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Hypercapnia (Respiratory Acidosis): Causes & Symptoms

Written by Ana Aleksic, MSc (Pharmacy) | Last updated:
Puya Yazdi
Medically reviewed by
Puya Yazdi, MD | Written by Ana Aleksic, MSc (Pharmacy) | Last updated:

Carbon dioxide is everywhere around us and our bodies are constantly at work to balance its blood levels. However, some illness or other factors may throw off this balance. Respiratory acidosis or hypercapnia is when the blood becomes too acidic from too much carbon dioxide. Read on to learn more about how and when this balance is disrupted and when you should see a doctor.

CO2 & Acid-Base Balance


CO2 is the chemical formula for carbon dioxide, a gas that occurs naturally in the Earth’s atmosphere and is produced by our bodies as well. When we burn the sugars, fats, and proteins we eat into energy, carbon dioxide is produced as a waste product. Carbon dioxide travels through the bloodstream to the lungs, where it is exhaled [1].

The majority of carbon dioxide in our bloodstream dissolves in water to form a weak acid called carbonic acid. This weak acid splits into a hydrogen ion (the acid component, H+) and a bicarbonate ion (the base component, HCO3-). Most of the carbon dioxide dissolved in our bloodstream exists in this split form [1, 2].

Carbon dioxide levels in the bloodstream are measured as a partial pressure. This is calculated by adding the amount of carbon dioxide breathed into the ratio between the amount of carbon dioxide produced and the amount eliminated [1, 2].

Carbon dioxide is also carried by the blood plasma and hemoglobin. Hemoglobin also carries oxygen in the blood. High CO2 levels interfere with oxygen levels, which causes hemoglobin to transport less oxygen [1].

The Acid-Base Balance

The balance between carbonic acid and bicarbonate ion contributes to the acid-base balance in the blood. It helps keep the acidity of the blood steady at around a pH of 7.4, staying in a narrow range of 7.35 to 7.45. This blood pH is calculated as the ratio between the concentration of bicarbonate ion, and the partial pressure of carbon dioxide in the blood [3, 1].

Normally, this acid-base balance acts as a buffer, which protects the blood against big changes in pH when acids or bases or added to the blood. The carbonic acid can neutralize any bases, and the bicarbonate ion can neutralize acids, keeping our blood pH stable [1].

High levels of CO2 make the blood more acidic. This is because as more CO2 dissolves in our bloodstream, there is also more carbonic acid. The carbonic acid naturally splits, producing more hydrogen ions and lowering the blood pH, which indicates higher acidity.

Changes in the acidity of our blood can severely harm many of the organs in our body. Having a stable blood pH around 7.4 is crucial for delivering oxygen to the whole body and helping proteins function normally [4, 3].

Carbon Dioxide Tests

In the body, two of the major types of blood vessels are arteries and veins. Arteries carry blood from the heart to other tissues and organs, and veins carry blood with other organs’ waste products to the lungs.

Since carbon dioxide is a common waste product, carbon dioxide levels will be different between arterial and venous blood.

One test to measure carbon dioxide levels in the blood is called venipuncture blood sampling or a bicarbonate test. This test only measures the level of bicarbonate ion in a blood sample, so it reports only carbon dioxide levels. This carbon dioxide test is a common part of a series of tests called an electrolyte panel [5].

Another test to measure carbon dioxide levels is called arterial blood gas (ABG) sampling. This test measures a blood sample for various factors that affect the acid-base balance in the blood, including the partial pressures of oxygen and carbon dioxide, blood pH, total hemoglobin, and how much of each gas is bound to the hemoglobin [4].

Normal CO2 levels

Lab results are commonly shown as a set of values known as a “reference range”, which is sometimes referred to as a “normal range”. A reference range includes the upper and lower limits of a lab test based on a group of otherwise healthy people.

Your healthcare provider will compare your lab test results with reference values to see if any of your results fall outside the range of expected values. By doing so, you and your healthcare provider can gain clues to help identify possible conditions or diseases.

Normal results from ABG sampling show a blood pH between 7.35 and 7.45, and a partial pressure of carbon dioxide between 35 to 45 mmHg (4.7 to 6.0 kPa).

