- Physiology, Carbon Dioxide Retention
- What is Carbon Dioxide (CO2)? Blood Tests & Normal Levels
- What is Carbon Dioxide?
- Why test CO2 levels?
- Test Types
- Total Carbon Dioxide Test
- Arterial Blood Gas Test
- Bicarbonate Test
- Carbon Dioxide (CO2) Normal Levels
- By Test Type
- Learn More
- Blood Test: Comprehensive Metabolic Panel (CMP)
- What Is a Comprehensive Metabolic Panel (CMP)?
- Why Are Comprehensive Metabolic Panels Done?
- How Should We Prepare for a CMP?
- How Is a CMP Done?
- Can I Stay With My Child During a CMP?
- How Long Does a CMP Take?
- What Happens After a CMP?
- When Are CMP Results Ready?
- Are There Any Risks From CMPs?
- Carbon Dioxide (CO2) in Blood: MedlinePlus Lab Test Information
- What Is a Carbon Dioxide Blood Test?
- Know Your ABGs – Arterial Blood Gases Explained
- What is an ABG?
- How it differs from a VBG
- How Is An ABG Drawn?
- Key Components
- How to Interpret an ABG?
- ABG Resources
Physiology, Carbon Dioxide Retention
Carbon dioxide (CO2) is a colorless gas that comprises approximately 0.04% of Earth’s atmosphere. In the human body, carbon dioxide is formed from the metabolism of carbohydrates, fats, and amino acids, in a process known as cellular respiration. While cellular respiration is notable for being a source of ATP, it also generates the waste product, CO2.
The body gets rid of excess CO2 by breathing it out. However, CO2 in its normal range from 38 to 42 mm Hg plays various roles in the human body. It regulates the pH of blood, stimulates breathing, and influences the affinity hemoglobin has for oxygen (O2).
Fluctuations in CO2 levels are highly regulated and can cause disturbances in the human body if normal levels are not maintained.
CO2 retention is known as hypercapnia. Hypercapnia is usually due to hypoventilation or increased dead space in which the alveoli are ventilated but not perfused. In a state of hypercapnia or hypoventilation, there is an accumulation of CO2. The increased CO2 causes a drop in pH, leading to a state of respiratory acidosis.
The chemoreceptor reflex is important in allowing the body to respond to changes in pO2, pCO2, and pH. Chemoreceptors can be categorized as peripheral or central. Peripheral chemoreceptors are positioned in the carotid and aortic bodies. Central chemoreceptors are located near the ventrolateral surfaces of the medulla.
While peripheral chemoreceptors are sensitive to changes in mostly O2 and CO2 and pH to a lesser degree, central chemoreceptors are sensitive to changes in pCO2 and pH. The glomus cells in the carotid and aortic bodies detect states of hypoxia, hypercapnia, and acidosis. On the other hand, central chemoreceptors do not detect states of hypoxia.
They detect a change in PCO2 very rapidly because CO2 diffuses through the blood-brain barrier (BBB) and into the CSF easily. On the other hand, central chemoreceptors take longer to detect a change in arterial pH because H+ does not cross the BBB. When a state of hypercapnia is introduced, central chemoreceptor activity is increased.
As a result, the sympathetic outflow to the vasculature is increased, and efforts are made to increase the respiratory rate.
Cellular respiration is the process of converting ingested nutrients by combining it with O2 in tissues to produce energy in the form of ATP. The following chemical reaction represents it.
C6H12O6 + 6O2 -> 6CO2 + 6H2O
The O2 needed for cellular respiration is obtained during inhalation. The CO2 that is generated is removed from the body during exhalation.
The respiratory and circulatory systems, in conjunction, play a remarkable role in the regulation of CO2.
While the respiratory system is responsible for gas exchange, the circulatory system is responsible for transporting blood and its components to and from the tissues. Gas exchange occurs in the lungs and the tissues.
During inspiration, air travels ultimately from the atmosphere into the alveoli where it begins the process of gas exchange. At the alveolar-capillary interface, O2 is released into blood and CO2 is taken up by the alveoli.
In contrast, gas exchange in the tissues releases the CO2 into the blood and picks up O2 from the blood so that O2 can be delivered to the tissues. The gases are exchanged via simple diffusion from an area of higher pressure to lower pressure.
CO2 is a regulator of blood pH. In the blood, CO2 is carried in several different ways. Approximately 80% to 90% of it dissolves in water, 5% to 10% dissolves in the plasma, and 5% to 10% is bound to hemoglobin.
