An Overview of Hypercapnia

Understanding its causes, treatment, and association with COPD

In This Article

Alveoli in the lungs.
Dorling Kindersley/Getty Images

Carbon dioxide (CO2) is produced as a normal byproduct of metabolic processes in cells and normally diffuses into the bloodstream to be exhaled from the lungs. There are times, however, when an excess amount can build up—a condition known as hypercapnia, which is also referred to as hypercarbia or carbon dioxide retention (CO2 retention). There are many possible causes of hypercapnia, including conditions that result in the production of more carbon dioxide (such as infections or trauma) and those that result in the reduced exhalation of the gas from the lungs (such as some lung diseases).


Ordinarily, the body carefully regulates the amount of carbon dioxide in the blood. When carbon dioxide levels become elevated, there are special receptors that detect the increase and, in turn, send messages to the brain to breathe more deeply or to move so that airflow is not obstructed (this is why people move at night if their mouth and nose become covered by a blanket).

Due to these regulatory pathways, medically significant hypercapnia in otherwise healthy individuals is rare.


To understand the possible medical causes of hypercapnia, it's helpful to first review the mechanisms that interfere with the body's ability to regulate CO2. They include:

  • Decreased ventilation (hypoventilation)
  • Increased dead space in the lungs (blood supply to the area of the lung is impaired so gases cannot be exchanged properly) such as noted with ventilation-perfusion (VQ) mismatch the lungs
  • Increased production of carbon dioxide by the body, such as with trauma
  • Disruption of the signal that tells the body to breathe more when oxygen levels are low, such as can occur when oxygen is given to chronic obstructive pulmonary disease (COPD) patients
  • Rebreathing exhaled carbon dioxide

Medical Conditions

There are a number of medical conditions that may be responsible for hypercapnia, and some of these may do so through more than one of the mechanisms at play when CO2 builds up in the blood.


Generally speaking, when we breathe, we inhale oxygen and exhale carbon dioxide. These two respiratory gases are exchanged deep within the lungs in tiny, grape-like clusters, or air sacs, called alveoli.

In people with COPD, this process is impaired because the alveoli are destroyed, leaving less surface area for oxygen to get from the lungs into the bloodstream and for carbon dioxide to get from the blood into the lungs to be exhaled. This results in a low amount of oxygen in the blood, a condition called hypoxemia, as well as hypercapnia.

This retention of carbon dioxide in people with COPD is due to the inadequate exchange of gases that results from V/Q mismatch in the lungs. COPD is a major cause of hypercapnia, though not everyone who has COPD—even severe or end-stage disease—will develop this concern.

Neurological Diseases and Overdose

With respiratory depression caused by an overdose of narcotics, or with degenerative neurological conditions, the brain does not respond to commands to increase breathing as it ordinarily would. As a result, CO2 can accumulate in the blood.

Examples of such concerns include:

  • Certain muscle or nervous system disorders like encephalitis, muscular dystrophy, amyotrophic lateral sclerosis (ALS), or myasthenia gravis
  • Drug overdoses such as with an opioid or benzodiazepine
  • Brainstem stroke


While COPD and neurologic issues are notable, there are other possible causes of hypercapnia as well:

Signs and Symptoms

When hypercapnia is mild, many people do not even realize they have the condition, especially when it develops slowly over time. When symptoms of mild hypercapnia occur, they may include:

  • Fatigue 
  • An inability to concentrate or think clearly
  • Headaches
  • Flushing
  • Dizziness
  • Mild dyspnea (shortness of breath) or an elevated respiratory rate (tachypnea)
  • Increased blood pressure

Since these symptoms are all non-specific, having awareness is important so that the condition is not missed.

On the other hand, symptoms of severe hypercapnia are more pronounced. Such cases may eventually lead to respiratory failure and possibly death. Symptoms and signs may include:

  • Paranoia, depression, and confusion, which may progress to coma
  • Muscle twitches
  • Seizures
  • Palpitations
  • Panic or a feeling of impending doom
  • Dilation or widening of superficial veins in the skin
  • Papilledema, swelling of the area surrounding the optic nerves that can be seen on an eye exam (a sign of increased pressure in the brain)


The diagnosis of hypercapnia begins with a careful history and physical exam, combined with having a high index of suspicion that the condition may exist.

The condition is usually diagnosed based on the results of an arterial blood gas, with a CO2 pressure of 45 mm Hg or above being diagnostic.

An increase in carbon dioxide in the blood also lowers the pH of the blood (causes acidosis), a condition referred to as respiratory acidosis. Other tests may be used to look for the underlying cause of hypercapnia.


The treatment of hypercapnia depends on its severity. Supportive care treatment options are often needed while physicians address and treat (when possible) the underlying cause of the condition.

Treating hypercapnia involves restoring ventilation so that carbon dioxide can be released from the body and may include:

  • Noninvasive ventilation: Noninvasive ventilation provides ventilatory support through the upper airways. It enhances the breathing process by giving the patient a mixture of air and oxygen from a flow generator through a tightly fitted facial or nasal mask.
  • Intubation and mechanical ventilation: Intubation is the process of inserting a special tube (an endotracheal tube) through the mouth and then into the airway. The tube then gets hooked up to a mechanical ventilator that takes over active breathing for the patient.

When to Call Your Doctor

Hypercapnia can lead to respiratory failure and death if left untreated. If you have COPD, a major risk factor for the condition, being aware of the symptoms of hypercapnia is key to early detection.

Call your doctor as soon as possible if you experience any change in your symptoms or general health, especially if you notice profound fatigue, a decrease in concentration or confusion, muscle twitches, or palpitations. Fortunately, it's fairly easy for a physician to detect the problem quickly if present.

A Word From Verywell

Talking about hypercapnia can be confusing, but the important take-home points are that people who have lung disease should have a high index of suspicion; seeking medical care if they note any symptoms at all and that urgent medical treatment is needed if carbon dioxide levels are elevated in people with other medical conditions.

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