The normal concentration of bicarbonate ion in the blood is 22-26 mEq/L in adults from ABG sampling or the bicarbonate test [4].

An increase in the partial pressure of carbon dioxide in the blood corresponds to a decrease in blood pH, which indicates an increase in blood acidity [4].

Some lab-to-lab variability occurs due to differences in equipment, techniques, and chemicals used. Don’t panic if your result is slightly out of range — as long as it’s in the normal range based on the laboratory that did the testing, your value is normal.

However, it’s important to remember that a normal test doesn’t mean a particular medical condition is absent. Your doctor will interpret your results in conjunction with your medical history and other test results.

Have in mind that a single test isn’t enough to make a diagnosis. Your doctor will interpret this test, taking into account your medical history and other tests. A result that is slightly low/high may not be of medical significance, as this test often varies from day to day and from person to person.

What is Hypercapnia?


Hypercapnia falls under the category of acid-base disorders. Also called respiratory acidosis, hypercapnia is when you have too much carbon dioxide in the blood. This occurs when the body doesn’t eliminate carbon dioxide as quickly as it is produced, which makes the blood more acidic [1, 6, 2].

In the carbon dioxide test (ABG sampling), doctors confirm hypercapnia by an increase in the partial pressure of carbon dioxide and a decrease in blood pH. The decrease in pH indicates higher acidity, hence the term “acidosis” [3].

It is called respiratory acidosis because the apparent cause originates in the lungs. When the lungs do not remove enough CO2, the partial pressure of carbon dioxide increases and causes acidosis [3].

If the blood pH decreases without an increase in the partial pressure of carbon dioxide, it is a different kind of acidosis — metabolic acidosis. Hypercapnia refers only to respiratory acidosis [3].


Symptoms may vary depending on the underlying cause. Because increased blood acidity can harm many organ systems, hypercapnia can result in a variety of different symptoms, including:

Ventilation (assisted breathing) and increased oxygen can reduce the symptoms of hypercapnia. Research suggests that symptoms are most severe at normal blood oxygen levels [8].

When to See a Doctor

Urgently see your doctor or go to the nearest hospital if you have any of the symptoms described above or if you are experiencing respiratory problems, such as difficulty breathing or hypoventilation (shallow breathing).

Hypercapnia Causes

Causes shown here are commonly associated with hypercapnia. Work with your doctor or other health care professional for an accurate diagnosis.

Hypercapnia can result from multiple underlying causes, which are divided into two major categories:

  • The body eliminates less carbon dioxide
  • The body produces more carbon dioxide

The first category is more common, whereas the second is more likely to result from metabolic rather than respiratory acidosis [2].

Higher levels of carbon dioxide in the environment block the body’s ability to eliminate carbon dioxide and can also cause hypercapnia. This happens because more CO2 in the air doesn’t allow as much CO2 to leave the blood when exhaling [10, 11].

Any disorder or illness that impairs breathing function can increase the risk of hypercapnia and respiratory acidosis. Breathing can be impaired by reduced breathing muscle function or problems with the brain’s control of breathing [6].

Physicians will decide on treatment based on the main cause or risk factor of hypercapnia.

Caused by Reduced Elimination of Carbon Dioxide

1) Chronic Obstructive Pulmonary Disease (COPD)

Chronic obstructive pulmonary disorder (COPD) is a lung disease that can cause hypercapnia. It causes hypercapnia by impairing lung function, either reducing the amount of air that reaches the lungs or blocking proper gas exchange between the lungs and the bloodstream [12].

Hypercapnia is associated with an increased risk of death in patients with COPD, demonstrated in an observational study of over 2000 COPD patients receiving long-term oxygen therapy [12, 13].

Studies suggest that patients with severe COPD (but not mild to moderate) are more susceptible to hypercapnia [14].

Scientists explain that when a patient has a difficult time getting enough air into their lungs, the body can decide to save energy by reducing breathing instead. This allows hypercapnia to develop by reducing CO2 elimination in a process called “submissive hypercapnia” [14].