An arterial blood gas (ABG) is needed to evaluate patients with suspected hypercapnia. Hypercapnia is defined as the PaCO2 being greater than 42 mm Hg. If the PaCO2 is greater than 45 mm Hg, and the PaO2 is less than 60 mm Hg, a patient is in hypercapnic respiratory failure.
When CO2 dissolves in water, it forms a weak acid known as carbonic acid, H2CO3. H2CO3 can dissociate into a hydrogen ion and bicarbonate ion. The following chemical reaction represents this process.
CO2 + H2O -> H2CO3 -> H+ + HCO3-
A solution is acidic when H+ ions are lost in solution. When CO2 levels are high, there is a right shift in the reaction mentioned above. As a consequence, the pH decreases, introducing a state of acidosis. In contrary, when CO2 levels are low, there is a left shift in the reaction.
In that case, the pH increases because of a decrease in the concentration of H+ ions, introducing a state of alkalosis. CO2 also allows for the formation of a buffer system that maintains blood pH in a normal range.
H2CO3 neutralizes the base that is responsible for increasing blood pH, whereas HCO3- neutralizes the acid that is responsible for decreasing blood pH.
An important enzyme that catalyzes the conversion of
CO2 + H2O -> H+ + HCO3-
is carbonic anhydrase. Carbonic anhydrase also helps maintain the acid-base balance in the blood and is present in high concentrations in erythrocytes. In response to increased or decreased levels of CO2 in the blood, the body can respond by hyperventilating or hypoventilating, respectively.
The CO2 that is bound to hemoglobin forms a carbamino compound. In circumstances where the CO2 and H+ concentrations are high, there is decreased the affinity of hemoglobin to carry O2.
If CO2 concentrations are low, there is increased the affinity of hemoglobin to carry O2. This is known as the Bohr effect.
On the other hand, if O2 concentrations are high, there is an increased capacity to unload CO2 from the tissues. This is known as the Haldane effect.
A thorough history should be taken to gain an understanding of any factors that may have precipitated signs and symptoms of hypercapnia. Patients with hypercapnia can present with tachycardia, dyspnea, flushed skin, confusion, headaches, and dizziness. If the hypercapnia develops gradually over time, symptoms may be mild or may not be present at all.
Other cases of hypercapnia may be more severe and lead to respiratory failure. In these cases, symptoms such as seizures, papilledema, depression, and muscle twitches can be seen. If a patient with COPD presents with signs and symptoms of hypercapnia, immediate medical attention should be attained before CO2 reaches life-threatening levels.
Hypercapnia should be managed by addressing its underlying cause. A noninvasive positive pressure ventilator may provide support to patients who are having trouble breathing normally. If a noninvasive ventilator is not efficient, intubation may be indicated. Bronchodilators may also be used in patients suffering from an obstructive airway disease.
In recent studies, the use of the esophageal balloon in managing hypercapnia in a patient with acute respiratory distress syndrome was also shown to be effective.
To access free multiple choice questions on this topic, click here.
