• What is hypoxia? Symptoms and causes of oxygen deprivation

    A flight attendant helping a passenger who is suffering from hypoxia

As you might well imagine, the environment in which we pilots operate is quite distinct from “life at sea level”. We undergo significant changes in pressure, speed, and other environmental conditions that pose adverse effects on our bodies. One such effect is hypoxia.

And what is hypoxia, you ask? Let’s just say it involves a lack of oxygen in the blood. However, in this article, we delve deeper. Will you join us?

What is hypoxia?

Perhaps you’ve heard of hypoxia, as well as anoxia, but do you know what it is? Hypoxia is a deficiency of oxygen in the blood, whereas anoxia is the complete absence of oxygen in the blood.

It’s known that air is composed of 21% oxygen, but it’s important to consider that, as we ascend, pressure decreases, hence air particles are more spread out.

What does this mean? According to Dalton’s Law, the partial pressure of oxygen decreases, and with fewer oxygen particles entering our bodies, there will be a lower concentration of oxygen in the blood.

How can oxygen deprivation occur during a flight?

Continuing with the theme of oxyhaemoglobin, then, why might an episode of hypoxia occur during a flight?

It’s because the external pressure decreases, changing the amount of gases present in the lungs, resulting in less oxygen in them. And when does the pressure change? Exactly! It decreases when ascending and increases when descending.

It can also occur during cabin depressurisation, when the pressure outside the aircraft plays a crucial role in oxygen transfer.

Oximeter, a tool used to measure the presence of oxygen in the blood

Causes of hypoxia

Apart from being at high altitudes, as can happen during a flight, hypoxia can be caused by:

  • Inhalation of smoke.
  • Carbon monoxide poisoning.
  • Pressure on the trachea.
  • Suffocation.
  • Diseases that prevent the correct functioning of the respiratory muscles or the cardiovascular system.
  • Very low blood pressure.
A passenger suffering from hypoxia symptoms

Symptoms of hypoxia

How can you identify hypoxia before it becomes anoxia? Symptoms of oxygen deficiency in the blood are:

  • Difficulty concentrating
  • Loss of consciousness, rapid mood changes, and euphoria
  • Lethargy and drowsiness
  • Headache, dizziness, and nausea
  • Loss of muscle coordination
  • Bluish colouration of the skin and pallor, especially in extremities.
  • Failure in the senses, such as in colour vision.

Euphoria, a feeling of complete well-being, is a symptom to be particularly cautious of when discussing hypoxia, as it can be experienced above 10,000 feet, and if it occurs to a solo-flying pilot, it can be very dangerous.

More about hypoxia

Moreover, the level of hypoxia can be affected by the individual’s physical condition, the level of decompression, the altitude of the flight, and the affected individual’s own tolerance.

An important fact is that a regular smoker may begin to suffer from hypoxia at 6,000 feet, and smoking is considered one of the risk factors since carbon monoxide binds to haemoglobin and blocks the transfer of oxygen.

As you see, to work in the air, one must be healthy to avoid all risk factors. Thus, pilots must regularly be checked to be fit to fly, and how is this achieved? By passing the exam to obtain the Aviation Medical Certificate.

  • A flight passenger suffering from hypoxia

How many types of hypoxia are there?

There are several types of hypoxia, such as perinatal hypoxia, hypoxic or anoxic hypoxia, haemic or anaemic hypoxia, ischaemic hypoxia, histotoxic or cytotoxic hypoxia, and hypoxaemic hypoxia.

Perinatal hypoxia

Affects neonates, especially due to complications during childbirth that interfere with oxygen supply, potentially resulting in long-term brain damage.

Hypoxic hypoxia

This is the most common in aviation, caused by insufficient partial oxygen pressure at high altitudes.
What causes it? As explained, exposure to high altitudes, though it can also be caused by lung diseases even during sleep apnoea and while diving. It can also occur when inhaling gases such as nitrous oxide or laughing gas.

