What Is Hypoxia

Oxygen is a vital element of our lives and without it, we as humans can only survive for a few minutes. When oxygen delivery is affected, organ function will gradually worsen and can potentially become life-threatening, with severe cases leading to coma and even death.¹ There is, however, a number of presentations, ranging from acute to chronic, which will be further discussed in this article.

Hypoxia is a state in which not enough oxygen can reach the human body’s tissues to maintain a balance that all the body systems need to survive and function correctly. There should be a balance between the body’s need for oxygen and available supplies of oxygen in order to maintain an even balance within the body.¹

The two main systems responsible for delivering oxygen to tissues are the cardiovascular and the respiratory systems, with their many components. If either of these two fail to function normally, the end result will be low oxygen levels in your blood and, subsequently, in your tissues.^1,2 This article will answer some common questions on the topic of hypoxia, providing an overview of types, causes, and management.

Overview

There are various levels of hypoxia, ranging from mild to severe, and it can present in either acute or chronic forms or a mix between the two. The body’s response to hypoxia varies, with some tissues being able to tolerate hypoxia for a longer time, while others can be severely damaged by low oxygen levels.^1,2

The causes of hypoxia can be split into; low blood flow to the tissues; or low oxygen content in the blood which is supplied to the tissues, the latter type also being known as hypoxaemia.³

For a better understanding of the mechanism of hypoxia, it is important to know that oxygen is carried in the blood by a molecule called haemoglobin, and for it to arrive at the haemoglobin, there is a direct interaction between the air in the lungs and the small blood vessels lining the lungs. This process can be completed at one of the following three points: 

• Perfusion - blood flow to the lungs
• Ventilation - airflow to the lungs
• Diffusion - gas exchange through the lungs’ tissue²

Types of hypoxia

There are several types of hypoxia, and they can be classified into the following:

Hypoxaemic hypoxia

In this particular type of hypoxia, the oxygen pressure in the blood reaching the tissues is not enough. This can either happen from a decreased quantity of oxygen in the air you inhale (for example, with people living at high altitude), or when there is failure of the heart and lungs, resulting in the lungs not being able to efficiently transfer oxygen from their tissues to the blood.

Anaemic hypoxia 

With this type of hypoxia, the amount of functional haemoglobin in the body is too low, hence the blood’s capacity to safely carry oxygen in satisfactory amounts is limited. This can happen if there is severe blood loss, or with certain conditions such as carbon monoxide poisoning.

Stagnant hypoxia

In this case, the blood may be normal, however the flow to the tissues is disrupted. This is usually the end result of conditions of the heart, where the heart muscle is affected and unable to supply tissues with the needed amount of blood and, subsequently, oxygen.

Histotoxic hypoxia

Finally, in histotoxic hypoxia, the body is unable to use oxygen safely, even though the amount in the blood may be normal. This is usually caused by agents which affect the tissue cells, making them unable to use oxygen properly. Agents causing this include narcotics, alcohol, acetone, and some anaesthetic agents.

Causes of hypoxia

As mentioned at the beginning of this article, hypoxia is caused either by low blood flow to the tissues, or low oxygen content in the blood. We will now explore the causes of hypoxia in further detail:

Hypoventilation

• Due to airway obstruction (for example in the case of foreign body inhalation, or laryngeal oedema, or certain airway diseases such as chronic obstructive pulmonary disease)
• Decreased respiratory drive - in the case of deep sleep or coma
• Reduced movement of the chest wall as in obesity hypoventilation syndrome or in massive ascites, or in some diseases of the spinal cord which will compress the lungs (ankylosing spondylitis)
• Neuromuscular diseases such as amyotrophic lateral sclerosis (ALS) or myasthenia gravis

Ventilation perfusion mismatch (also known as V/Q mismatch)

• A decreased V/Q mismatch, also known as high perfusion, can be found in certain diseases such as chronic bronchitis, mucus plugging and pulmonary oedema. These impair the ventilation, thus leading to a decrease in the ratio of ventilation to perfusion
• An increased V/Q ratio, also known as impaired perfusion, can be found in pulmonary embolism or in increased ventilation, such as the case of emphysema where the surface area for gas exchange is decreased, thus causing increased ventilation when compared to perfusion

Right to left shunt

• This denomination signifies the state where blood crosses from the right to the left side of the heart without receiving oxygen, which can be found in conditions such as pulmonary arteriovenous malformations, fistulas, hepato-pulmonary syndrome and others.
• In some cases, the shunting can be physiological, such as in pneumonia for example, blood passing through non-ventilated tissue at the lung level. In this case, this is caused primarily by inflammation.