1.Vasileiadis I, Alevrakis E, Ampelioti S, Vagionas D, Rovina N, Koutsoukou A. Acid-Base Disturbances in Patients with Asthma: A Literature Review and Comments on Their Pathophysiology. J Clin Med. 2019 Apr 25;8(4) [PMC free article: PMC6518237] [PubMed: 31027265]2.Baillieul S, Revol B, Jullian-Desayes I, Joyeux-Faure M, Tamisier R, Pépin JL. Diagnosis and management of central sleep apnea syndrome. Expert Rev Respir Med. 2019 Jun;13(6):545-557. [PubMed: 31014146]3.Bigatello L, Pesenti A. Respiratory Physiology for the Anesthesiologist. Anesthesiology. 2019 Jun;130(6):1064-1077. [PubMed: 30998510]4.de Carvalho M, Swash M, Pinto S. Diaphragmatic Neurophysiology and Respiratory Markers in ALS. Front Neurol. 2019;10:143. [PMC free article: PMC6393326] [PubMed: 30846968]5.Eikermann M, Santer P, Ramachandran SK, Pandit J. Recent advances in understanding and managing postoperative respiratory problems. F1000Res. 2019;8 [PMC free article: PMC6381803] [PubMed: 30828433]6.Athayde RAB, Oliveira Filho JRB, Lorenzi Filho G, Genta PR. Obesity hypoventilation syndrome: a current review. J Bras Pneumol. 2018 Nov-Dec;44(6):510-518. [PMC free article: PMC6459748] [PubMed: 30726328]7.Comellini V, Pacilli AMG, Nava S. Benefits of non-invasive ventilation in acute hypercapnic respiratory failure. Respirology. 2019 Apr;24(4):308-317. [PubMed: 30636373]8.Frat JP, Coudroy R, Thille AW. Non-invasive ventilation or high-flow oxygen therapy: When to choose one over the other? Respirology. 2019 Aug;24(8):724-731. [PubMed: 30406954]9.Thille AW, Frat JP. Noninvasive ventilation as acute therapy. Curr Opin Crit Care. 2018 Dec;24(6):519-524. [PubMed: 30299309]10.Berbenetz N, Wang Y, Brown J, Godfrey C, Ahmad M, Vital FM, Lambiase P, Banerjee A, Bakhai A, Chong M. Non-invasive positive pressure ventilation (CPAP or bilevel NPPV) for cardiogenic pulmonary oedema. Cochrane Database Syst Rev. 2019 Apr 05;4:CD005351. [PMC free article: PMC6449889] [PubMed: 30950507]11.Diaz Milian R, Foley E, Bauer M, Martinez-Velez A, Castresana MR. Expiratory Central Airway Collapse in Adults: Anesthetic Implications (Part 1). J. Cardiothorac. Vasc. Anesth. 2019 Sep;33(9):2546-2554. [PubMed: 30279064]
What is Carbon Dioxide (CO2)? Blood Tests & Normal Levels
Carbon dioxide is in the air, in the ocean, and even in soda. You can also find carbon dioxide in your blood, where it plays an important role in regulating your breathing and maintaining the pH of your blood. Learn about the different ways to test carbon dioxide levels and what the results mean.
What is Carbon Dioxide?
Carbon dioxide, which has the chemical formula CO2, is a gas that is naturally found in the Earth’s atmosphere and the human body. When cells convert sugars and fats into energy, they create CO2 as a byproduct. This CO2 then travels through the blood and into the lungs, where it is exhaled the body .
Most of the CO2 in the body (usually over 90%) is actually in the form of bicarbonate (HCO3-). The rest is dissolved in the blood or found in the form of carbonic acid (H2CO3). CO2 naturally switches between these different forms depending on how acidic or basic the blood is, otherwise known as blood pH [1, 2].
CO2, especially in the form of bicarbonate, plays an important role in maintaining the pH of the blood. Bicarbonate acts as a buffer, preventing blood from becoming too acidic or basic .
The breathing rate is also regulated by CO2. A high level of CO2 in the blood tells the body that it’s not getting enough oxygen, which will trigger the urge to breathe .
Why test CO2 levels?
CO2 levels are most commonly used to evaluate the acid-base balance in the blood, which may be disturbed by issues in the lungs or kidneys. Three pieces of information are needed to get a full picture of acid-base status, which include :
- Blood pH, which is the concentration of hydrogen ions in the blood. Hydrogen ions are released when an acid reacts with water and is a marker for acidity.
- Bicarbonate (HCO3-), the most common form of CO2 in your body. It plays an important role in maintaining blood pH.
- Partial pressure of CO2 (pCO2), which represents the pressure that CO2 gas exerts in the blood.
Besides acid-base balance, CO2 levels are also helpful in monitoring lung diseases and the effectiveness of oxygen therapy .
There are several different tests that measure CO2 levels in the blood. Each test measures a different form or aspect of CO2 and sometimes multiple tests are needed to get a full picture of an individual’s health.
Total Carbon Dioxide Test
The serum total carbon dioxide test (TCO2 test) measures the total amount of CO2 (in all forms) in the blood, including :
- CO2 bound to other compounds
This test requires a blood sample, which a medical professional will usually take from a vein in the arm .
The TCO2 test does not measure the components typically used to evaluate acid-base balance (blood pH, HCO3-, and pCO2 are what is needed) .
However, since so much of a person’s CO2 is in the form of HCO3-, the total CO2 is often used as a surrogate for HCO3- levels. In fact, the terms CO2 and HCO3- are sometimes used interchangeably in terms of these tests .
Because the TCO2 test is fairly quick and easy to perform, it is usually the first test used to screen for acid-base disorders. If results are abnormal, additional tests may be performed .
