The medical reasoning behind hypoxia during flight

Published: May 23, 2019

14DAYPILOT

Why does hypoxia occur? How many types of hypoxia are there? To know more about hypoxia we must first know the science behind the transport of oxygen throughout the body.

Let’s step onto the surface of the respiratory system. We all know that our body takes in oxygen and exhales CO2. All of these occurs inside the lungs however, there are more steps involved to this where oxygen is brought to all parts of the body and exchanged with carbon dioxide. The human blood contains haemoglobin (Hb) that carries the oxygen molecules from the lungs to the body tissues around the body such as body organs. The functionality of the body depends on the amount of haemoglobin and the amount of oxygen saturation present.

 

Types of hypoxia

There are 4 types of hypoxia; Hypoxic Hypoxia, Anaemic Hypoxia, Stagnant or Hypokinetic Hypoxia, and Histoxic Hypoxia. Hypoxic Hypoxia, or otherwise known as Altitude Hypoxia, occurs when there is a reduced partial pressure of oxygen in air. This means that there is less oxygen molecules that are breathed in. This could be due to being in higher altitudes or due to some other natural, or unnatural, occurrences.

Anaemic Hypoxia occurs when the body cannot transport as much oxygen to its fullest capability as there may be less haemoglobin present in the body. This sort of problem occurs in those that are anaemic, meaning they have a lower red blood cell count than the norm. This may, however, be caused by poor nutrition, lack of nitrates or sulfates, or by carbon monoxide poisoning. Hb has a higher affinity towards carbon monoxide, hence, Hb will bind with carbon monoxide more preferably compared to oxygen, reducing the amount of oxygen carried around the body. Carbon monoxide poisoning is highly dangerous as it is irreversible, meaning the haemoglobin will forever be bound to the carbon monoxide.

Stagnant or Hypokinetic Hypoxia occurs when there are circulatory system problems or COPD such as heart failure, or may also occur during flights as blood pools in the lower limbs under high g manoeuvres. This would be true when the blood is stuck in a certain area for a certain amount of time.

Histoxic Hypoxia occurs when there is a reduction in the absorption of oxygen by the body tissues due to an external force such as the consumption of alcohol, narcotics or poisons. Alcohol gives a desaturation effect which reduces the amount of oxygen carried by the haemoglobin causing a suffocation effect in the body’s tissues as they are not receiving adequate amounts of oxygen. This may cause further complications where the brain is no longer receiving enough oxygen. All 4 types of hypoxia may be found during flight however, Hypoxic Hypoxia is the most common type of hypoxia as it is correlated to the altitude the person is in.

 

Why is hypoxia found in higher altitudes?

The higher the altitude, the lower the air pressure as atmospheric pressure is inversely proportional to altitude. However, atmospheric pressure is directly proportional to the partial pressure of oxygen (pO2), hence the pO2 is also reduced. Below 10 000 ft, the saturation of haemoglobin with oxygen is only slightly affected and does not show any, or in some cases only little, effect to the functionality of the human body. At altitudes of greater than 10 000 ft., the oxygen levels in the air reduces and so does the oxygen level in the body, reducing rapidly. When the altitude level reaches 20 000 ft., the pO2 in blood is nearly half saturated and the human body’s functionality is deterred.

Such scenarios can be found during flight or during mountain hikes, such as the climbing of Mount. Everest. To counter effects such as these during climbing, one must stay in the lower levels for several months, increasing the altitude after a few months. This allows the body to adapt to the changes in the pO2 present in the air which may be done by increasing lung capacities, producing more red blood cells, producing red blood cells at a faster rate, etc. Although, when it comes to flying, the human body cannot adapt to the changes as it does during hikes as there is no time for the body to adapt. This is why hypoxia occurs at higher altitudes.

But why is hypoxia is not experienced by everyone during flying? Aircrafts are pressurised in order to prevent hypoxia from occurring as hypoxia poses as a dangerous threat to the aircrew and passengers. If hypoxia affects the pilots, the plane may lose control such as the incident of helios airways [link to instagram post]. In these cases, the aircraft may have had a malfunctioned pressurising system, damage in the aircraft’s body structure, etc.


 

Symptoms of Hypoxia

The symptoms of hypoxia varies depending on the person affected due to the range of age, size, fitness of the person. The most common visible symptom of hypoxia are blue lips and/or fingertips. Another type of symptom would be feeling feverish although another person may feel cold instead.

Other signs of hypoxia would be breathlessness, excessive yawning - caused by the body searching for more oxygen, unexplained fatigue, lack of ability in doing tasks, loss of consciousness, difficulty in concentrating, and many more.

With all these in mind, it is good to have had prior understandings to hypoxia as this will definitely help you to spot the “unseen” symptoms more quickly and prevent anything worse from occurring. All aircrew are required to learn about hypoxia as aircrews are the first help of any aircraft passengers that are experiencing hypoxia and to other aircrews.

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