2) Lung Scarring

Lung scarring, or cystic fibrosis, is another lung disease that can cause hypercapnia. In an observational study of over 300 cystic fibrosis patients waiting for lung transplants, hypercapnia was associated with an increased risk of death [12, 15].

3) Pneumonia

Community-acquired pneumonia is a common lung infection. In a study on approximately 450 patients hospitalized with pneumonia, those with hypercapnia (as measured by ABG sampling) had an increased need for care as well as increased risk of death [12, 16].

4) Anorexia

In a study (observational) of 45 teenagers hospitalized for anorexia, mild respiratory acidosis was common [17, 18, 19].

Hypercapnia was suspected as a result of impaired lung function due to weakened lung muscles and is improved by bed rest and adjusted food intake. A different study found similar results [17, 18].

In a separate study of 22 women, anorexia was associated with multiple lung issues, but not with the acid-base imbalance in the blood. However, more severe cases of anorexia may worsen impaired lung function, disrupt the acid-base balance, and cause respiratory acidosis [20].

5) Sleep Apnea

Research suggests that obstructive sleep apnea syndrome may lead to hypercapnia during sleep. Sleep apnea can be serious and refers to when breathing repeatedly starts and stops during sleep. In some cases, hypercapnia may point to obstructive sleep apnea [21].

A study of 9 children with obstructive sleep apnea found that most children before surgery displayed moderate hypercapnia, which persisted for almost half a year afterward [22].

6) Obesity

Obesity can cause various breathing problems, including obesity hypoventilation syndrome, which is a breathing disorder characterized by a combination of obesity and excessively slow or shallow breathing. This can lead to respiratory failure (with low blood levels of oxygen) and cause hypercapnia [23].

This breathing disorder leads to sudden hypercapnia in the short term and can progress into chronic hypercapnia when left unaddressed in the long term, according to a review [24, 25].

Obesity is also the leading cause of obstructive sleep apnea, which is separately associated with hypercapnia as well. Patients with obstructive sleep apnea often had daytime hypercapnia, more severe in more obese patients and those with other obesity-related breathing disorders in an observational study of over 1000 patients [26, 27].

Scientists are also exploring whether hypercapnia increases fat cell growth in test tubes, which subsequently increases fat mass. They hypothesize that a vicious cycle may be at play: breathing disorders caused by obesity result in hypercapnia, which might increase fat cell growth, worsening of obesity [26].

7) Alcohol Abuse

Alcohol abuse was linked to the severity of hypercapnia and respiratory failure in a study of 33 patients (observational). Those who were chronic heavy alcohol abusers and had breathing issues had a greater chance of developing respiratory failure with hypercapnia [28].

In a study on young teenagers, alcohol intoxication commonly led to mild acidosis. It is unclear whether the acidosis results from respiratory or metabolic causes, though [29].

Alcohol intake also worsens symptoms of obstructive sleep apnea, which is separately linked to hypercapnia [30].

8) Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is a disorder characterized by progressive muscle weakening, eventually causing impaired breathing. Even without respiratory failure, this breathing impairment can cause hypercapnia. ALS can also cause sleep breathing disorders, which can separately cause hypercapnia [31, 32, 33].

9) Medication

Certain drugs may cause respiratory acidosis by interfering with the body’s control of breathing, muscle function, or causing very fast breathing (hyperventilation). Very fast, intense, breathing reduces the amount of carbon dioxide exhaled and oxygen inhaled.

Common drugs that can impair muscle function include anesthetics and sedatives. Drugs that may cause hyperventilation or interfere with the body’s control over breathing include epinephrine, nicotine, opioids, and sedatives (barbiturates) [34, 35, 6].

In one case study, a patient hospitalized for opioid overdose had a severe case of hypercapnia (ABG sampling). Opioids can depress the brain’s breathing center, reducing oxygen intake and CO2 exhalation. Assisted breathing and providing oxygen in the aftermath of opioid overdoses is considered to be important in reducing the complications of hypercapnia [36, 37].

Please discuss your medications with your doctor.