Arterial Blood Gas Test
The arterial blood gas (ABG) test is also commonly performed. It measures pCO2, pO2, and blood pH. In addition, it is possible to calculate HCO3- levels using the results of this test. This means the ABG test provides all the information needed to fully evaluate acid-base balance .
The ABG test is also a little more difficult to perform compared to other tests. This test requires a blood sample from the artery, which can be more painful and potentially has more complications compared to drawing blood from the veins [1, 6].
Despite being harder to perform, the ABG test is still one of the best ways to evaluate blood pH imbalances and is commonly performed whenever an acid-base disorder is suspected [1, 6].
The bicarbonate test measures only the amount of HCO3- in the blood using samples from the veins. It is less commonly used than the ABG test because it provides less information. However, a venous HCO3- test is also easier and cheaper to perform than an ABG test, making it sometimes more popular in rural areas or less developed countries [1, 6].
Carbon Dioxide (CO2) Normal Levels
Normal ranges can vary between laboratories due to differences in equipment, techniques, and chemicals used. If your results are outside of the normal range, it may not necessarily mean there is something wrong. However, a normal result also doesn’t mean a particular medical condition is absent. Always talk with your doctor to learn more about your test results.
By Test Type
For total CO2 in the blood, the normal range is about 23 to 30 mEq/L .
For HCO3- in the blood, the normal range is about 22 to 26 mEq/L .
For pCO2 in the arteries, the normal range is about 35 to 45 mmHg .
However, these ranges can vary depending on many factors, such as :
- Gender: men usually have slightly higher CO2 levels than women
- Blood sample source: venous blood has slightly higher CO2 levels than arterial blood
- Altitude: higher elevations may lower CO2 levels
This post is part of a three-part series about carbon dioxide. Read the other parts to learn about:
Blood Test: Comprehensive Metabolic Panel (CMP)
A blood test is when a sample of blood is taken from the body to be tested in a lab. Doctors order blood tests to check things such as the levels of glucose, hemoglobin, or white blood cells. This can help them detect problems a disease or medical condition. Sometimes, blood tests can help them see how well an organ (such as the liver or kidneys) is working.
What Is a Comprehensive Metabolic Panel (CMP)?
The comprehensive metabolic panel (CMP) is a blood test that gives doctors information about the body's fluid balance, levels of electrolytes sodium and potassium, and how well the kidneys and liver are working.
Why Are Comprehensive Metabolic Panels Done?
A CMP is done to learn information about the levels of:
- Glucose, a type of sugar used by the body for energy. High glucose levels may point to diabetes.
- Calcium, which plays an important role in how muscles and nerves work.
- Sodium, potassium, carbon dioxide, and chloride, which help control the body's fluid levels and its acid-base balance. Normal levels of these electrolytes help keep cells in the body working as they should.
- Blood urea nitrogen (BUN) and creatinine, which are waste products filtered the blood by the kidneys. These levels show how well the kidneys are working.
- Albumin and total protein, which are needed to build and maintain muscles, bones, blood, and organ tissue. Low levels may be seen with liver or kidney disease, or nutritional problems.
- Liver tests: Alkaline phosphatase (ALP), alanine aminotransferase (ALT), aspartate aminotransferase (AST), and bilirubin. When these levels are high, it can be a sign of a problem with the liver.
How Should We Prepare for a CMP?
Your child may be asked to stop eating and drinking for 8 to 12 hours before a CMP. Tell your doctor about any medicines your child takes because some drugs might affect the test results.
Wearing a T-shirt or short-sleeved shirt for the test can make things easier for your child, and you also can bring along a toy or book as a distraction.
How Is a CMP Done?
Most blood tests take a small amount of blood from a vein. To do that, a health professional will:
- clean the skin
- put an elastic band (tourniquet) above the area to get the veins to swell with blood
- insert a needle into a vein (usually in the arm inside of the elbow or on the back of the hand)
- pull the blood sample into a vial or syringe
- take off the elastic band and remove the needle from the vein
In babies, blood draws are sometimes done as a “heel stick collection.” After cleaning the area, the health professional will prick your baby's heel with a tiny needle (or lancet) to collect a small sample of blood.
Collecting a sample of blood is only temporarily uncomfortable and can feel a quick pinprick.
Can I Stay With My Child During a CMP?
