What I Learned in Pilot Training: Hypoxia

I realized I have not been holding up my end of the bargain when it comes to all of the things I am learning at pilot training, so I am going to do my best to start to remedy that.

For the first two weeks of training the vast majority of what we talked about was the physiological aspects of flying airplanes.  I talked about some of this in my last post, but there is one area that I feel warrants its own separate discussion, and that is hypoxia.

A quick Google search tells us that hypoxia is a “deficiency in the amount of oxygen reaching the tissues”.  This definition is pretty general, but that is because the effects of hypoxia can vary dramatically.  That is one of the scary things about hypoxia is that it is different for everyone, so there is no way to give anyone a surefire way to know it is happening.

Maybe even scarier is that some of the potential symptoms can be exact opposite.  Some people will feel euphoric, or extra happy.  Other people will feel sick and miserable.  Some of the other symptoms are tingling in the extremities, headache, feeling warm, decreased visual acuity, cyanosis (blueness in the lips), and loss of mental capacity.

The last one may be the biggest threat to pilots, but if you are struggling to think clearly, how likely are you to notice it?  Your brain isn’t working well, but somehow it is supposed to realize that it isn’t working well?

As part of our training, we go into an altitude chamber to experience the effects of hypoxia, and determine what our own individual symptoms are.  It is invaluable training and helps you to recognize how you respond so that should it ever happen in flight, you are more likely to notice it.  To add to the challenges of hypoxia, your symptoms can change over time.

When I did the altitude chamber in navigator school, my only symptom was a very slight headache across my forehead that took forever to appear.  When I did it this time it was tingling in my fingers and feeling warm.  You may notice that most of these symptoms are things that could happen for dozens of different reasons.  Lots of people get headaches when they fly because they are dehydrated or a little sick, or because they had too much fun at the bar the night before.

Which brings me to some of the different reasons that you can experience hypoxia.  There are four general types of hypoxia: histotoxic hypoxia, stagnant hypoxia, hypemic hypoxia, and hypoxic hypoxia

Histotoxic hypoxia is the type that would be created by having too much fun at the bar the night before.  Substances like alcohols, narcotics, and other drugs impact the cell’s ability to accept oxygen and even though there may be enough in the air, it can’t get into the cells to do work.

Stagnant hypoxia happens when the blood is not flowing to the tissues properly.  In the military world this is usually driven by pulling G’s which causes the blood to pool in the lower extremities and thus not flow to your upper body.  It can also happen when you get cold and your circulation decreases.

Hypemic hypoxia is similar to histotoxic, but instead of the cells not accepting oxygen, it relates to the blood’s inability to carry oxygen.  In aviation this is most often caused by the inhalation of carbon monoxide either from a bad heater, or even just the exhaust on the ground before you get in and takeoff.

Hypoxic hypoxia is when the air simply does not have enough oxygen for your to breathe, which happens at higher altitudes.  Clearly an issue for flying an airplane.  It is also the most common type of hypoxia experienced in aviation.

We can all agree that hypoxia is a bad thing as it prevents us from thinking clearly and from using our body to safely operate an airplane.  So what can we do to prevent it, or recognize it when it does happen?

First, prevention because it is ALWAYS better to prevent a bad situation that it is to deal with it after it arises.  Being physically prepared to fly is the simplest way to prevent hypoxia, and a host of other issues.  That means that if you have not gotten enough rest, or you have been feeling sick, or you are hung over, or drunk, then you should not be flying.  We all want to fly as much as possible, but you have to know your limits and never cross them.  Even in the military we regularly talk about how there are almost no flights that cannot wait until another day if the crew is not physically fit to fly.

Another part of prevention is ensuring that you have the proper equipment for the flying you will doing.  If your flight is going to be mostly around sea level hypoxic hypoxia is not very likely, but if you have a broken exhaust system, or a cabin heat system that is not working properly, you put yourself at very real risk of hypemic hopxia, and that is every bit as dangerous.  Hypoxia is not only a matter of flying at high altitude!

Generally speaking we talk about oxygen deficiency starting at around 10,000 feet MSL.  So if you are planning to fly anywhere near that altitude for very long you need to have the right equipment.  That could be a pressurized cockpit, or it could be supplemental oxygen.  If you will be flying at night, some people push that altitude as low as 5,000 feet MSL.  Once again, everyone is different, and has different tolerances.  If in doubt bring along the extra oxygen because it may save your life.

I realize that going to an oxygen chamber is not realistic for most people, nor is it really necessary for someone cruising around in a 172, but if you get the opportunity I would highly recommend it.  That being said, there is still plenty of education and experience you can get to learn more about the effects of hypoxia and how it can affect you.

My only experience with hypoxia, other than in the controlled environment of an altitude chamber, was actually on a hiking trip.

I was climbing Mt. Fuji in Japan, which only goes up to about 12,300 feet, not that much above 10,000.  As we got to the summit it was rainy and cold and I was miserable and I had a headache.  But, I didn’t even realize it until we were on our way back down and I started to feel better.  Sure I recognized that I was tired, but it wasn’t until I headed lower and started to lose my symptoms that I realized I had even had symptoms.

The odds of this happening in aviation are not very high, at least if it has become a critical issue.  If the symptoms are minor then descending may be enough to clear your head and fix your symptoms, but descending may not always be an option, and if it has to do with poor equipment, it is not likely that the equipment will just fix itself.

So how do you recognize that you are having issues?  The key to this is not waiting for extra signs.  At the first indication that you may be hypoxic you need to take action.  Some people can go hours with symptoms before it becomes life threatening, but for others the symptoms may come on and only minutes later the person will be incapacitated.  So if you even think there is a small chance you might be hypoxic, try to fix it immediately.

If you are fortunate enough to be flying with someone else, ask them how they are feeling, or if they see anything different with you. Sometimes just bringing it up will key the whole crew off that something is not right and you may save everyone.  Maybe it is just you having an issue though, and they can help you remedy the situation and get back to normal.  The point is to not keep it a secret if you don’t feel right.

Many people do most of their flying alone so what can they do?  In many cases, you can still tell someone, whether that be ATC or anyone else that may be on frequency with you.  Again, just mentioning that something doesn’t feel right can help key off your mind that something isn’t right and can help you down the path to fixing it.

If there is legitimately no one around then just say it out loud to no one.  The brain is pretty amazing and just vocalizing it may be enough.  Most importantly, if you are by yourself, don’t hesitate to take some action if you feel even the slightest possibility that something isn’t right because you have no idea how far you are from just passing out.

This ended up being way longer than I expected, but I would argue that I didn’t even scratch the surface on the topic, or at least on aerospace physiology.  There are people whose entire career is in that field.

Hopefully what I have done is given you some things to think about when you fly, and encouraged you to spend some extra time thinking about how your body is performing when you fly.  We spend a lot of time checking on the plane, and our equipment but most of us don’t take enough time to check on our physical ability to fly.

It doesn’t have to be a huge drawn out process, but even taking the time to stop before you step to the plane and just ask yourself how you are feeling may be what saves your life.  I would also encourage you to spend some time learning more about aerospace physiology.  That could be through articles, or YouTube, or seminars, or other classes.  No matter where it comes from, understanding how our bodies work when they are in the unnatural state of flying will only make you a better aviator.