Hypoxemia is a term that describes low levels of oxygen in the blood, specifically within arterial blood. The condition may indicate a problem with blood circulation or breathing and may present with symptoms such as a shortness of breath.

A doctor can measure a person’s blood oxygen level through blood tests or a small device that clips onto the finger. Hypoxemia can occur for a number of reasons, which can involve underlying medical conditions such as chronic obstructive pulmonary disease (COPD), asthma, and anemia.

In this article, we will explore different types and causes of hypoxemia, how it differs from hypoxia, and how a doctor may diagnose and treat the condition.

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Hypoxemia refers to a reduction in the amount of oxygen in the bloodstream. This means that the level of oxygen falls below the normal range and there is not enough present in the blood.

A medical professional can determine the oxygenation of blood through an arterial blood gas analysis and measuring the partial pressure of oxygen (PaO2). A healthy PaO2 value varies between 75–100 millimeters of mercury (mm Hg), and the medical community typically considers an oxygen level below 60 mm Hg as hypoxemia.

While some people may use the terms hypoxemia and hypoxia interchangeably, they are different. Though they both refer to low oxygen levels, they occur in different tissues. Specifically, hypoxemia describes low oxygen levels in the blood, while hypoxia describes low levels of oxygen in the tissues of the body.

While the conditions are different, it is possible for them to occur together. If hypoxemia is present, it may result in hypoxia, as low levels of oxygen in the blood may be unable to supply sufficient oxygen to the body’s tissues. However, it is also possible for one condition to occur without the other.

There are various reasons for hypoxemia to occur. Some of these include:

Ventilation/perfusion mismatch

Ventilation is the air that a person breathes in and out, while perfusion is the circulation of blood. A normal ventilation/perfusion (V/Q) ratio is roughly around 1.

A V/Q mismatch occurs when parts of the lungs receive oxygen but no blood circulation, or they receive blood circulation but no oxygen. Evidence suggests that a V/Q mismatch is the most common cause of hypoxemia.


Hypoventilation occurs when breathing is insufficient for the exchange of oxygen and carbon dioxide in the body. When hypoventilation occurs, there is a high level of carbon dioxide (CO2) and a low level of oxygen in the blood. As a result, hypoxemia is often present, especially during sleep.

A person who is experiencing hypoventilation may have slow and shallow breathing.

Click here to read more about hypoventilation.

Right-to-left shunt

A shunt refers to when blood from the heart’s right side enters the left side without any exchange of oxygen or CO2.

In normal conditions, blood that has no oxygen enters the right side of the heart and travels to the lungs to obtain oxygen. Once it has done so, the blood travels to the left side of the heart to distribute the oxygen to the rest of the body.

Diffusion impairment

Diffusion impairment occurs when there is dysfunction, or limitation, in the oxygen transport between the alveoli and the capillaries. The alveoli are sacs in the lungs where gas exchange takes place and capillaries are the blood vessels that oxygen diffuses into.

Low PaO2

A low environmental PaO2 may result in hypoxemia. For example, the partial pressure of oxygen is lower at higher altitudes. This means that a person accustomed to breathing at sea level may experience hypoxemia at higher altitudes, as the amount of oxygen available is lower.

Symptoms of hypoxemia can include:

  • shortness of breath
  • irritability
  • confusion
  • drowsiness
  • rapid or irregular heartbeat
  • rapid and shallow breathing
  • blue or gray discoloration of the skin

A person should seek the advice of a medical professional for an accurate diagnosis of hypoxemia. A doctor will first carry out a physical exam, which may include looking at the skin and nails for color changes and listening to the heart and lungs.

The doctor may then carry out additional tests to determine the amount of oxygen in the blood. This may include:

  • Arterial blood gas test: This test involves drawing blood, then analyzing the sample in a lab to determine the level of oxygen and carbon dioxide present.
  • Pulse oximetry: A pulse oximeter is a device that measures how saturated the blood is with oxygen. The device normally attaches to the fingers, ears, or toes. The device shines a light through the skin and analyzes it to establish the percentage of oxygen in the blood.
  • Lung function tests: These are tests that analyze how well the lungs work. An example of a lung function test is spirometry. This involves blowing into a tube known as a spirometer to determine how much air a person can breathe in and out of their lungs.

As hypoxemia involves a reduction in blood oxygen levels, the aim of treatment is to restore the level of oxygen to a healthy level.

To do so, a person may receive oxygen therapy. This is a treatment option that provides supplemental oxygen that they can receive in a hospital or at home. There are a variety of different ways that a person may receive oxygen therapy, such as through a face mask or a tube that fits inside the nose, known as a nasal cannula.

If a person is experiencing an underlying condition that is causing hypoxemia, such as COPD or asthma, then additional treatment may be necessary to treat these conditions.

Untreated hypoxemia may lead to other complications in the body, as without adequate oxygen in the blood, the body cannot optimally function. Complications may include:

  • brain damage and brain death
  • heart problems
  • lung complications
  • kidney problems

People can try a few lifestyle changes to help reduce the likelihood of hypoxemia or prevent conditions that could result in hypoxemia. This can include:

  • quitting smoking, as this influences ventilation
  • getting regular exercise
  • avoiding potential pollutants
  • preventing respiratory infections by receiving vaccines and wearing face coverings
  • practicing breathing exercises
  • taking medication as prescribed for underlying conditions that may cause hypoxemia
  • using supplemental oxygen if necessary

A person should seek the advice of a doctor if they experience symptoms of hypoxemia, such as heartbeat or breathing irregularities and discoloration of the skin. Early detection and treatment may help prevent the condition from progressing and causing more serious complications.

Hypoxemia describes low levels of oxygen in the bloodstream. This is different from hypoxia, which is a low level of oxygen in the body’s tissues. Hypoxemia may occur for a number of reasons, such as a right-to-left shunt, a V/Q mismatch, low PaO2, diffusion impairment, and hypoventilation. Additionally, conditions such as COPD may also result in hypoxemia.

Symptoms of hypoxemia can include shortness of breath, confusion, a fast heartbeat, and drowsiness. A doctor may diagnose hypoxemia through a physical examination as well as tests such as an arterial blood gas analysis.

A person should contact their doctor as soon as possible if they experience symptoms of hypoxemia, as untreated hypoxemia can lead to further complications on the major organs, such as the brain and the heart.