Parents usually can stay with their child during a blood test. Encourage your child to relax and stay still because tensing muscles can make it harder to draw blood. Your child might want to look away when the needle is inserted and the blood is collected. Encourage your child to relax by taking slow deep breaths or singing a favorite song.
How Long Does a CMP Take?
Most blood tests take just a few minutes. Occasionally, it can be hard to find a vein so the health professional may need to try more than once.
What Happens After a CMP?
The health professional will remove the elastic band and the needle and cover the area with cotton or a bandage to stop the bleeding. Afterward, there may be some mild bruising, which should go away in a few days.
When Are CMP Results Ready?
Blood samples are processed by a machine, and it may take a few hours to a day for the results to be available. If the test results show signs of a problem, the doctor might order other tests to figure out what the problem is and how to treat it.
Are There Any Risks From CMPs?
A comprehensive metabolic panel is a safe procedure with minimal risks. Some kids might feel faint or lightheaded from the test. A few kids and teens have a strong fear of needles. If your child is anxious, talk with the doctor before the test about ways to make the procedure easier.
A small bruise or mild soreness around the blood test site is common and can last for a few days. Get medical care for your child if the discomfort gets worse or lasts longer.
If you have questions about the CMP, speak with your doctor or the health professional doing the blood draw.
Carbon Dioxide (CO2) in Blood: MedlinePlus Lab Test Information
URL of this page: https://medlineplus.gov/lab-tests/carbon-dioxide-co2-in-blood/
Carbon dioxide (CO2) is an odorless, colorless gas. It is a waste product made by your body. Your blood carries carbon dioxide to your lungs.
You breathe out carbon dioxide and breathe in oxygen all day, every day, without thinking about it. A CO2 blood test measures the amount of carbon dioxide in your blood.
Too much or too little carbon dioxide in the blood can indicate a health problem.
Other names: carbon dioxide content, CO2 content, carbon dioxide blood test, bicarbonate blood test, bicarbonate test, total CO2; TCO2; carbon dioxide content; CO2 content; bicarb; HCO3
A CO2 blood test is often part of a series of tests called an electrolyte panel. Electrolytes help balance the levels of acids and bases in your body.
Most of the carbon dioxide in your body is in the form of bicarbonate, which is a type of electrolyte. An electrolyte panel may part of a regular exam.
The test may also help monitor or diagnose conditions related to an electrolyte imbalance. These include kidney diseases, lung diseases, and high blood pressure.
Your health care provider may have ordered a CO2 blood test as part of your regular checkup or if you have symptoms of an electrolyte imbalance. These include:
- Difficulty breathing
- Prolonged vomiting and/or diarrhea
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.
You don't need any special preparations for a CO2 blood test or an electrolyte panel. If your health care provider has ordered more tests on your blood sample, you may need to fast (not eat or drink) for several hours before the test. Your health care provider will let you know if there are any special instructions to follow.
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
Abnormal results may indicate that your body has an electrolyte imbalance, or that there is a problem removing carbon dioxide through your lungs. Too much CO2 in the blood can indicate a variety of conditions including:
- Lung diseases
- Cushing’s syndrome, a disorder of the adrenal glands. Your adrenal glands are located above your kidneys. They help control heart rate, blood pressure, and other body functions. In Cushing’s syndrome, these glands make too much of a hormone called cortisol. It causes a variety of symptoms, including muscle weakness, vision problems, and high blood pressure.
- Hormonal disorders
- Kidney disorders
- Alkalosis, a condition in which you have too much base in your blood
Too little CO2 in the blood may indicate:
- Addison's disease, another disorder of the adrenal glands. In Addison's disease, the glands don't produce enough of certain types of hormones, including cortisol. The condition can cause a variety of symptoms, including weakness, dizziness, weight loss, and dehydration.
- Acidosis, a condition in which you have too much acid in your blood
- Ketoacidosis, a complication of type 1 and type 2 diabetes
- Kidney disorders
If your test results are not in the normal range, it doesn't necessarily mean you have a medical condition requiring treatment. Other factors, including certain medicines, can affect the level of CO2 in your blood. To learn what your results mean, talk to your health care provider.
Some prescription and over-the-counter medicines can increase or decrease the amount of carbon dioxide in your blood. Be sure to tell your health care provider about any medicines you are taking.
- Hinkle J, Cheever K. Brunner & Suddarth's Handbook of Laboratory and Diagnostic Tests. 2nd Ed, Kindle. Philadelphia: Wolters Kluwer Health, Lippincott Williams & Wilkins; c2014. Total Carbon Dioxide Content; p. 488.
