CO2 (Carbon Dioxide)

Marker Name: CO₂ (Carbon Dioxide)

REFERENCE RANGES FOR CO2:

Laboratory reference range: 18–29 mmol/L

Functional reference range: 25–30 mmol/L

DESCRIPTION:

Carbon dioxide (CO₂) is the waste product of cellular respiration, either aerobic respiration or fermentation. Chemoreceptors in the carotid bodies and aortic bodies and on the ventral lateral surface of the medulla oblongata sense carbon dioxide.¹ The relative level of carbon dioxide detected by these chemoreceptors can influence respiration rate.¹ A molecule closely related to carbon dioxide, bicarbonate (HCO₃), is an important buffer in blood. Physiologic mechanisms alter the function of the lungs and the kidneys to tightly regulate pH in the blood through changes in carbon dioxide and bicarbonate levels.²

The major buffer system in blood comprises bicarbonate and carbonic acid. In other words, hydrogen ion concentration (pH) in the blood may be affected by changes in circulating bicarbonate levels or carbonic acid levels.² Carbonic acid maintains an equilibrium with carbon dioxide and water. Thus, increases or decreases in carbon dioxide levels, as may occur through decreased or increased respiration, respectively, can drive levels of carbonic acid up or down. Carbonic acid is also converted into bicarbonate and hydronium ions. In the kidneys and lungs, the enzyme carbonic anhydrase catalyzes this reaction.³

An elevated partial pressure of carbon dioxide in arterial blood is called hypercapnia. An elevation in venous blood CO₂ is not necessarily hypercapnia, since elevated venous CO₂ could correspond to either alkalosis or acidosis. Venous bicarbonate is a surrogate measure of CO₂.² Thus, an elevated bicarbonate level in the blood suggests an acid-base disturbance of some sort, either a metabolic alkalosis or a compensatory respiratory acidosis.

CO₂ levels in blood may be abnormally high when the lungs cannot exhale sufficient amounts of carbon dioxide produced by metabolically active tissue. Examples include chronic obstructive pulmonary disease (COPD) and severe asthma. Restrictive lung diseases, such as interstitial lung disease and sarcoidosis, may also cause elevated CO₂ levels. Extensive vomiting is a common cause of metabolic alkalosis and accompanying elevations in blood CO₂ levels.

An abnormally low carbon dioxide level in the venous blood may be caused by metabolic acidosis or respiratory alkalosis. Metabolic acidosis usually involves a problem with the kidneys, such as renal tubular acidosis, or some acidification of the blood as may be caused by drugs or intoxicants.² The chief concern of respiratory alkalosis is hyperventilation, which can be related to a medical or mental health condition (e.g., panic disorder, generalized anxiety disorder).

CO₂ is measured in venous blood along with the other components of the basic metabolic panel or complete metabolic panel. It is not possible to accurately diagnose an acid-base disturbance from an isolated total CO₂ measurement.² An acid-base disturbance can be determined if one has knowledge of pH, partial pressure of carbon dioxide (PaCO₂), total CO₂, sodium, and chloride levels.² Sodium and chloride are used to determine the anion gap. The causes of acidosis are different depending on whether the anion gap is normal or increased.

PATHOLOGICAL/CONVENTIONAL RANGE INDICATIONS:

High in:^2,4,5

• Obstructive airway disease (e.g., COPD, severe asthma)
• Restrictive airway disease (e.g., interstitial lung disease, sarcoidosis)
• Metabolic alkalosis
  • Chronic vomiting
  • Conn syndrome
  • Hyperaldosteronism
  • Cushing syndrome
  • Hypokalemia
  • Volume contraction
  • Diuretics
  • Excessive licorice ingestion

Low in:^2,4,5

• Respiratory alkalosis (e.g., hyperventilation)
• Metabolic acidosis
  • Kidney failure
  • Renal tubular acidosis (Types I, II, IV)
  • Ketoacidosis
  • Lactic acidosis
  • Protracted diarrhea
  • Addison disease
• Drugs and toxins
  • Ethylene glycol poisoning
  • Methanol poisoning
  • Alcohol intoxication
  • Salicylate intoxication (e.g., aspirin overdose)
  • Paraldehyde
  • Acetazolamide

FUNCTIONAL RANGE INDICATIONS:

High in:

• Emphysema
• Diuretic use
• Aldosteronism
• Hyperemesis

Low in:

• Functional dysglycemia (check other markers of dysglycemia)
• Salicylate and diuretic use
• Fasting or malnutrition

References:

1. http://www.ncbi.nlm.nih.gov/pubmed/26335642
2. http://www.ncbi.nlm.nih.gov/books/NBK308/
3. http://www.ncbi.nlm.nih.gov/pubmed/22607884
4. https://labtestsonline.org/understanding/analytes/blood-gases/tab/test/
5. https://labtestsonline.org/understanding/analytes/co2/tab/test/