Overview and Types of Hypoxia

Elderly woman breathing through oxygen mask during home therapy.
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You may hear the term hypoxia when your doctor discusses your medical condition. What exactly is hypoxia, what are the symptoms and signs, and what are the possible causes?


Hypoxia literally means "low oxygen," but is defined as a deficiency in the amount of oxygen that reaches the tissues of the body. It differs from hypoxemia, which means an inadequate amount of oxygen traveling in the blood.

Hypoxia may be due to hypoxemia, for example, if an inadequate amount of oxygen reaches the tissues because there is an inadequate amount of oxygen in the blood, or it may be due to other causes.

The lack of oxygen in tissues is also known as "oxygen starvation." If a complete lack of oxygen occurs in the tissues, it is termed anoxia.

Hypoxia may affect the whole body (generalized hypoxia), or a specific organ or area of the body (tissue hypoxia). It can also be classified as acute or chronic, with acute meaning a rapid onset, and chronic meaning that hypoxia has been ongoing for some time.


As noted above, there are different types of hypoxia or reasons that there is not enough oxygen in the tissues of the body. These include:

  • Hypoxic hypoxia (hypoxemic hypoxia): In this type of hypoxia, the tissues do not have enough oxygen because there is a lack of oxygen in the blood flowing to the tissues. Hypoxic hypoxia can be caused by inadequate breathing as well as other causes.
  • Anemic hypoxia: In the setting of anemia, low hemoglobin levels result in a reduced ability of the blood to carry oxygen that is breathed in, and hence, a diminished supply of oxygen available to the tissues. Anemia can, in turn, be caused by many conditions.
  • Stagnant hypoxia (circulatory hypoxia): This form of hypoxia is caused by inadequate blood flow, which results in less oxygen available to the tissues.
  • Histiotoxic hypoxia: With histiotoxic hypoxia, an adequate amount of oxygen is inhaled through the lungs and delivered to the tissues, but the tissues are unable to use the oxygen that is present.
  • Metabolic hypoxia: Metabolic hypoxia occurs when there is more demand for oxygen by the tissues than usual. Oxygen may be absorbed, transported, and used properly by the tissues, but due to a condition that raises metabolism, it is still not enough. An example of this is sepsis (a serious and overwhelming infection).


    The signs and symptoms of hypoxia can vary between different people, and by how long the symptoms have been present. Some of them include:

    • Dizziness or fainting (syncope)
    • Shortness of breath (dyspnea)
    • Confusion, lethargy, and/or lack of judgment
    • Headaches
    • Rapid heart rate (tachycardia)
    • Elevated respiratory rate (tachypnea)
    • Euphoria and a sense of well-being
    • Tingling, warm sensations
    • Elevated blood pressure (hypertension)
    • Lack of coordination
    • Visual changes, such as tunnel vision
    • Elevated red blood cell count (polycythemia) in people with chronic hypoxia
    • A bluish tinge to the lips and extremities (cyanosis)


    The organs most affected by hypoxia are the brain, the heart, and the liver. If the hypoxia is severe, irreversible damage can begin within four minutes of the onset. Coma, seizures, and death may occur in severe cases. Chronic, milder hypoxia can also cause damage to the major organs of the body.

    When hypoxia is acute, symptoms often include motor incoordination and impaired judgment. Due to these symptoms, a person with hypoxia is sometimes erroneously thought to be intoxicated with alcohol.

    Chronic hypoxia tends to have different symptoms, such as fatigue, apathy, a delayed reaction time, or reduced work capacity.

    Causes of Hypoxia

    There are different causes of hypoxia depending on the mechanism by which lower amounts of tissue of the body. These causes can span the spectrum from problems at the level of the heart to non-medical reasons such as traveling to a region where the altitude is higher than at home.