10) Diving

Diving for extended periods of time may increase the diver’s inhalation of carbon dioxide by depleting the oxygen in their air supply, which will increase the levels of carbon dioxide in the bloodstream and potentially cause hypercapnia. High pressures underwater can also cause hypercapnia accompanied by low oxygen levels [38, 39].

In a study of 5 divers, hypercapnia was most likely a result of a problem transferring carbon dioxide from the blood to the lungs during exhalation, leading to a build-up of CO2 in the blood [40].

Repeated diving could lead to carbon dioxide build-up and hypercapnia even infrequent divers like seals [41].

Caused by Increased Production of Carbon Dioxide

11) Extreme Heat

Heat stroke can cause hypercapnia by increasing the body’s production of carbon dioxide. Research suggests that if the heat stroke is accompanied by impaired breathing and lung gas exchange, the risk is even higher [2, 6].

Heat-related illnesses such as heat stroke have been associated with increased blood acidity, a hallmark of hypercapnia, in rats. Heat-related illnesses can cause issues with breathing, which can interfere with how the body maintains regular carbon dioxide levels, which further impairs the body’s ability to maintain temperature [42].

However, in a study of 21 patients with heat stroke, metabolic acidosis was more common than respiratory acidosis [43].

Treatment of Hypercapnia

The treatment of hypercapnia/respiratory acidosis depends on the underlying cause.

Symptoms of hypoventilation can be similar to the symptoms of other serious health conditions, which is why it’s paramount to seek medical help if you experience them.

Some treatments your doctor might recommend are covered below.

Ventilation (Assisted Breathing)

Ventilation, or assisted breathing, is the standard method of management for respiratory failure resulting from hypercapnia. Non-invasive ventilation is the primary strategy used, which refers to breathing support using a mask or similar device instead of inserting an artificial airway [44, 45].

In a randomized controlled trial of approximately 200 patients with severe COPD and chronic hypercapnia, adding ventilation (assisted breathing through a face mask) to a standard treatment improved the survival rate, especially when used to reduce hypercapnia [46, 45].

This type of ventilation was particularly effective in reducing death in patients with respiratory failure with hypercapnia (review) [46, 45].

In a case study, mechanical breathing support helped one patient with ALS and respiratory failure after they were hospitalized for hypercapnia. Guidelines for the treatment of ALS provided by the American Academy of Neurology also recommend the use of breathing support by ventilation as a method of treatment [47, 48].

In a man with drug-induced hypercapnia, likely hospitalized due to opioid overdose, non-invasive positive pressure ventilation reduced hypercapnia. Treatment was paired with a dosage of naloxone to reverse the opioid overdose [37].

Oxygen Therapy

Oxygen therapy is often used to increase low oxygen in the blood. In a review of hypercapnia in patients with COPD, physicians recommended carefully measured oxygen therapy to avoid hypercapnia. Lung diseases like COPD may cause problems with the breathing control center, which is why hypercapnia may result from oxygen therapy in these patients [49].

In a study of around 4500 patients with COPD on long-term oxygen therapy, hypercapnia was not associated with a significantly increased risk of death. In fact, some evidence suggests that long-term oxygen therapy may reduce the negative effects of hypercapnia on COPD [50].


For some patients with COPD, ventilation is not a viable option. Certain medications may help treat respiratory failure from hypercapnia, including the following:

  • Antibiotics may be needed to treat pneumonia or other respiratory infections
  • Bronchodilators may be prescribed to open the airways and ease breathing
  • Corticosteroids can reduce airway inflammation
  • Acetazolamide may stimulate breathing

Acetazolamide causes mild metabolic acidosis, which makes the body increase the amount of air breathed in. In one study of 45 patients with hypercapnia on diuretic or steroid therapy, acetazolamide reduced CO2 (after stopping the diuretic/steroid treatment). Other medications that act similar to acetazolamide may also be an option [51, 52].

Modafinil, taken orally, may stimulate breathing and reduce hypercapnia, but its effectiveness for this indication hasn’t been confirmed in proper studied [53].

Complementary Approaches to Hypercapnia


You may try the complementary approaches listed below if you and your doctor determine that they could be appropriate.

Hypercapnia can cause severe symptoms and requires urgent medical treatment.

