camera, it should make for a funny home video that you can dust off during family events. 🙂  https://www.faa.gov/pilots/training/airman_education/aerospace_physiology/

References: 

AIM Page 535 – 536 http://www.faraim.org/aim/aim-4-03-14-535.html

FAA Beware of Hypoxia By Larry Boshers https://www.faa.gov/pilots/training/airman_education/topics_of_interest/hypoxia/

FAR 91.211 https://www.law.cornell.edu/cfr/text/14/91.211