An Overview of Hypercapnia

Hypercapnia is defined as an excess amount of carbon dioxide (CO2) in the blood, and may also be referred to as hypercarbia or carbon dioxide retention (CO2 retention). Carbon dioxide 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 many possible causes of hypercapnia, including conditions that result in the production of more carbon dioxide (such as infections or trauma) and conditions that result in the reduced exhalation of the gas from the lungs (such as some lung diseases or a reduced breathing rate). Hypercapnia can be diagnosed by finding a partial pressure of CO2 in the blood of 45 mm Hg or higher based on the results of arterial blood gases. Treatment includes addressing the underlying cause, as well as supporting respiration with noninvasive ventilation or intubation with mechanical ventilation.

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 breath more deeply or to move so that airflow is not obstructed (this is why people move at night if their mouth and nose become obstructed by a blanket). Due to these regulatory pathways, medically significant hypercapnia in otherwise healthy individuals is rare.

Hypercapnia in Neurological Diseases and Overdose

We will discuss a number of mechanisms that can lead to hypercapnia below, but two of the major causes are decreased ventilation and COPD. With respiratory depression caused by an overdose of narcotics, or with degenerative neurological conditions such as amyotrophic lateral sclerosis (ALS), the brain does not respond to commands to increase breathing as it ordinarily would.

Hypercapnia in COPD

With COPD, the cause of hypercapnia is different. 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, and high levels of carbon dioxide in the blood, a condition called hypercapnia. 

This retention of carbon dioxide in people with COPD is due to a process called ventilation-perfusion mismatch (V/Q mismatch) in the lungs. This occurs when the capillaries and alveoli in the lungs don't line up as well as usual for the proper exchange of gases to take place. That said, not everyone who has COPD, even severe or end-stage COPD, will develop hypercapnia.

When hypercapnia is mild, many people do not even-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 nonspecific, having an awareness is important so that the condition is not missed.

On the other hand, symptoms of severe hypercapnia are more pronounced. Severe hypercapnia 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 and that is a sign of increased pressure in the brain

To understand the possible medical causes of hypercapnia, it's helpful to look at the mechanisms which can cause it to occur.

Mechanisms

Mechanisms that may lead to the development of hypercapnia include:

• Decreased ventilation (hypoventilation)
• Increased dead space in the lungs (such as noted with VQ mismatch above)
• Increased production of carbon dioxide by the body (such as with trauma)
• Oxygen-induced: In people with COPD who have adapted to low oxygen levels, giving oxygen can disrupt the signal that tells the body to breathe more, leading to hypoventilation (or, when severe, respiratory arrest) and hypercapnia
• Rebreathing exhaled carbon dioxide
• Exposure to high levels of carbon dioxide in the atmosphere (for example, near geothermal features such as a volcano)

Medical Conditions That May Result in Hypercapnia

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 above:

• Lung diseases such as COPD or end-stage interstitial lung diseases like idiopathic pulmonary fibrosis
• Drug overdose, such as with an opioid or benzodiazepine
• Brainstem stroke
• Severe illness or trauma
• Brainstem stroke
• Hypothermia
• Certain metabolic disorders (for example, low phosphate and magnesium levels in the bloodstream)
• Certain muscle or nervous system disorders like encephalitis, muscular dystrophy, ALS, or myasthenia gravis
• Improper settings on a ventilator
• Obstructive sleep apnea
• Obesity hypoventilation syndrome
• Scuba diving

Hypercapnia is also thought to play a role in sudden infant death syndrome.

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 a 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.

Supportive Treatment Options

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, 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 (via arterial blood gases) 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.