These strategies should only be used if a doctor has confirmed the exact cause of hypercapnia and you are receiving adequate treatment.

None of the complementary approaches mentioned below should ever be done in place of what your doctor recommends or prescribes.

Additionally, some of the approaches listed here — particularly dietary interventions — lack proper evidence from large human studies.

1) Stop Smoking

Smokers are exposed to and inhale more carbon dioxide (about 350 times more than in the normal air!), which can lead to hypercapnia. Additionally, nicotine can worsen hypercapnia by interfering with the brain’s control over breathing [54, 35].

To reduce hypercapnia, stop smoking. Limit your overall exposure to smoke — even passive exposure to cigarette smoke can increase hypercapnia.

In a study (observational) of 70 infants, infants whose mothers smoked were more sensitive to hypercapnia. In another study of patients with COPD and hypercapnia, smoking reduced survival [55, 56].

2) Lose Extra Weight

Obesity worsens hypercapnia. One way to potentially improve hypercapnia and overall health if you are obese is to lose weight. A study of very obese patients with sleep apnea and hypercapnia suggested that weight loss could potentially reduce hypercapnia [57].

3) Diet

Eat a healthy, nutrient-dense, calorically-balanced diet. Talk to your doctor and nutritionist about the best approach you should take. Dietary modifications discussed below lack evidence, but we bring them up for informational purposes.

Decreasing Dietary Calcium?

No valid clinical evidence backs up this approach.

Hypercapnia as a result of increased carbon dioxide inhalation can lead to increased calcium in the urine (hypercalciuria) — the leading cause of kidney stones [11].

The authors of a small study of 4 healthy male volunteers hypothesized that increased carbon dioxide inhalation from the environment led to more calcium retained in the body, without any health benefits for bone health. This is controversial and has yet to be confirmed in larger studies [11].

Based on this experimental stance, people exposed to high CO2 levels in the environment can eat more low-calcium foods. The impact of a low-calcium diet on hypercapnia is unknown, though.

Some healthy examples of lower-calcium foods include chicken, eggplant, apples, beets, grapes, pineapple, strawberries, cantaloupe, and asparagus. On the other hand, high-calcium foods are dairy products, grains, legumes, and meat [58].

Decreasing Oxalates?

Clinical evidence is lacking to support this approach.

Scientists are investigating whether hypercapnia can reduce kidney cell survival and worsen kidney stone formation from calcium oxalate stones in kidney cells [59].

However, the impact of lowering dietary oxalates or increasing foods that anecdotally help detox the kidneys on calcium oxalate stones or hypercapnia is unknown.

Some foods that purportedly help detoxify the kidneys include asparagus, melon, and parsley. Some foods high in oxalates include leafy greens, vegetables, fruits, cocoa, nuts, and seeds [60].

Potential Benefits of Hypercapnia?

Research Areas

Aside from numerous harms, the potential benefits of hypercapnia for some disorders are also under investigation.

In a study (SB-RT) of 50 patients without hypercapnia who underwent lung surgery, temporary hypercapnia from inhaling carbon dioxide during ventilation reduced inflammation by reducing the number of inflammatory cells. There were no major side effects, aside from increased heart rate and blood pressure for some patients during surgery [61].

Hypercapnia reduced lung injuries caused by sepsis and pneumonia at the early stages in various rat studies. However, over a long time, hypercapnia could also worsen lung injuries by suppressing the immune system [62, 63, 64, 65, 66].

Hypercapnia also reduced inflammation in a mice study with skin transplants. Mice exposed to higher concentrations of carbon dioxide in the air had increased survival after receiving incompatible skin transplants [67].

Scientists are also investigating whether hypocapnia reduces lung injuries in cells, but it seems to slow down the healing process [68].

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About the Author

Ana Aleksic

Ana Aleksic

MSc (Pharmacy)
Ana received her MS in Pharmacy from the University of Belgrade.
Ana has many years of experience in clinical research and health advising. She loves communicating science and empowering people to achieve their optimal health. Ana spent years working with patients who suffer from various mental health issues and chronic health problems. She is a strong advocate of integrating scientific knowledge and holistic medicine.


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