- Lab Tests Online [Internet]. Washington D.C.: American Association for Clinical Chemistry; c2001–2017. Bicarbonate: The Test; [updated 2016 Jan 26; cited 2017 Mar 19]; [about 4 screens]. Available from: https://labtestsonline.org/understanding/analytes/co2/tab/test
- Lab Tests Online [Internet]. Washington D.C: American Association for Clinical Chemistry; c2001–2019. Cushing Syndrome; [updated 2017 Nov 29; cited 2019 Feb 4]; [about 2 screens]. Available from: https://labtestsonline.org/conditions/cushing-syndrome
- Merck Manual Consumer Version [Internet]. Kenilworth (NJ): Merck & Co., Inc.; c2017. Addison Disease; [cited 2017 Mar 19]; [about 2 screens]. Available from: https://www.merckmanuals.com/home/hormonal-and-metabolic-disorders/adrenal-gland-disorders/addison-disease
- Merck Manual Consumer Version [Internet]. Kenilworth (NJ): Merck & Co., Inc.; c2017. Overview of Acid-Base Balance; [cited 2017 Mar 19]; [about 2 screens]. Available from: https://www.merckmanuals.com/home/hormonal-and-metabolic-disorders/acid-base-balance/overview-of-acid-base-balance
- National Cancer Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; NCI Dictionary of Cancer Terms: adrenal gland; [cited 2017 Mar 19]; [about 3 screens]. Available from: https://www.cancer.gov/publications/dictionaries/cancer-terms?cdrid=46678
- National Cancer Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; NCI Dictionary of Cancer Terms: carbon dioxide; [cited 2017 Mar 19]; [about 3 screens]. Available from: https://www.cancer.gov/publications/dictionaries/cancer-terms?cdrid=538147
- National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; Types of Blood Tests; [updated 2012 Jan 6; cited 2017 Mar 19]; [about 4 screens]. Available from: https://www.nhlbi.nih.gov/health-topics/blood-tests#Types
- National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; What Are the Risks of Blood Tests?; [updated 2012 Jan 6; cited 2017 Mar 19]; [about 6 screens]. Available from: https://www.nhlbi.nih.gov/health-topics/blood-tests#Risk-Factors
- National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; What Do Blood Tests Show?; [updated 2012 Jan 6; cited 2017 Mar 19]; [about 7 screens]. Available from: https://www.nhlbi.nih.gov/health-topics/blood-tests
- National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; What To Expect with Blood Tests; [updated 2012 Jan 6; cited 2017 Mar 19]; [about 5 screens]. Available from: https://www.nhlbi.nih.gov/health-topics/blood-tests
- University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; c2017. Health Encyclopedia: Carbon Dioxide (Blood); [cited 2017 Mar 19]; [about 2 screens]. Available from: https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid;=carbon_dioxide_blood
What Is a Carbon Dioxide Blood Test?
Your body’s metabolic functions produce a waste product known as carbon dioxide (CO2), a gas that has no odor or color.
Your blood is responsible for carrying CO2 to your lungs where you exhale it without conscious effort. The majority of CO2 found in your blood is comprised of a form called bicarbonate (HCO3).
Bicarbonate serves a vital purpose in your blood—it helps keep the body’s acids and bases in check.
In a healthy individual, the presence of CO2 in the blood stays within a normal range and doesn’t present any problems. However, if your CO2 levels rise too high or fall too low, this may be an indication that you have a health condition that needs to be addressed. A carbon dioxide blood test may be known as other names, including:
- Carbon dioxide content
- CO2 content
- Bicarbonate blood test
- Bicarbonate test
- Total CO2
- CO2 test-serum
Typically, a CO2 blood test is done in conjunction with an electrolyte panel, which measures sodium, potassium, and chloride levels, or as part of a metabolic panel. Electrolytes are an integral part of the way your body regulates its fluid balance and maintains appropriate acid-base (pH) levels.
The purpose of the test is to confirm whether or not there’s a fluctuation in your CO2 levels and an electrolyte imbalance in your body.
Additionally, your healthcare provider may use this test to monitor other health conditions, such as those that affect the kidneys, liver, blood pressure, and more. It may also be a helpful test in monitoring the effects of some medications.
