Tutorial: Hypoxia 2 – Effects

“With Silent, Lifting Mind I’ve TroD…”

“Oh! I have slipped the surly bonds of Earth
And danced the skies on laughter-silvered wings;
Sunward I’ve climbed, and joined the tumbling mirth
of sun-split clouds, — and done a hundred things
You have not dreamed of – wheeled and soared and swung
High in the sunlit silence. Hov’ring there,
I’ve chased the shouting wind along, and flung
My eager craft through footless halls of air….

Up, up the long, delirious, burning blue
I’ve topped the wind-swept heights with easy grace.
Where never lark, or even eagle flew —
And, while with silent, lifting mind I’ve trod
The high untrespassed sanctity of space,
– Put out my hand, and touched the face of God.”

Read the poem ‘High Flight’ by John Gillespie Magee Jr in a room full of aviators and you can’t help but notice glances turn wistful, gazes drift to the floor or the odd tear fall. Written as an attempt to put into words the pure joy and freedom of flight, the poem has a bittersweet tone, made even more poignant perhaps by the loss of the talented pilot and poet a mere 4 months later to a mid-air collision at the age of just 19. It has come to be an important part of the aerospace community and is often recited at memorial services to other aviators who, like Gillespie Magee Jr, have taken their last flight.

Like many pilots in WW2, the journey from deciding to fly to sitting in a fully operational aircraft was short. Having joined the Royal Canadian Air Force in October of 1940, he trained on Harvards in Canada, receiving his wings in June 1941. By 7 August 1941 Gillespie Magee Jr had been assigned to No 53 Operational Training Unit at RAF Llandow and took his first flight in the most advanced UK fighter of the time, the Spitfire.

11 days later he flew his 7th sortie – an instrument check – taking the MkV higher than he had ever been before – all the way to 33,000ft. The story goes that as he climbed words from Cuthbert Hicks’ poem ‘The Blind Man Flies’ – “For I have danced the streets of heaven, And touched the face of God” stuck in his head. Shortly after landing he penned his now famous words.

This beautiful verse gives a glimpse into the mind of a pilot experiencing the pure joy of flying high above the clouds. But it may well tell us something more about High Flight. Gillespie Magee Jr wrote in his logbook previously that he experienced some symptoms of hypoxia before descending below 10,000ft. To many the words reflect the sensation of calm, disconnection and even euphoria that can come when the oxygen supply to the brain runs low.

We may never know what inspired Gillespie Magee Jr to write the poem. But if you are lucky enough to fly above the white carpet of clouds (fully protected from hypoxia by modern equipment) on a sunny day you may just get a glimpse of the ‘untrespassed sanctity of space’ that draws so many people back.

To see the original handwritten version of High Flight (written mere moments after landing!) and to learn more about John Gillespie Magee Jr, click here for a great article by Elinor Florence.

“IT’s GETTING HARDER AND HARDER TO BREATHE“

Last tutorial we talked about the 4 actions required to get oxygen to where it is needed most and how an interruption in any of these pathways can lead to hypoxia. We also covered the variety of ways this can occur in aviation, concluding that the most common by far is due to the fall in barometric pressure as we ascend to altitude. This time we’ll cover what happens when we get hypoxic and why preventing this is one of the key roles of the Aerospace Medicine specialist team.

When there is a global cause of hypoxia such as the hypoxic (or absorption) hypoxia caused by hypobaria (low atmospheric pressure) it is going to affect every tissue in the body. How quickly it does depends on the match between oxygen supply and demand of that tissue. It is how hypoxia impacts the normal role of that tissue that affects our ability to recover an aircraft to a safe, lower altitude.

There are a huge number of physiological changes that occur due to hypoxia. Most of these can be seen as an active homeostatic response of the body; an attempt to increase oxygen delivery to tissues and reverse the hypoxia. As we will see in our next tutorial, sometimes this works and sometimes it just serves to make things worse.

Hypoxia affects all tissues but for this tutorial I want to focus on one particular organ which is critical to flight safety. This is, of course, the brain…

“HOW DARE YOU SAY THAT MY BEHAVIOUR IS UNACCEPTABLE?”