    • Altitude sickness: The FAA recommends supplemental oxygen for flights over 10,000 feet and over 6,000 feet at night (because a pilot's vision is very sensitive to low oxygen).
    • Lung conditions: Inadequate air exchange in the lungs due to illnesses such as COPD, asthma, lung cancer, pneumonia, rheumatoid lung disease, and pulmonary hypertension.
    • Hypoventilation: Hypoventilation simply means “not breathing enough.” This can be “central” if the brain doesn’t tell the lungs to breathe, such as when under anesthesia, due to a stroke or head injury, as a side effect of medications for pain, or as a reaction to illegal drugs. It can be “peripheral” due to an airway obstruction that interferes with breathing, such as choking (when a foreign body becomes lodged in the trachea or large airways of the lungs), drowning, suffocation, or in a cardiac arrest when breathing stops. Hypoventilation is also present in obstructive (COPD, asthma, cystic fibrosis, bronchiectasis) and restrictive (pulmonary fibrosis including rheumatologic causes, scarring) lung conditions.

      Causes of Anemic Hypoxia

      In the setting of anemia, low hemoglobin levels result in a reduced ability of the blood to carry oxygen that is breathed in, and hence, a diminished supply of oxygen available to the tissues. Causes include:

      • Anemia of any cause: This can include iron deficiency anemia, pernicious anemia, and chemotherapy-induced anemia.
      • Hemorrhage: Hemorrhage can be obvious, such as from injuries sustained in an accident, or hidden due to internal bleeding.
      • Methemoglobinemia: Methemoglobinemia, also known as affinity hypoxia, is an abnormal hemoglobin that doesn't bind oxygen very well.
      • Carbon monoxide poisoning: With Carbon monoxide poisoning, hemoglobin is unable to bind oxygen.

      Causes of Circulatory/Stagnant Hypoxia

      This form of hypoxia is caused by inadequate blood flow, which results in less oxygen available to the tissues. Causes include:

      • Edema: Edema, a swelling of the tissues (like from heart failure), can limit the ability of oxygen present in the blood to adequately reach the tissues.
      • Ischemic hypoxia: Obstruction to the flow of blood carrying oxygen, like from a clot in a coronary artery (a heart attack), can prevent the tissues from receiving oxygen.

      Causes of Histiotoxic Hypoxia

      With histiotoxic hypoxia, an adequate amount of oxygen is inhaled through the lungs and delivered to the tissues, but the tissues are unable to use the oxygen that is present. Cyanide poisoning is a possible cause.

      Tests Your Doctor May Order

      There are many different tests your doctor may order to get a better grasp on why you are experiencing symptoms of low oxygen (a low level of oxygen in your tissues). Even if the cause of your hypoxia is known, laboratory and radiological studies can be used to get to the ultimate cause of your symptoms. Some tests that may be done if you have hypoxia include:

      • Oximetry (with a pulse oximeter) to monitor the oxygen level in your blood.
      • Arterial blood gases to evaluate your respiratory and metabolic status.
      • Blood tests: Lab tests that are drawn may include a complete blood count (CBC) to look for anemia (low red blood cell count) or signs of infection.
      • EKG to look for any signs of heart damage, or an irregular heartbeat.
      • X-ray or a CT of your chest to look for lung diseases, a pneumothorax, or infection.
      • Bronchoscopy to look for a foreign body or another cause of obstruction in the airways, such as a tumor.
      • CT or MRI of your head to look for brain abnormalities that could suppress breathing such as tumors, bleeds, or strokes.
      • Echocardiogram in order to observe the motion of the heart and look for damage or abnormalities in the heart or heart valves.


      The treatment of hypoxia will depend upon the underlying cause. As noted earlier, there is a wide range of conditions and mechanisms that affect those conditions, between different people. The "end organ" manifestations often differ as well.

      While you and your doctor are working together to determine the cause, she may recommend oxygen therapy if you are short of breath or having other symptoms suggestive of moderate or severe hypoxia. If your symptoms are severe, mechanical ventilation with a ventilator may be needed.

      Hyperbaric oxygen treatment is sometimes used for severe tissue hypoxia. The increased levels of oxygen provided under pressure can sometimes improve tissue perfusion in a way that is otherwise not possible.

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      Article Sources

      • Kasper, Dennis L.., Anthony S. Fauci, and Stephen L.. Hauser. Harrison's Principles of Internal Medicine. New York: Mc Graw Hill education, 2015. Print.