An Overview of Hypercapnia

Having too much carbon dioxide in the blood can cause serious symptoms

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Hypercapnia occurs when carbon dioxide (CO2 levels) are too high in the bloodstream. High CO2 levels in the blood may happen with conditions that make it hard to remove CO2 from the body. These include chronic obstructive pulmonary disease (COPD), certain neurological and muscle diseases, metabolic disruptions, or damage to areas of the brain that regulate your breathing.

Hypercapnia can be a life-threatening health crisis, and severe hypercapnia can lead to death.

This article explains both mild and severe effects of hypercapnia, along with some of the possible underlying causes. It discusses how the condition is diagnosed and some of the hypercapnia treatment options.

Alveoli in the lungs
Dorling Kindersley / Getty Images

How Your Lungs Work

The respiratory system is simple and complex at the same time: As you inhale, air enters your lungs, Oxygen then moves into the blood which carries it to your body systems.

Once you have inhaled oxygen, you exhale the waste product carbon dioxide. This process is called gas exchange. Medical issues arise when not enough oxygen enters the lungs. However, problems can also occur if too little carbon dioxide is exhaled, which causes the body's CO2 level to spike.

This state is known as hypercapnia—sometimes called hypercarbia or carbon dioxide retention.

Hypercarbia vs. Hypercapnia and Hypoxemia

The terms hypercarbia and hypercapnia both describe conditions in which carbon dioxide levels in the body are elevated. They are often used interchangeably.

Hypercapnia and hypoxemia mean two different things because hypoxemia refers to low levels of oxygen in the blood. It can lead to hypoxia, which describes low oxygen levels in your body tissues and organs.

Mild Hypercapnia Symptoms

Many people are unaware that they have mild hypercapnia. When you consider the common, everyday nature of the symptoms, you might understand why.

Hypercapnia symptoms can include:

  • Dizziness
  • Fatigue 
  • Flushing
  • Headaches
  • Inability to concentrate or think clearly
  • Increased blood pressure
  • Muscle twitches
  • Rapid breathing (tachypnea)
  • Shortness of breath (dyspnea)

When CO2 levels become elevated, special receptors in your brain detect the increased blood level. These receptors send messages to your lungs to make you breathe more deeply and/or faster until your CO2 reaches a normal level.

With severe hypercapnia, though, the symptoms and complications can become life-threatening.

What Happens When CO2 Levels Are Too High?

Your CO2 level may be too high even if your oxygen level is normal. In many cases, a higher CO2 level leads to mild symptoms including headache and fatigue. When the mechanisms designed to protect this balance in your body no longer work, more severe symptoms of difficulty breathing, respiratory failure, seizure, and coma can occur.

Hypercapnia Complications

Severe hypercapnia can cause noticeable and distressing effects. Unlike mild hypercapnia, the body is incapable of restoring CO2 balance, which can lead to urgent symptoms:

  • Acute paranoia, depression, or confusion
  • Coma
  • Dilation (widening) of blood vessels in the skin
  • Fainting
  • Hyperventilation
  • Panic attack
  • Respiratory failure
  • Seizure
  • Swelling of the optic nerve (papilledema)

Severe cases of hypercapnia can lead to respiratory failure and coma if left untreated. So call 911 if you experience any of these symptoms.

Hypercapnia Causes

Hypercapnia results from excess CO2 production or reduced CO2 exhalation from the lungs. Some health issues may be at the root of these dynamics, and a number of risk factors may increase the odds of the condition developing.

It's rare for someone who is healthy to develop severe (also known as "medically significant") hypercapnia. But certain types of health issues can trigger the condition.

Metabolic Changes

Illnesses, infections, and severe trauma can cause an alteration in the body’s metabolism, resulting in excess CO2 production. If your breathing can’t catch up with your need to exhale CO2 from your body, you can develop an elevated blood CO2 level.

Causes of excess CO2 production include:

  • Severe illness, infection, or trauma
  • Hypothermia (too-low body temperature)
  • Scuba diving (due to pressure changes)
  • Improper settings on a ventilator

Onset of Lung Disease

Lung disease can interfere with CO2 removal. A situation called ventilation/perfusion (V/Q) mismatch occurs when you have severe lung damage that prevents the flow of blood and/or air in your lungs.

The following chronic conditions can cause CO2 gas to build up in the body:

COPD is a major cause of hypercapnia. But even people with severe or end-stage COPD may not develop hypercapnia.

Muscle Weakness

Neuromuscular diseases such as amyotrophic lateral sclerosis (ALS) and muscular dystrophy can make it a struggle to breathe, resulting in a buildup of blood CO2 levels.

Myasthenia gravis is another neuromuscular cause of hypercapnia.

Brain Disorders

Conditions that impair your brain’s ability to regulate breathing may result in CO2 accumulation in your blood (central hypoventilation). Your respiratory control can be impaired by:

Diagnosing Hypercapnia

Often, the symptoms of mild hypercapnia are simple enough for an experienced healthcare provider to address. They are worth monitoring, particularly if they linger or get worse.

Severe hypercapnia requires a more assertive approach. Efforts usually focus on a search for the underlying cause, and the search begins with a careful history and physical examination.

You will likely need a blood test that measures your CO2 level. An arterial blood gas (ABG) test measures your blood oxygen, CO2, bicarbonate, and pH.

Typically, blood tests use blood samples taken from a vein. An ABG test requires a sample of blood from your artery.

You may also need other diagnostic tests. In the meantime, you may require intervention with medication and/or assistance with breathing from a breathing mask or mechanical ventilator.

Hypercapnia Diagnosis

Hypercapnia is usually diagnosed when CO2 pressure measures at 45 mm Hg (millimeters of mercury) or above.