Impaired oxygen diffusion

• This usually happens when the oxygen diffusion is impaired locally, at the lung level, between the lung and the pulmonary blood vessels. Causes of this would include oedema, inflammation or fibrosis.^1,2

Signs and symptoms of hypoxia

As mentioned in the introduction, hypoxia can be acute or chronic. Acutely, this will present with shortness of breath and an increased rate of breathing. Depending on how severe the hypoxia is, the symptoms will also be more or less severe. One can also experience an increased heart rate, as the heart is trying to compensate for the low oxygen supply by providing more oxygen to the tissues.

Signs and symptoms also vary with the presentation of the individual. In the case of upper airway obstruction, one can experience stridor (noisy breathing due to obstructed airway). Cyanotic skin (blue to purple skin colour) can also indicate severe hypoxia.

Once oxygen delivery is very compromised, organ function will gradually worsen, which can be seen through symptoms including restlessness, confusion, and headaches, leading to coma and, if not treated correctly, death.

When one presents with hypoxia to a doctor, symptoms usually include shortness of breath on mobility.¹ Depending on the underlying condition which causes hypoxia, one might present with a productive cough and fever in the case of a lung infection or leg oedema in the case of heart failure. 

A specialist doctor will usually listen to the patient’s lungs with a stethoscope if they present with symptoms of hypoxia, as this again provides very useful information for the clinician.^1,2

Management and treatment for hypoxia

The management and treatment of hypoxia can be divided into three categories: maintaining patent airways, increasing the oxygen levels in the air we inspire, and improving the capacity of diffusion.¹

Maintaining patent airways

This is done by a trained clinician and aims to ensure patency of the upper airways (i.e., not blocked), usually by using manoeuvres, which prevent the throat from closing and stopping one from performing the act of breathing. 

Some chronic conditions which cause hypoxia, such as obesity hyperventilating syndrome, require the use of specialised equipment in the form of small machinery, which includes a breathing mask. This machine will help achieve the necessary pressure for ventilation and is called CPAP or BiPAP.

Medication or chest physiotherapy are also often used to maintain the patency of the airways.¹

Increasing the oxygen levels in the air 

This is usually done by using an external supply of oxygen, which is given to the person in need via nasal cannulae or oxygen masks.

Improving the capacity of diffusion

With this option, the idea is to treat the underlying cause of hypoxia, for example:

• Diuretic medication (medication which removes excess fluid from the body) can be used in pulmonary oedema
• Steroid medication can be used in certain inflammatory diseases of the lungs^1,2

Diagnosis

In order to diagnose hypoxia, a useful tool which clinicians use is pulse oximetry. Using this allows the evaluation of arterial oxygen saturation (SaO2), i.e., the amount of oxygen in your blood. 

The arterial oxygen saturation signifies the quantity of oxygen connected to haemoglobin in the arterial blood, and this quantity is given as a percentage. When we rest, SaO2 under 95% is considered abnormal. With exercise, a drop in oxygen saturation of more than 5% is also considered abnormal. However, one should always look at these values with clinical signs and symptoms.^4,5,6

Another useful tool in the evaluation of hypoxia is arterial blood gas, which can help indicate the cause of hypoxia. Clinicians also use imaging techniques when diagnosing hypoxia, which are usually imaging studies of the chest, such as X-rays or CT scans, which will help point towards the likely cause of hypoxia. CT scans can give detailed images when compared to X-rays; however, the latter are less costly.

FAQs

How can I prevent hypoxia?

The best way one can prevent hypoxia is to prevent underlying conditions which can lead to low oxygen levels, such as conditions of the lungs or heart. If you are aware of having any lung or heart conditions, talk to your doctor about hypoxia prevention.

How common is hypoxia?

Hypoxia has variable prevalence; however, it is a common disorder with many potential causes. Some of these are very common, such as pneumonia, while others are rarer, such as hypoxia caused by cyanide poisoning. All in all, the prevalence of this condition can vary greatly.

Who is at risk of hypoxia?

Most conditions which reduce the oxygen levels in your blood can cause hypoxia. Such conditions include heart or lung problems, such as COPD or asthma. Some infections can cause hypoxia as well, such as pneumonia or COVID-19.

What can I expect if I have hypoxia?

Hypoxia means low oxygen levels in your body. Symptoms you may experience include difficulty breathing, increased heart rate, blue to purple skin colour, and you may even show signs such as restlessness and confusion.

When should I see a doctor?

Hypoxia can become life-threatening very quickly if the cause is not well under control. If you experience any symptoms of hypoxia, such as difficulty breathing, confusion, rapid heart rate or restlessness, seek advice from your treating physician as soon as possible.

Summary

With oxygen being a vital element of life, a lack of it, in the case of hypoxia, can cause a very rapid worsening of one’s health that has the potential to become life-threatening. When oxygen delivery is affected, organ function starts to gradually worsen. As with most things in life, maintaining a balance is vital, and in this case, it is a balance between available supplies of oxygen and the human body’s oxygen needs. As there are many health conditions which can cause the oxygen tissue supply to be insufficient, it is good practice to seek advice from your physician if you experience any symptoms of hypoxia.