Regardless of the reason it occurs, whenever the supply of oxygen to the brain can no longer reach its demands, normal cognitive processes will be affected. This includes functions that are critical to flying an aircraft such as memory, logical reasoning and visuospatial processing. Just as critically, it affects our judgement, changing risk taking behaviors and leading to a sense of calm or even euphoria. Ultimately, if severe enough or left uncorrected, hypoxia will lead to a loss of consciousness and death. Most importantly, due to these effects on cognition, most people are entirely unaware that they are becoming hypoxic.

The severity of these symptoms and the order in which they may appear depends on a number of factors:

Exposure:

Exposure to a greater level of hypoxia will clearly result in worse symptoms. In aviation this generally is dictated by the pressure altitude to which you are exposed.

Early pioneers of aviation medicine began the difficult job of trying to assess at what altitude different cognitive effects will occur. Due to the complexity of measuring psychomotor function (the combination of attention, problem solving and response), research in this area continues and a true consensus is difficult, if not impossible to find.

The debate about at what altitude hypoxia becomes unacceptable still continues (Hint: it depends on what you are doing there and the risk you are willing to take!). To even summarise the conflicting studies and points of view would be a series of tutorials in and of itself.

Despite that, we can use some key findings to illustrate the changing affects as we ascend.

There are a few things to bear in mind with tables like the one above. Firstly, it represents someone unprotected from hypoxia. Secondly it is only a reference for average symptoms; there is a huge variability between individuals and different ways to measure cognition so these numbers can differ. Finally, it represents someone slowly climbing in altitude, for reasons that will become clear soon!

Destin from Smarter Every Day made a great video showing the effects of a relatively rapid exposure to hypoxia (1,5000ft/min to 25,000ft) . You can skip ahead to 2 min 50 if you want to see the chamber run on its own, but I recommend watching the whole thing.

This video nicely demonstrates some really important points:

1) Destin shows some clear symptoms of more severe hypoxia – he is no longer able to do a very simple task (a child’s toy), he has some shakes – he almost seems drunk which is a common comment.

2) Once Destin puts his oxygen back on his symptoms resolve incredibly rapidly.

3) Compare the responses of Don and Destin. Don notices his symptoms early (“a Looney Tunes kind of thing”) and corrects for his hypoxia in the right way by restarting his oxygen. Destin, distracted by other tasks, misses this point and becomes so hypoxic he is no longer able to put on his mask turn on his oxygen by himself even when he is told he is going to die (see Time of Useful Consciousness below)!

Time:

The time between the onset of hypoxia and maximum exposure has a big effect on what order signs/symptoms will occur. If the onset is gradual enough the pattern of symptoms may be relatively close to those outlined in the table above. If the onset is rapid then the sign/symptoms at the maximum exposure may present at the same time as the others or loss of consciousness may be the very first sign.

Take the table of symptoms above as an example. Whilst it suggests that that at 16-18,000ft PA you might see some reduced performance and increased reaction times, if you were to be exposed to this altitude rapidly, a sudden loss of consciousness could occur.

At least in part this is because a rapid exposure to hypoxia does not allow compensatory physiological mechanisms to take effect before the deficit occurs. We will cover these mechanisms in more detail in our next tutorial but you can compare it to the effect on blood pressure of a slow, trickling blood loss vs a fast hemorrhage. When blood is lost slowly, reflex mechanisms like the baroreceptor response have time to kick in, causing vasoconstriction, tachycardia and increased cardiac contractility. This helps to ‘prop’ up the blood pressure for a bit longer. However, unless incredibly slow when longer term changes come into play, the stressor will outstrip the body’s ability to compensate.

Put simply, this is why mountaineers can reach the peak of Mount Everest (approx 27,450ft PA) without oxygen (although this is highly inadvisable) whilst a pilot rapidly exposed to this altitude would expect to have about 3-6 minutes before they lose the ability to correct for their own hypoxia.*

*This is known as Time of Useful Consciousness (or TUC) and is the key difference between the outcomes of Don and Destin in the video above – Destin was so hypoxic he was unable (and unwilling) to correct for this. It serves as a useful way of communicating the effects of hypoxia. However, for all the reasons we are currently discussing, it is rather inaccurate and is only really a useful measure to compare the effects of different altitudes. Numbers vary depending on source but some often quoted figures are:

