Introduction

Have you heard of “hypoxia” and “hypoxemia” and thought they were synonyms? Even though they might sound similar and they may often be interchanged while speaking, they are not the same thing. 

Hypoxia is defined as low levels of oxygen in the body’s tissues, while hypoxemia is when there is not enough oxygen in your blood. The latter can lead to the former and they can co-occur, but it is not always the case, as one can also happen without the other.^1,2 

This article aims to delve deeper into explaining hypoxia, and hypoxemia, their causes, symptoms and other information of interest. So keep reading to learn more!

Hypoxia

When the tissues in your body do not receive enough oxygen, that is known as hypoxia. When you inhale, oxygen enters your nose or mouth and travels to your lungs. Once there, it enters the air sacs (alveoli) where the gas exchange takes place. Then the blood vessels take the oxygen and carry it to your organs, and the carbon dioxide, a waste gas, moves from your blood to your lungs and exits your system when you exhale (breathe out). Therefore, if your tissues do not get an appropriate quantity of oxygen, you will become hypoxic.¹

This issue is quite common and is often seen if you have an underlying condition. If you suffer from it, you need to seek medical help immediately, and usually, it goes away once the health problem causing it is treated. 

Causes

Hypoxia can happen because of an underlying condition that affects blood circulation (blood does not flow properly to your tissues) or your breathing (decreased levels of oxygen in the blood - hypoxemia). The conditions that might lead to this include:¹

• Hypoventilation
• Laryngeal oedema
• Foreign body inhalation
• Bronchial asthma
• Chronic obstructive pulmonary disease (COPD)
• Obesity hypoventilation syndrome (OHS)
• Neuromuscular diseases
• Chronic bronchitis
• Pulmonary embolism
• Heart shunt
• Pneumonia
• Fluid accumulation in the lungs (oedema)
• Fibrosis
• Deep sedation or coma
• Cyanide poisoning
• High altitude

Types of hypoxia

Hypoxia can be differentiated into four groups:^1,3 

• Hypoxemic hypoxia: this is the most common type, and it happens because you have low oxygen levels in your blood (hypoxemia). This often occurs if you have hypoventilation, heart shunting and also if you’re at high altitudes
• Anaemic hypoxia: this happens because you do not have enough red blood cells in your body that can deliver oxygen to your tissues. You can develop anaemia if your body does not produce enough healthy red blood cells
• Circulatory hypoxia: this occurs despite you having enough oxygen because your heart is not able to pump the blood properly, into the organs. Vessel blockage or blood clots could be a risk factor as well
• Histotoxic hypoxia: in this case, you might get the needed oxygen into your system but for some reason, your cells are unable to use it effectively. This often occurs when you have cyanide poisoning 

Symptoms

Hypoxia might present itself in an acute or chronic form. Alongside its underlying causes, the form will determine what symptoms you might experience. In addition, the severity of this health problem will also result in more serious symptoms. Some manifestations you could feel include:¹

• Shortness of breath
• Rapid and shallow breathing
• Restlessness 
• Headache
• Confusion

In severe cases, you might notice:

• Blue or grey skin or lips (cyanosis)
• Tachycardia
• Altered mental status
• Coma

You may also present other symptoms that may appear as a consequence of the underlying health problem, such as:¹

• Wet cough 
• Fever
• Chest pain
• Leg swelling and oedema

Diagnosis

To diagnose hypoxia, your general practitioner (GP) will conduct a physical examination, auscultating your heart and lungs. Furthermore, they may also order certain medical tests to find the underlying cause of this issue, including:¹

• Pulse oximetry: a noninvasive and painless procedure that measures your blood oxygen levels (oxygen saturation), with a small device attached to your finger. There is also a version of this test which gets the needed data while you sleep, called the nocturnal oximetry test
• Arterial blood gas (ABG) test: measures your blood oxygen and carbon dioxide levels. This test is done by extracting a sample from an artery
• Imaging: different imaging studies like chest X-rays, CT scans or VQ scans might be performed to determine the underlying cause of hypoxia
• Pulmonary function test (PFT): this procedure, where you breathe into a tube connected to a machine, informs you how well your lungs work
• Six-minute walk test: this test evaluates your heart and lung function while you are on a six-minute self-paced walk
• Haemoglobin test: this blood test measures your levels of haemoglobin 

