Blood Carbon Dioxide Level

Definition
Purpose
Precautions
Description
Preparation
Aftercare
Risks
Results

Definition

Carbon dioxide is the waste product of the respiratory system. It is a gas that is exchanged for oxygen n the body’s tissues, transported to the lungs, and then breathed off during exhalation.

Carbon dioxide travels throughout the body in the form of bicarbonate, or HCO₃-. Bicarbonate levels are involved in keeping the body in appropriate acid-base balance (pH level). When the kidneys sense that the body’s acid-base balance is tending towards the acidic, the kidneys secrete more bicarbonate, in order to neutralize the acid. When the kidneys sense that the body’s acid-base balance is tending towards the more alkaline, the kidneys reabsorb bicarbonate from the bloodstream, in order to decrease the body’s alkalinity.

On a cellular level, bicarbonate works in concert with sodium, chloride, and potassium to attain and maintain appropriate pH balance within cells.

A blood carbon dioxide level reflects the presence of all three forms of carbon dioxide in the blood, including bicarbonate (HCO₃-), carbonic acid (H₂CO₃) and dissolved CO₂. The level of bicarbonate present, therefore, is extrapolated from the overall blood carbon dioxide level; it is not an exact measurement, but an estimate based on the total blood carbon dioxide level measured.

Purpose

A blood carbon dioxide level is usually drawn as part of a larger panel of electrolytes. Other measurements in the electrolyte panel include chloride, potassium, and sodium. Sometimes the blood carbon dioxide level is drawn along with an arterial blood gas, and the results are correlated with each other to help determine whether the acid-base imbalance is due to respiratory causes or metabolic causes. Respiratory acid-base imbalances are due to an imbalance in the intake of oxygen relative to the output of carbon dioxide. Metabolic acid-base imbalances are due to inappropriate amounts of bicarbonate in the blood. Excess bicarbonate results in metabolic alkalosis; a shortage of bicarbonate results in metabolic acidosis.

Precautions

There are no precautions necessary prior to having a blood carbon dioxide level drawn. Patients can continue their usual diet, activities, and medications.

KEY TERMS

Acid— Any chemical or compound that lowers the pH of a solution below 7.0, meaning that there is a surplus of hydrogen ions dissociated within that solution.

Alkaline— Any chemical or compound that raises the pH of a solution above 7.0, meaning that there is a relative shortage of hydrogen ions dissociated within that solution.

Metabolic— Pertaining to metabolism, the physical and chemical processes of living things that produce energy.

Neutralize— The way the body addresses acidity or alkalinity: adding acid to an alkaline environment to arrive at a neutral pH value, or adding bicarbonate to an acidic environment to arrive at a neutral pH value.

pH— A measure of the acidity or alkalinity of a solution, relative to a standard solution. A neutral pH value is 7.0. An acidic pH value is below 7.0. An alkaline pH value is above 7.0.

Patients who are taking anticoagulant medications should inform their healthcare practitioner, since this may increase their chance of bleeding or bruising after a blood test.

Description

This test requires blood to be drawn from a vein (usually one in the forearm), generally by a nurse or phlebotomist (an individual who has been trained to draw blood). A tourniquet is applied to the arm above the area where the needle stick will be performed. The site of the needle stick is cleaned with antiseptic, and the needle is inserted. The blood is collected in vacuum tubes. After collection, the needle is withdrawn, and pressure is kept on the blood draw site to stop any bleeding and decrease bruising. A bandage is then applied.

Preparation

There are no restrictions on diet or physical activity, either before or after the blood test.

Aftercare

As with any blood tests, discomfort, bruising, and/or a very small amount of bleeding is common at the puncture site. Immediately after the needle is withdrawn, it is helpful to put pressure on the puncture site until the bleeding has stopped. This decreases the chance of significant bruising. Warm packs may relieve minor discomfort. Some individuals may feel briefly woozy after a blood test, and they should be encouraged to lie down and rest until they feel better.

Risks

Basic blood tests, such as blood carbon dioxide levels, do not carry any significant risks, other than slight bruising and the chance of brief dizziness.

Results

In adults, a normal blood carbon dioxide level is 23-29 millimoles per liter (mmol/L). In children a normal blood carbon dioxide level is 20-28 mmol/L. In infants, a normal blood carbon dioxide level is 13–22 mmol/L.

A number of drugs may affect the results of the test. Blood carbon dioxide levels may be elevated in patients who are using steroid medications, barbiturates, bicarbonates, and loop diuretics. Blood carbon dioxide levels may be decreased in patients who are using methicillin, nitrofurantoin, tetracycline, thiazide diuretics, and triamterene. It is important that the healthcare provider take into consideration the effects that these drugs may have on the blood carbon dioxide level.

High levels

High blood carbon dioxide levels may be due to:

• chronic obstructive pulmonary disease;
• emphysema;
• pneumonia;
• Cushing’s disease;
• Conn’s syndrome;
• alcoholism; or
• vomiting.

Low levels

Low blood carbon dioxide levels may be due to:

• pneumonia;
• cirrhosis of the liver;
• liver failure;
• hyperventilation (fast, shallow breathing);
• diabetes;
• kidney failure;
• liver failure;
• salicylate (aspirin) overdose;
• shock states;
• chronic diarrhea;
• dehydration;
• chronic severe malnutrition; or
• ingestion of toxins such as antifreeze (ethylene glycol) or wood alcohol (methanol).

Resources

BOOKS

Brenner, B. M., and F. C. Rector, eds. Brenner & Rector’s The Kidney, 7th ed. Philadelphia: Saunders, 2004.

Goldman L., D. Ausiello, eds. Cecil Textbook of Internal Medicine, 23rd ed. Philadelphia: Saunders, 2007.

Mason, R. J., V. C. Broaddus, J. F. Murray, and J. A. Nadel. Murray & Nadel’s Textbook of Respiratory Medicine, 4th ed. Philadelphia: Saunders, 2005.

McPherson R. A., and M. R. Pincus, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods, 21st ed. Philadelphia: Saunders, 2006.

OTHER

Medical Encyclopedia. Medline Plus. U.S. National Library of Medicine and the National Institutes of Health. January 2, 2008. http://www.nlm.nih.gov/medlineplus/encyclopedia.html (February 10, 2008).

ORGANIZATIONS

American Association for Clinical Chemistry, 1850 K Street, NW, Suite 625, Washington, DC, 20006, (800) 892-1400, http://www.aacc.org.

Rosalyn Carson-DeWitt, M.D.

Blood clot prevention seeVenous thrombosis prevention

Blood count seeComplete blood count

Blood crossmatching seeType and screen