Sometimes it is useful to measure the amount of carbon dioxide in the arteries along with the venous HCO3 measurement (most often helpful in people with a lung disorder to determine how well the lungs are functioning). This is called an arterial blood gases (ABG) test and the blood is taken from an artery rather than a vein.
The carbon dioxide blood test should be similar to any blood tests you’ve had done in the past, and the risks associated it are typically considered low. There may be some circumstances that make it more difficult for a technician or doctor to obtain a blood sample on the first try.
For example, if your veins are difficult to locate or they shift their position in the process of getting a sample, the needle might have to be inserted more than once to draw blood.
The chances of other problems arising with a blood draw are small, but they could include:
- A slight poke or stinging feeling at the insertion site of the needle
- Bruising at the site
- Feeling lightheaded or faint
- The formation of a hematoma (when the blood pools under the skin)
- Excessive bleeding
- A throbbing feeling after the test, especially if the needle was inserted more than one time
- Phlebitis (also known as a swollen vein)
- An infection
In general, the risks and contraindications for the CO2 test are minimal. If you experience minor discomfort following the blood draw, your symptoms can often be alleviated with a few simple steps.
Keep the bandage for the length of time the clinician instructs you—this will reduce the risk of bruising. Should you experience phlebitis, you should alert your health provider.
They may recommend elevating the affected arm and warm compresses.
Be sure to tell your doctor about any medications you’re taking, even over-the-counter ones. Drugs corticosteroids or antacids can skew the lab results, and your doctor will want to gather the most helpful information to figure out why you’re not feeling well. If you have other tests done at the same time, you may be asked to fast for several hours before the blood sample is taken.
Your physician should provide you with any specific instructions you need to follow on the day of the test.
Additionally, you’ll want to make sure you have your insurance card with you and a form of identification so that there’s no delay in your ability to get the test.
Most CO2 blood tests are often run as part of routine blood work, but you might want to speak with your insurance carrier regarding the out-of-pocket expenses you could be responsible for paying so that you’re not surprised by an unexpected bill.
Your blood will be drawn for a CO2 blood test in the typical way that many blood tests are done. Most ly, you’ll be seated in a chair and your healthcare provider will wrap an elastic band around your arm, which will temporarily restrict blood flow and allow them to locate a vein.
Once they have found a vein, they’ll prepare the area with a disinfectant using an alcohol pad or an alcohol-moistened cotton ball prior to inserting the needle to draw your blood.
After the needle has entered your vein, the clinician or technician will place a vial at the end of the syringe to collect your blood sample.
When they have drawn enough blood for the test, they’ll remove the elastic band and place a piece of gauze or a cotton ball over it.
They may ask you to put pressure on the site for a minute or two before covering it with an adhesive. Generally, the test takes only a few minutes to complete.
When you receive your test results, remember that they are a guide to help your doctor figure out what’s going on with you and why you may not be feeling well. Also, tests performed at different labs may yield different results.
Keep in mind that a test that falls outside the normal values of the reference range doesn’t automatically indicate that you have a medical condition.
There can be other elements, such as medications you might be taking, that contribute to your results.
If you have too much CO2 in your blood, this could suggest:
- Frequent use of medications antacids
- Lung conditions pulmonary edema and chronic obstructive pulmonary disease (COPD)
- Congestive heart failure with diuretic use
- Disorders affecting the adrenal glands Cushing's disease
- Impaired kidney function (this can also cause too little CO2, as well)
If your blood levels indicate your CO2 is too low, this may point to:
- Hyperventilation, which causes respiratory alkalosis and a compensatory metabolic acidosis
- Excessive alcohol or drug consumption
- Complications from type 1 or type 2 diabetes such as ketoacidosis
- Impaired kidney function
- Adrenal gland insufficiency Addison’s disease
If you have any questions or concerns regarding the carbon dioxide blood test, be sure to talk with your doctor ahead of time so that you can feel informed about the procedure, any pre-test instructions that the doctor may have for you, and what to expect from the test results.
Thanks for your feedback!
What are your concerns?
Know Your ABGs – Arterial Blood Gases Explained
Arterial Blood Gases (ABG) can be difficult and confusing to understand at first. However, it's a crucial skill for nurses, physicians, respiratory therapists, and nursing students to learn.
An ABG is the most accurate representation of a patient’s clinical status and correct interpretation can lead to quicker and more accurate changes in the plan of care.
