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Excess Carbon Dioxide in the Blood in Dogs

Hypercapnia in Dogs

Hypercapnia refers to an elevation in the arterial blood’s partial pressure of carbon dioxide. Carbon dioxide constitutes a natural component of both the atmosphere and the mammalian body’s chemical composition. It serves as the byproduct of aerobic cellular metabolism, crucial for cells reliant on oxygen. Furthermore, carbon dioxide acts as the primary stimulus for breathing, activating central chemoreceptors in the medulla oblongata, situated in the brainstem’s lower region. Within the bloodstream, carbon dioxide exists in three forms: approximately 65 percent as bicarbonate, 30 percent bound to hemoglobin, and 5 percent dissolved in plasma.

Carbon dioxide undergoes constant addition to and removal from the air cells within the lungs, reflecting its status as a standard atmospheric component. Normally, arterial blood contains 35-45 mm Hg of carbon dioxide, measured in pressure units. However, an excess of carbon dioxide in the blood can precipitate abnormal conditions, manifesting symptoms ranging from dizziness to convulsions. If left untreated, hypercapnia can progress to fatal outcomes.

Hypercapnia is synonymous with hypoventilation, which denotes insufficient inhalation of fresh air. Alveolar hypoventilation, marked by the lungs’ failure to adequately intake clean oxygen, typically underlies this condition. Additionally, hypercapnia may be associated with lung ailments or environmental factors leading to elevated carbon dioxide levels in breathable air. This disorder can affect dogs of any breed, age, or gender.


Symptoms of hypercapnia predominantly affect the brain, resulting in a variety of nervous system manifestations. Additional signs include:

  • Irregular breathing patterns
  • Weakness
  • Severe cases may entail a decreased heart rate and slow breathing
  • Obstruction in the upper airway
  • Lung parenchymal disease
  • Hypoventilation due to muscle weakness or neuropathy
  • Accumulation of excess abdominal fluid


Causes of hypoventilation leading to hypercapnia may stem from a decrease in alveolar ventilation and include:

  • Anesthesia
  • Muscular paralysis
  • Obstruction in the upper airway
  • Presence of air or fluid in the pleural space
  • Restrictions in the movement of the thoracic (chest) cage
  • Diaphragmatic hernia, characterized by a hole in the diaphragm allowing abdominal organs to protrude into the chest cavity, often impeding breathing
  • Pulmonary parenchymal disease affecting lung tissue
  • Central nervous system disease
  • Administration of sodium bicarbonate, found in certain foods and medications, particularly those used to treat acidosis, which converts into carbon dioxide in instances of inadequate ventilation

Spontaneous occurrences may arise during patient inhalation of anesthesia or due to heightened inhaled carbon dioxide levels, such as those experienced through rebreathing exhaled gases. However, the most frequent cause is attributed to depleted carbon dioxide absorbent in anesthesia machines.


Diagnosing hypercapnia involves a thorough process, as there are various potential causes. Your veterinarian will likely employ a differential diagnosis approach. This method involves carefully examining outward symptoms, systematically eliminating each of the more common causes until the correct disorder is identified and can be treated accordingly.

If your dog is conscious, the veterinarian will assess for hyperthermia (elevated body temperature), hypoxemia (insufficient oxygen levels), and head trauma. In cases where the dog is unconscious, particularly due to anesthesia, the vet will assess for hypoxemia.

If none of these conditions are found to be the root cause of the symptoms, the veterinarian may conduct an upper airway endoscopy to rule out laryngeal mass or paralysis of the larynx muscles.


The primary treatment involves addressing the underlying cause, discontinuing inhalation anesthesia, or ensuring sufficient ventilation during anesthesia. Your veterinarian will initiate treatment by ensuring adequate ventilation in the lung air cells. If your dog is under anesthesia, the veterinarian will manually or mechanically ventilate the lungs using an anesthesia ventilator.

Non-anesthetized dogs with severe pulmonary or central nervous system disease may require mechanical ventilation using a critical care ventilator, although sedation might be necessary for this treatment. The administration of supplemental oxygen will depend on the primary disease, as providing oxygen alone without adequate ventilation typically does not resolve hypercapnia.

Living and Management

Your veterinarian will evaluate the efficacy of both supportive (ventilation) and definitive treatments. This assessment should lead to a reduction in respiratory effort. Arterial blood gas analysis will be conducted to gauge improvement and to evaluate your dog’s capacity to intake sufficient oxygen as required.

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