What Is Partial Pressure of Carbon Dioxide (PaCO2)?

Evaluates Impact of CO2 on Obstructive Lung Disease

Doctor Auscultating a patient
What is the partial pressure of carbon dioxide (PaCO2) and why is it important in COPD?. Getty Images/Universal Images Group

If you have COPD, your doctor may want to know what your partial pressure of carbon dioxide (PaCO2) level is. PaCO2 is one of several tests used to measure arterial blood gasses in people with lung disease and other illnesses. It evaluates how well carbon dioxide (CO2) moves from the lungs into the blood.

PaCO2 is only one of the things measured in arterial blood gasses (ABG) test. It also evaluates the partial pressure of oxygen (PaO2), bicarbonate (HCO3), and the pH level of blood.

Why Measuring PaCO2 Is Important

Every time we inhale, oxygen is brought into the lungs and is delivered to the alveoli. Alveoli are where the transfer of oxygen into the blood and carbon dioxide out of the blood take place.

If the partial pressure of oxygen and carbon dioxide is normal, the molecules will move from the alveoli into the blood and back as they should. Changes in that pressure can result in getting too little oxygen in the blood or accumulating too much carbon dioxide in the blood. Neither is considered good.

Having too much carbon dioxide is called hypercapnia, a condition common in people with later-stage COPD. Too little CO2 can lead to alkalosis, a condition where you have too many bases in your blood (CO2 is an acid).

What Causes Changes in PaCO2

There are a number of factors that can affect blood gas levels. From a broad perspective, changes in atmospheric pressure (such as climbing a mountain, scuba diving, or even sitting a commercial flight) can exert pressure on the body which can alter how well or poorly blood moves from the lungs to the capillaries and back.

Diseases can work the same way, altering the partial pressure that ensures the balanced transfer of CO2 molecules. Several conditions can alter these levels:

  • obstructive lung diseases such as COPD and asthma
  • central nervous system impairment (including head injuries and drug use)
  • neuromuscular disease such as ALS
  • low concentration of hemoglobin used to transport oxygen and carbon dioxide through the blood

Normal and Abnormal PaCO2 Levels

An ABG test is usually performed on the radial artery in the wrist or the femoral artery in the groin. It is generally an uncomplicated procedure but can be painful given that arteries are located deeper in the body than veins. Swelling and bruising can sometimes occur.

The normal range of PaCO2 is between 40 and 45 mm Hg. If it is higher than 45 mm Hg, then you have too much carbon dioxide in your blood. Under 40 mm Hg, and you have too little.

Elevated CO2 levels are commonly seen in cases of:

  • obstructive lung disease
  • severe vomiting
  • overuse of mercury-based diuretics
  • aldosteronism (a type of hormonal disorder that causes high blood pressure)

By contrast, decreased CO2 is frequently seen with:

  • kidney dysfunction or failure
  • severe diarrhea
  • anorexia/starvation
  • overuse of chlorothiazide diuretics (used to reduce stroke and heart attack risk)
  • diabetic acidosis

Importance of PaCO2 in COPD

Carbon dioxide is in equilibrium with bicarbonate (HCO3) in the blood. When CO2 is elevated, it creates an acidic environment. In people with COPD who have serious breathing problems, the increased CO2 level can result in what we call respiratory acidosis.

When this happens in late-stage COPD (when a person has severely weakened respiratory muscles), the condition can lead to respiratory failure.

Sources:

Abdo, W. and Heunks, L. "Oxygen-induced hypercapnia in COPD: myths and facts." Critical Care. 2012. 16(5):323.

U.S. National Library of Medicine. Medline Plus. "Blood gasses." MedLine Plus. Bethesda, Maryland; Updated August 25, 2014.

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