Before learning how to analyze an ABG, it’s important to understand what exactly it is, the different components and their corresponding values, and how it differs from a Venous Blood Gas (VBG).
What is an ABG?
An ABG is a blood test that measures the acidity, or pH, and the levels of oxygen (O2) and carbon dioxide (CO2) from an artery.
The test is used to check the function of the patient’s lungs and how well they are able to move oxygen and remove carbon dioxide.
This test is commonly performed in the ICU and ER setting; however, ABGs can be drawn on any patient on any floor depending on their diagnosis.
How it differs from a VBG
A VBG on the other hand, tests the venous blood and can accurately determine pH and CO2 but is unable to provide reliable O2 data. For this reason, arterial testing has become the gold standard in sick patients who are at risk for sudden decompensation or those with a respiratory component.
ABGs are drawn for a variety of reasons. These may include concern for:
- Lung Failure
- Kidney Failure
- Uncontrolled diabetes
- Chronic Obstructive Pulmonary Disease
- Drug Overdose
- Metabolic Disease
- Chemical Poisoning
Show Me Nursing Programs
How Is An ABG Drawn?
An Arterial Blood Gas requires the nurse to collect a small sample of blood – generally a minimum of 0.5 ml, but a full 1 ml is preferred. Blood can be drawn via an arterial stick from the wrist, groin, or forearm.
The radial artery is most commonly used to obtain the sample. However, the femoral artery and brachial artery can be used if necessary. If the patient already has a pre-existing arterial line, this can be used to obtain the sample.
Once the blood is obtained, it is either sent to the hospital’s central lab for analysis or tested by the respiratory therapist on the unit’s blood gas analyzer. Most ICUs have one on the unit for a quick turnaround.
While arterial samples are the best for diagnostic reasons, they do provide some challenges for nurses and providers.
The main issue is if the patient does not have a functioning arterial line, a frontline clinician has to draw the arterial sample.
Some hospitals allow specially trained nurses or phlebotomists to perform this skill but only after an intense training program. If a provider is not available to perform the arterial stick, treatment could be delayed.
There are five key components to an ABG. They include:
- Partial pressure of oxygen (PaO2)
- Partial pressure of carbon dioxide (PaCO2)
- Bicarbonate (HCO3)
- Oxygen saturation (O2 Sat)
Blood gases can also measure hemoglobin and hematocrit as well as electrolyte values such as potassium, calcium, and sodium. If a clinician wishes to obtain these values, a SuperGas is run.
The aforementioned five components all have different normal values and represent different aspects of the blood gas. According to the National Institute of Health, typical normal values are:
- pH: 7.35-7.45
- Partial pressure of oxygen (PaO2): 75 to 100 mmHg
- Partial pressure of carbon dioxide (PaCO2): 35-45 mmHg
- Bicarbonate (HCO3): 22-26 mEq/L
- Oxygen saturation (O2 Sat): 94-100%
Show Me Nursing Programs
How to Interpret an ABG?
The first value a nurse should look at is the pH to determine if the patient is in normal range, above, or below. If a patient’s pH > 7.45, the patient is alkalotic. If the pH < 7.35, then the patient is acidotic.
Next, examine the PaCO2. This will determine if the changes in the blood gas are due to the respiratory system or metabolically driven. In combination with the HCO3, the nurse will be able to fully comprehend the blood gas.
Below is a chart that contains the different values and determining if the patient is suffering from a respiratory or a metabolic component. This will enable the medical team to treat the patient adequately.
|Respiratory acidosis with metabolic compensation||↓||↑||↑|
|Respiratory alkalosis with metabolic compensation||↑||↓||↓|
The acronym ROME is used to help nurses remember the relationship between pH and CO2.
The CO2 is the respiratory component of the blood gas: if the CO2 is low and the pH is high then the patient would have respiratory alkalosis. These two values move in opposite directions.
On the other hand, the HCO3 is the metabolic component of the blood gas. If the HCO3 is low and pH is low also the patient is in metabolic acidosis. These two values move in the same direction.
There are many different ways to remember how to analyze arterial blood gas. It’s a nurse’s responsibility to be able to identify key components in order to be prepared for the next step. Asking the help of more senior clinical nurses and respiratory therapists will allow novice nurses to master this skill.
There’s a wealth of information available for nurses seeking to deepen their understanding of this important test. Here are some great resources you might use for further study.
MediStudents ABG Tutorial
Analysis and Monitoring of Gas Exchange