15,000ft PA = >30 mins
25,000ft PA = 3-6 mins
30,000ft PA = 1-3 mins
35,000ft PA = 30 sec – 1 min
40,000ft PA = 12-15 sec

There is another phenomenon which is worth mentioning. If you are unlucky enough to suddenly be exposed to a high enough altitude very quickly then the PO₂ that you inhale into your alveoli may be lower than the PO₂ in the blood in your pulmonary capillaries. Fick’s Law will still operate but now the direction of diffusion is reversed. This means in certain circumstances you can end up paradoxically losing oxygen through the lungs rather than gaining it, at least until equilibrium is reached. As with any equilibrium the end state will be the same but you will just get there a lot more quickly.

Individual Variation

Look at any study into hypoxia and you will note that, within a certain range, the error bars for both physiological and cognitive effects tend to be quite large. Clearly if you expose people to a severe enough hypoxia they will all become unconscious, but its what happens before this which varies significantly between individuals.

The results from the table above highlight how widely symptoms can vary between individuals who are exposed to the same severity of hypoxia at the same rate. This is due to a number of variables. Some of these are known including the modifiable factors listed below and how the individual’s own protective mechanisms respond. Others are less clear and likely represent the normal spread of individual physiology.

All the subjects in McFarland’s study were healthy individuals, bnut clearly any medical condition that can affect any of the mechanisms required to get oxygen to the cells will tend to result in worse symptoms for any given altitude. This is why medical assessment for fitness to fly is so important for people with these conditions.

Other Factors

A number of modifiable factors can affect your response to hypoxia:

Exercise – Think back to our notes about diffusion limitation. If you are exercising, the increased cardiac output will reduce the time for diffusion in the pulmonary capillaries, especially when a lower diffusion gradient is present during hypoxia. This leads to a double effect where, not only is the potential equilibrium lowered but it is not even reached before blood leaves the lung.

Temperature – being cold makes you more prone to hypoxic symptoms. Not good when we remember unprotected exposure to altitude generally means exposure to low temperatures too!

Illness – concurrent illness also affects how much hypoxia will impact you. This is just one of many reasons why fitness to fly is such an important issue.

Drugs/Alcohol – this can impact your response to hypoxia in three key ways. First is by lowering your threshold for hypoxic symptoms. Secondly, the effects of drugs and alcohol can mask the early symptoms of hypoxia. Finally, being under the influence of certain drugs or alcohol will affect your ability to correctly perform key safety actions.

“Up, Up The Long, Delirious, Burning Blue…”

Now let’s think about what can happen if this occurs in flight. One of the most dangerous ways hypoxia can affect aircrew is when it is unnoticed.

If you suddenly lose a canopy or a hole appears in the side of your aircraft you are likely to notice and – if correctly trained – perform your corrective actions quickly. However, if there is a slow leak, or if the aircraft fails to pressurise as you ascend, you may not notice the early symptoms of hypoxia (particularly if distracted by other tasks, like Destin was in the video above).

Once you pass your Time of Useful Consciousness you will not be able to correct for your own hypoxia and, unless someone else can step in to help, the point of no return has been crossed.

Listen to the ATC recording below to see how this can play out in a real life scenario:

Scary, no?

This is why all aviation regulatory bodies limit how high you can fly when unprotected from hypoxia. These tend to contain an upper do not exceed altitude with a second, slightly lower, altitude above which only a limited amount of time can be spent.

As we mentioned earlier, how much hypoxia is acceptable is down to how long you are going to be exposed for and what sort of tasks you are doing. If you are a passenger with no safety critical roles you can tolerate a slightly greater amount of cognitive decrement before safety is affected. If you are in control of the aircraft or are responsible for passenger safety this is a very different story. And even lower levels may be accepted in certain circumstances, such as if you are performing higher workloads in flight. This explains why rules for military aircrew are generally rather different than for civil flying.

“IS AnyONe OUt There..?“

Clearly aircraft aren’t being flown about the skies with pilots that appear to be drunk and we don’t all get hypoxic every time we fly to our holidays at 39,000ft PA. So surely there must be ways around this…Indeed there are, but you’ll have to wait until next time to find out.

Until then…

Yours,

JB

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Check out this tutorial on the effects of hypoxia by #NGAM at NextGenAsM.wordpress.com

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