Haemic hypoxia

What is haemic hypoxia? This type arises from a reduction in the haemoglobin’s capacity to transport oxygen, either through carbon monoxide poisoning, whether from smoking or other causes, or due to severe anaemia and blood loss from haemorrhage or donation.

Ischaemic hypoxia

Results from inadequate perfusion, where blood flow to the tissues is insufficient, especially affecting organs like the brain and heart.

Histotoxic hypoxia

Occurs when cells are unable to utilise oxygen due to intoxication, whether from alcohol, cyanide, or drugs.

Hypoxaemic hypoxia

It is due to a decrease in the oxygen saturation of haemoglobin, despite adequate lung function.

A woman suffering from first stage of hypoxia

Stages of hypoxia

As mentioned before, partial pressure decreases with altitude, but from what altitude does this become significant?

Well, this depends on each person, their health state, and habits but generally, we can divide the body’s reaction to this decrease in oxygen into three stages:

Reaction stage

This threshold is reached when the decrease in the amount of oxygen reaching the body starts to be noticeable, around 7,000 feet. Here we can observe how performing complex tasks becomes more difficult, and there’s a slight increase in heart rate and respiration. Additionally, night vision is also affected starting from 5,000 feet.

Disturbance stage

It occurs between 10,000 and 12,000 feet when blood oxygen saturation significantly decreases, and the body begins to activate defence mechanisms to avoid the effects of hypoxia.
When we reach this stage, the body struggles to perform tasks and maintain attention and alertness. From 12,000 feet we can suffer euphoria, drowsiness, dizziness, fatigue, and headache.

Critical stage

It occurs around 22,000 feet, and the most significant aspect is that blood oxygen saturation decreases to 65-60%, considering that the normal saturation at sea level is 95-100%.
This significant decrease triggers a rapid deterioration of our mental functioning with great confusion and dizziness, even potentially leading to loss of consciousness.

  • A flight attendant during an hypoxia episode

How is it solved?

And bearing all this information in mind, how do we remedy it? If hypoxia is triggered by the decrease in pressure with altitude, then the solution is to descend below 10,000 feet and apply oxygen to the affected individuals.

Be careful, when recognising an episode of hypoxia, it can be confused with hyperventilation. Since increasing the breathing rate reduces the diameter of the arteries that supply the brain, and there’s an imbalance between carbon dioxide and oxygen present.

The symptoms of hyperventilation are very similar to those of hypoxia: dizziness, anxiety, disturbed visual capacity, decreased performance, loss of consciousness… But in case of doubt, it’s advisable to apply the protocol for hypoxia, which, as we’ve said, is to apply oxygen and descend below 10,000 feet.

A flight attendant in an hypoxia situation

How to prevent that lack of oxygen?

Prevention depends on the cause:

  • In the context of flights, prevention includes ensuring adequate pressurisation in airplanes and having oxygen supplies for emergency situations. For individuals with pre-existing conditions, medical consultation before the trip is crucial. Moreover, if an aeroplane operates at more than 10,000 feet in height, it must be equipped with oxygen masks.
  • If hypoxia has occurred due to pulmonary diseases or anaemia, it’s vital to follow medical treatment, and use supplemental oxygen if necessary.
  • And if it’s due to carbon monoxide poisoning, we recommend you check stoves, heaters, and also ensure ventilation in those areas.

Aviation is a sector formed by professionals from all fields who combine all their knowledge to make aviation a safe and efficient mode of transport, plus both pilots and cabin crew are trained to recognise symptoms and know how to act in case it occurs.

  • Meaning of hypoxia

Now you know what hypoxia is

We hope this article has helped you to understand in depth what hypoxia is and how it can affect a flight. And always remember that air professionals are prepared to face these episodes should they occur.

Did you enjoy this post? Don’t miss our aviation blog, where you can read this and more facts about travelling by plane. We look forward to seeing you!

You may be interested…