Excess Carbon Dioxide in the Blood in Cats


PetMD Editorial

Updated Aug. 24, 2022

Hypercapnia in Cats


Hypercapnia is synonymous with hypoventilation, or inadequate inhalation of fresh air. It is generally the result of alveolar hypoventilation, a failure of the air cells in the lungs to take in adequate amounts of clean oxygen. It may also be related to lung disease or to environmental conditions that result in increased levels of carbon dioxide in the breathable air.


Hypercapnia is characterized by an increase in the partial pressure of carbon dioxide in the arterial blood. Carbon dioxide is a normal part of the atmosphere, and a normal component of the chemical make-up of the mammalian body. Carbon dioxide is the end product of aerobic cellular metabolism (the function of cells that require oxygen to operate). It is considered the primary drive to breath, by stimulation of central chemoreceptors in the medulla oblongata (the lower portion of the brainstem). It is carried in the blood in three forms: 65 percent is as a bicarbonate; 30 percent is bound to hemoglobin; and 5 percent is dissolved in plasma. As a natural part of the atmosphere and the air that is inhaled, carbon dioxide is constantly being added to and removed from the air cells in the lungs. The normal amount of carbon dixide in the arterial blood is 35-45 mm Hg (a measurable unit of pressure).


However, an excess of carbon dioxide in the bloodstream can lead to an abnormal condition, causing symptoms ranging from dizziness to convulsions. Left untreated, a state of hypercapnia can lead to death.


Any breed, age, or gender of cat can be affected by this disorder.



Because the brain is primarily affected by this condition, nervous system signs abound. Other symptoms include:

  • Abnormal breathing pattern
  • Weakness
  • Severe condition may lead to slow heartbeat and slow breathing
  • Upper airway obstruction
  • Pulmonary parenchymal (internal cells in the lungs) disease
  • Hypoventilation owing to muscle weakness or neuropathy
  • Excess abdominal fluid




Hypoventilation that results from a decrease in alveolar ventilation; may be the result of one of the following:

  • Anesthesia
  • Muscular paralysis
  • Upper airway obstruction
  • Air or fluid in the pleural space
  • Restriction in movement of the thoracic (chest) cage
  • Diaphragmatic hernia (where there is a hole in the diaphragm, allowing a any one of the abdominal organs to push through the hole into the chest space, often interfering with breathing in the process)
  • Pulmonary parenchymal disease (disease of the lung tissue)
  • Central nervous system disease
  • Administration of sodium bicarbonate (used in some foods and medications, esp. some medicines that are used for treating acidosis), which dissociates into carbon dioxide when there is inadequate ventilation

It may also occur spontaneously in patients during inhalation of anesthesia or due to increased inhaled carbon dioxide, such as what occurs from rebreathing gases that had been exhaled. The most common cause, however, is due to an exhausted carbon dioxide absorbent in the anesthesia machine is the most common cause.




Because there are several possible causes for this condition, your veterinarian will most likely use differential diagnosis. This process is guided by deeper inspection of the apparent outward symptoms, ruling out each of the more common causes until the correct disorder is settled upon and can be treated appropriately. If your cat is conscious, your doctor will check your cat for symptoms of hyperthermia (body temperature that is too high), hypoxemia (lack of oxygen), and head trauma.  If your cat is not conscious, especially if it is due to being anesthetized, your veterinarian will check your cat for hypoxemia. 


If none of these disorders are found to be the cause of the symptoms, your veterinarian will perform an upper airway endoscopy to rule out a laryngeal mass or paralysis of the larynx (muscles of the throat). 




The definitive treatment is to treat the primary cause, discontinue inhalation anesthesia, or provide adequate ventilation during anesthesia. Your veterinarian will begin by providing adequate ventilation into the air cells of the lungs. If your cat is anesthetized, your doctor will accomplish ventilation manually or mechanically with an anesthesia ventilator.

Non-anesthetized cats with severe pulmonary or central nervous system disease can be treated by mechanical ventilation with a critical care ventilator, but the cat may require heavy sedation for this treatment. Supplemental oxygen will be determined by the primary disease, since providing supplemental oxygen without providing ventilation generally will not correct hypercapnia.


Living and Management


Your doctor will assess the effectiveness of supportive (ventilation) and definitive treatment. This should result in a decrease in the respiratory effort. Arterial blood gas will be evaluated to determine improvement, and to assess the adequacy of your cat's ability to take in adequate amounts of free oxygen as needed.

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