What Is Hypercapnia?

Causes of Elevated Carbon Dioxide Level in the Blood

man wearing an oxygen mask who may have hypercapnia
What is hypercapnia, what are the mechanisms that cause this symptom, what are the causes, and how is it treated?. ERproductions Ltd

Hypercapnia refers to an increased amount of carbon dioxide, the waste product of respiration, in the blood.  An elevated carbon dioxide level is defined as a PaCO2 level of 45 mm Hg or greater, with levels over 75 mm Hg being very severe.

What are the mechanisms that cause hypercapnia and what are some of the conditions in which this occur?

Understanding Carbon Dioxide Levels of the Blood

Carbon dioxide is produced as a waste product of metabolic processes in the cells.


To understand this it can help to understand one of the primary reactions that takes place in cell metabolism.

Cells take in oxygen and sugar as the "reactants" on the left side of this chemical reaction.  The reaction results in "products" including energy (to keep the cells functioning), water, and carbon dioxide.  The reaction simplistically looks like this:

Oxygen + Sugar (glucose) = Energy (ATP) + Water + Carbon Dioxide

Carbon dioxide is removed from the body during respiration.

Carbon dioxide that is produced in this reaction then diffuses into the bloodstream.  It is carried to the capillaries in the lungs, at which time it diffuses into the alveoli (the smallest units of the lungs through which gas exchange takes place) and is exhaled from the body through progressively larger airways and then out of the mouth or nose.

Mechanisms of Hypercapnia - An Elevated Carbon Dioxide Level in the Blood

There are several mechanisms by which someone may end up with an elevated carbon dioxide level in their blood.

  These include:

  • Hypoventilation - If someone is not breathing often enough, or breaths that are taken are too shallow, carbon dioxide can build up in the blood.  This can result from many conditions, ranging from a decreased level of consciousness to muscular diseases which interfere with respiratory muscles.
  • Increased dead space - This occurs when areas of the lung are ventilated that aren't able to participate in the exchange of oxygen and carbon dioxide, such as obstructive lung diseases like chronic bronchitis, emphysema, and bronchiectasis.
  • Rebreathing - If someone breathes in a mixture of gases with an elevated carbon dioxide content.  This type of mechanism is primarily a problem with anesthesia and some scuba divers.
  • Increased production of carbon dioxide - This can occur with a fever, severe infections, and some conditions such as thyroid storm.
  • Metabolic alkalosis - If the acid content of the blood is too low (if the pH of the blood is too alkaline) the body may try to increase carbon dioxide levels in the blood to regulate acid-base balance in the body.  On other words, in this setting an increased level of carbon dioxide level of the blood is "normal" and part of the body regulating its pH.

Hypercapnia Respiratory and Alkalosis

Hypercapnia refers to an elevated carbon dioxide level in the blood.  This carbon dioxide is in equilibrium with carbonic acid in the blood and can lower the pH of the blood so that it becomes too acidic (acidotic.)  The body may respond by using metabolic processes to neutralize the lowered pH, in other words, respiratory acidosis can result in a compensatory metabolic alkalosis.

Symptoms of Hypercapnia

Symptoms of hypercapnia can range from mild to severe and include:

  • Rapid, shallow breathing (tachypnea)
  • Dyspnea (the sensation of shortness of breath)
  • A rapid heart rate (tachycardia)
  • Hyperventilation and feeling panicky
  • Confusion and loss of consciousness

Causes of Hypercapnia

There are many causes of hypercapnia or CO2 retention.  Some include:

  • Obstructive lung diseases such as chronic bronchitis, emphysema, and bronchiectasis
  • Asthma
  • Drug overdoses
  • Brain conditions such as brainstem strokes, tumors, and other conditions
  • Muscular diseases such as muscular dystrophy, ALS, myasthenia gravis, and other myopathies and neuropathies
  • Obesity hypoventilation syndrome

Hypoxia and Hypercapnia

An elevated level of carbon dioxide can occur alone (with normal oxygen delivery in the blood) or with a low level of oxygen in the blood (hypoxemia.)  When a decreased oxygen level occurs in the blood, the result can by hypoxia - a decrease in oxygen present in the tissues which result in tissue damage and cell death.

Hypercapnic Respiratory Failure

The occurrence of respiratory failure due to hypercapnia is unfortunately all too common.  It's thought that patients with COPD are particularly prone to hypercapnia due to ventilation-perfusion mismatch (V/Q mismatch) in the lungs. This occurs when, simplistically, the capillaries and alveoli in the lungs don't line up as well as usual for the proper exchange of gases to take place.

There are several types of respiratory failure - some in which a low oxygen level (PaO2 less than 60) is responsible, and some in which hypercapnia (PaCO2 greater than 45) is responsible. Sometimes both of these occur together.

Treatments for Hypercapnia

The treatment for hypercapnia depends upon the underlying condition and mechanism.  Most of the time treatment will include either non-mechanical ventilation such as CPAP or BiPAP or mechanical ventilation (respirator support).


Feller-Kopman, D., and R. Schwartzstein. The evaluation, diagnosis, and treatment of the adult patient with acute hypercapnic respiratory failure. UpToDate. Updated 03/17/16. http://www.uptodate.com/contents/the-evaluation-diagnosis-and-treatment-of-the-adult-patient-with-acute-hypercapnic-respiratory-failure

Poon, C., Tin, C., and G. Song. Submissive hypercapnia: Why COPD patients are more prone to CO2 retention than heart failure patients. 2015. 216:86-93.

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