Your blood carries oxygen to the organs and tissues of your body. Hypoxemia is when you have low levels of oxygen in your blood.

Hypoxemia can be caused by a variety of conditions, including asthma, pneumonia, and chronic obstructive pulmonary disease (COPD). It’s a serious medical situation and requires prompt medical attention.

Hypoxia vs. hypoxemia

Hypoxia and hypoxemia refer to two different things. While hypoxemia refers to low oxygen levels in your blood, hypoxia refers to low levels of oxygen in the tissues of your body.

The two can sometimes, but not always, occur together.

Generally, the presence of hypoxemia suggests hypoxia. This makes sense because if oxygen levels are low in your blood, the tissues of your body are also probably not getting enough oxygen either.


There are several different types of hypoxemia, and the type depends on the mechanism through which blood oxygen levels are lowered.

Ventilation/perfusion (V/Q) mismatch

This is the most common type of hypoxemia. Ventilation refers to the oxygen supply in the lungs, while perfusion refers to the blood supply to the lungs.

Ventilation and perfusion are measured in a ratio, called V/Q ratio. Normally, there’s a small degree of mismatch in this ratio, however if the mismatch becomes too great, problems can occur.

There are two causes of ventilation perfusion mismatch:

  • The lungs are getting enough oxygen, but there’s not enough blood flow (increased V/Q ratio).
  • There is blood flow to the lungs, but not enough oxygen (decreased V/Q ratio).


Normally, deoxygenated blood enters the right side of the heart, travels to the lungs to receive oxygen, and then travels to the left side of the heart to be distributed to the rest of the body.

In this type of hypoxemia, blood enters the left side of the heart without becoming oxygenated in the lungs.

Diffusion impairment

When oxygen enters the lungs, it fills small sacs called alveoli. Tiny blood vessels called capillaries surround the alveoli. Oxygen diffuses from the alveoli into the blood running through the capillaries.

In this type of hypoxemia, the diffusion of oxygen into the bloodstream is impaired.


Hypoventilation is when oxygen intake occurs at a slow rate. This can result in higher levels of carbon dioxide in the blood and lower levels of oxygen.

Low environmental oxygen

This type of hypoxemia typically occurs at higher altitudes. Available oxygen in the air decreases with increasing altitude.

Therefore, at higher altitudes each breath provides you with lower levels of oxygen than when you’re at sea level.


There are many conditions that can cause hypoxemia. These can include:

  • acute respiratory distress syndrome (ARDS)
  • anemia
  • asthma
  • a blood clot in the lung (pulmonary embolism)
  • a collapsed lung
  • congenital heart defects or disease
  • COPD
  • fluid in the lung (pulmonary edema)
  • high altitudes
  • interstitial lung disease
  • medications that lower breathing rate, such as some narcotics and anesthetics
  • pneumonia
  • scarring in the lungs (pulmonary fibrosis)
  • sleep apnea

Different conditions can cause hypoxemia in different ways. Let’s look at a few examples:

  • COPD is a chronic condition in which the flow of air in the lungs is obstructed. Destruction of the walls of alveoli and surrounding capillaries in COPD can lead to problems with oxygen exchange, which can lead to hypoxemia.
  • Anemia is a condition in which there aren’t enough red blood cells to effectively carry oxygen. Because of this, a person with anemia may have low levels of oxygen in their blood.

Additionally, hypoxemia can be a symptom of another condition such as respiratory failure.

Respiratory failure occurs when not enough oxygen passes from your lungs to your blood. Therefore, low blood oxygen levels can be an indicator of respiratory failure.


Someone with hypoxemia may experience the following symptoms:

  • shortness of breath
  • coughing or wheezing
  • headache
  • rapid heartbeat
  • feeling confused or disoriented
  • a blue coloration to the skin, lips, and fingernails


In order to diagnose hypoxemia, your doctor will perform a physical examination during which they’ll check your heart and lungs. They may also check the color of your skin, fingernails, or lips.

There are some additional tests that they can perform to assess your oxygen levels and breathing. These can include:

  • Pulse oximetry, which uses a sensor placed on your finger to measure blood oxygen levels.
  • Arterial blood gas test, which uses a needle to draw a blood sample from an artery to measure blood oxygen levels.
  • Breathing tests, which may evaluate your breathing through a machine or by breathing into a tube.


Since hypoxemia involves low blood oxygen levels, the aim of treatment is to try to raise blood oxygen levels back to normal.

Oxygen therapy can be utilized to treat hypoxemia. This may involve using an oxygen mask or a small tube clipped to your nose to receive supplemental oxygen.

Hypoxemia can also be caused by an underlying condition such as asthma or pneumonia. If an underlying condition is causing your hypoxemia, your doctor will work to treat that condition as well.


The organs and tissues of your body require oxygen in order to function properly.

Damage can occur to vital organs such as the heart and brain in the absence of enough oxygen. Hypoxemia can be fatal if it goes untreated.

When to see a doctor

You should always seek emergency medical attention if shortness of breath appears suddenly and impacts your ability to function.

In some other cases, shortness of breath on its own can still warrant a doctor’s visit. If you experience any of the following symptoms, you should be sure to make an appointment with your doctor to discuss them:

  • shortness of breath that occurs with minimal activity or when you’re at rest
  • shortness of breath that occurs with exercise and gets worse
  • waking suddenly from sleep with shortness of breath

The bottom line

Hypoxemia is when you have low levels of oxygen in your blood. There are several different types of hypoxemia and many different conditions can cause it.

Hypoxemia is a serious condition and can lead to organ damage or even death if left untreated.

You should always seek emergency medical care if you have shortness of breath that occurs suddenly and affects your ability to function.


  • Abdo WF, et al. (2012). Oxygen-induced hypercapnia in COPD: Myths and facts. DOI:
  • Dalbak LG, et al. (2015). Should pulse oximetry be included in GPs’ assessment of patients with obstructive lung disease? DOI:
  • Fahy B, et al. (2018). Pulse oximetry.
  • Mayo Clinic Staff. (2018). Hypoxemia (low blood oxygen): Definition.
  • Pluddemann A. (2011). Pulse oximetry in primary care: Primary care diagnostic technology update. DOI:
  • Understanding pulse oximetry: SpIO2 concepts. (2013).
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