Treatment

Your treatment will depend on the underlying condition that is causing hypoxia. However, the management of this health issue mainly focuses on maintaining patent airways (the ability of the airway to remain open and allow appropriate airflow), increasing the oxygen levels of the inhaled air and improving the diffusion capacity of the lungs. Some measures taken to achieve this are:¹

• Positive pressure ventilation like CPAP or BiPAP
• Bronchodilators 
• Aggressive pulmonary hygiene
• Diuretics for pulmonary oedema
• Steroids 

Hypoxemia

When you have lower levels of oxygen in the blood than normal, that is known as hypoxemia. Normally, you should have an oxygen saturation of 95%, and anything below that is considered abnormal.⁴ So if you cannot get enough oxygen when you breathe, or the gas cannot reach your blood, you’re hypoxemic.

Hypoxemia may eventually lead to hypoxia, but it is not always the case. You can be hypoxemic without being hypoxic, and vice versa.

Causes

Hypoxemia occurs due to underlying health issues that cause your blood oxygen to be lower than it should be. The diseases that can cause hypoxemia are:²

• Asthma
• Chronic obstructive pulmonary disease (COPD)
• Bronchiectasis
• Cystic fibrosis 
• Interstitial lung disease (ILD)
• Pulmonary hypertension
• Pulmonary embolism
• Sleep apnea
• Acute respiratory distress syndrome (ARDS)
• Bronchoconstriction
• Idiopathic pulmonary fibrosis
• Heart shunt 

Furthermore, there are five main causes or mechanisms of hypoxemia:²

• Ventilation/perfusion (V/Q) mismatch: this is the most common cause. You need ventilation (airflow) and perfusion (blood flow) in your lungs for oxygen to get to your blood. If this is mismatched, it means that your lung either receives blood flow without oxygen or oxygen without blood
• Right-to-left shunt: normally, deoxygenated blood is sent from your right side to your left side, where it becomes oxygenated and flows to the rest of your body. In hypoxemia, the blood does not get the oxygen
• Diffusion impairment: sometimes, despite having adequate levels of oxygen, it is difficult for the gas to diffuse from the lungs into your blood vessels 
• Hypoventilation: this is when not enough oxygen enters your lungs
• Low-inspired oxygen: when there is not enough oxygen in the area around you to breathe in, like at high altitudes, your blood cannot be oxygenated 

Symptoms

The lack of oxygen can seriously impair how your body functions. The symptoms you present will depend on the severity of this condition, as well as the underlying pathology. Some signs that you may notice are:⁴

• Shortness of breath or difficulty breathing
• Rapid breathing
• Chest pain
• Coughing
• Wheezing
• Fatigue
• Headaches
• Confusion
• Blue tongue, lips or skin (cyanosis - only appears when oxygen saturation is at 67%)

Diagnosis

Your general practitioner will perform a medical examination, looking for abnormalities in your heart or lungs as these could be a sign of decreased oxygen in your blood. However, your medical provider might perform some additional tests as some signs might not be visible during the physical exams. These are:⁴

• Pulse oximetry test
• Arterial blood gas (ABG) test
• Six-minute walk test

Treatment

It is extremely important to treat hypoxemia as the lack of oxygen could lead to adverse effects on individual organs like the brain, the heart or the kidneys. Treatment will focus on targeting the co-morbidities causing hypoxaemia, as well as increasing oxygen quantity in the blood. Some treatments include:⁴

• Supplemental oxygen 
• Bronchodilators
• Medications like diuretics
• Pulmonary rehabilitation

Summary

Hypoxia and hypoxemia are two terms that are often used interchangeably, but they are not the same condition. Hypoxia is when your tissues do not receive enough oxygen, whereas hypoxemia is when your blood oxygen levels are lower than they should be, which is below 95%. Despite hypoxia developing from hypoxemia, they do not always present together, so you can be hypoxic while not being hypoxemic and the other way around.

Both of these conditions often occur due to underlying health issues that tend to be associated with the cardiovascular and respiratory systems. Treatment of these conditions is focused on targeting the co-morbidities and increasing the oxygen levels in your tissues and blood. Treating hypoxia and hypoxemia is extremely important, as the lack of oxygen can have negative consequences on your body and how it functions. Please refer to your GP or healthcare provider for extra information.