Diagnostic Tests

Aside from blood tests, certain go-to tests include:

  • Imaging tests: A chest X-ray and chest computerized tomography (CT) scan can help to evaluate the severity of pulmonary conditions like emphysema and pneumonia. If your healthcare provider thinks the cause may be related to the brain, you may need a brain imaging test, such as magnetic resonance imaging (MRI).
  • Pulmonary function tests (PFTs): Several measures of your respiratory function can help to assess your lung function. These include your vital capacity (the maximum amount of air that can be inhaled or exhaled from the lung) and forced expiratory volume in 1 second (FEV1). This test measures how much air you can forcefully exhale in 1 second.
  • Pulse oximetry: Your oxygen level can read as normal even when you have hypercapnia, but pulse oximetry is a noninvasive test that can be used to monitor sudden changes.

Hypercapnia Treatment

Treating hypercapnia focuses on improving ventilation so that you can get rid of excess CO2. The type of treatment used depends on the severity of the condition.

The options include:

  • Intubation requires that an endotracheal tube be placed in your mouth and down into your airway. You are unable to breathe or speak while you are intubated. You may need this type of respiratory assistance temporarily while a severe medical illness is being treated.
  • Mechanical ventilation involves the use of a mechanical ventilator that takes over the act of breathing for you.
  • Noninvasive ventilation provides breathing support through the upper airways. A tightly fitted mask is placed over your face or nose. The mask is connected to a machine that delivers gentle air pressure and oxygen from a flow generator, though you can still breathe on your own.
  • Oxygen therapy gives you freedom of movement while you're being treated for hypercapnia. You must wear a device that resembles an over-the-shoulder purse or backpack with a tube (cannula) that delivers oxygen into your nose.
  • Pulmonary rehab is another treatment option. It can vary greatly from one patient to another but can include a mix of breathing exercises, physical exercises, and diet and nutrition consultations.


People who have hypercapnia due to a chronic condition tend to have a poorer prognosis than those who do not, even with the same underlying cause. Early diagnosis and effective treatment may contribute to better outcomes.


Mild hypercapnia can be hard to spot. Having too much carbon dioxide in the body can cause nonspecific symptoms like headache, fatigue, and muscle twitches. Often, it clears up quickly on its own.

With severe hypercapnia, though, the body can't restore CO2 balance and the symptoms are more serious. An underlying health condition usually triggers hypercapnia, and the sooner it's identified, the sooner you can get treatment.

A Word From Verywell

If you're at risk for hypercapnia, it's crucial to learn about the condition's red flags so that you can get proper medical supervision. Mild symptoms can become severe symptoms, and these can be life-threatening.

12 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. National Heart, Lung and Blood Institute. How the lungs work.

  2. Davis JS, Allais L, Abdallah C. Caffeine Use in the Anesthetic Management of a Patient With Congenital Central Hypoventilation. Cureus. 2022 Jul 7;14(7):e26646. doi:10.7759/cureus.26646.

  3. Patel S, Miao JH, Yetiskul E, Anokhin A, Majmundar SH. Physiology, carbon dioxide retention. StatPearls.

  4. Adler D, Pépin JL, Dupuis-Lozeron E, et al. Comorbidities and subgroups of patients surviving severe acute hypercapnic respiratory failure in the intensive care unit. Am J Respir Crit Care Med. 2017;196(2):200-207. doi:10.1164/rccm.201608-1666OC

  5. Hogea SP, Tudorache E, Fildan AP, Fira-Mladinescu O, Marc M, Oancea C. Risk factors of chronic obstructive pulmonary disease exacerbations. Clin Respir J. 2020 Mar;14(3):183-197. doi:10.1111/crj.13129.

  6. Vogt S, Schreiber S, Kollewe K et al. Dyspnea in amyotrophic lateral sclerosis: the Dyspnea-ALS-Scale (DALS-15) essentially contributes to the diagnosis of respiratory impairment. Respir Med. 2019;154:116-121. doi:10.1016/j.rmed.2019.06.014

  7. Burns GP. Arterial blood gases made easy. Clin Med (Lond). 2014;14(1):66-8. doi:10.7861/clinmedicine.14-1-66

  8. Xia J, Gu S, Lei W, Zhang J, Wei H, Liu C, et al. High-flow nasal cannula versus conventional oxygen therapy in acute COPD exacerbation with mild hypercapnia: a multicenter randomized controlled trial. Crit Care. 2022 Apr 15;26(1):109. doi:10.1186/s13054-022-03973-7.

  9. American Lung Association. Lung procedures, tests & treatments.

  10. Pollock JM, Deibler AR, Whitlow CT, et al. Hypercapnia-induced cerebral hyperperfusion: an underrecognized clinical entity. AJNR Am J Neuroradiol. 2009;30(2):378-385. doi:10.3174/ajnr.A1316

  11. Morales-Quinteros L, Camprubí-Rimblas M, Bringué J, Bos LD, Schultz MJ, Artigas A. The role of hypercapnia in acute respiratory failure. Intensive Care Med Exp. 2019 Jul 25;7(Suppl 1):39. doi:10.1186/s40635-019-0239-0

  12. Yang H, Xiang P, Zhang E, Guo W, Shi Y, Zhang S, et al. Is hypercapnia associated with poor prognosis in chronic obstructive pulmonary disease? A long-term follow-up cohort study. BMJ Open. 2015 Dec 15;5(12):e008909. doi:10.1136/bmjopen-2015-008909.

By Deborah Leader, RN
 Deborah Leader RN, PHN, is a registered nurse and medical writer who focuses on COPD.