PaCO2: What is it? Purpose, Preparation, Procedure, Risks, Normal and Abnormal Levels

Arterial blood gases (ABGs) are diagnostic tests performed on blood drawn from an artery that contains oxygen and carbon dioxide.

PaCO2 is one of several tests used to measure arterial blood gases in people with lung and other diseases. It assesses how well carbon dioxide (CO2) moves from the lungs into the blood.

PaCO2 is just one of the values ​​measured in the arterial blood gas (ABG) test.

The purpose of the ABG

ABGs measure how well the lungs can provide adequate oxygen to the body and subsequently remove carbon dioxide. Blood gas analysis helps assess a person’s respiratory and metabolic status. ABGs also measure the pH of the blood and the integrity of the body’s acid-base balance.

How to prepare for an ABG

No special preparation is needed for an ABG.


After cleaning the area with an antiseptic , a needle will be used to collect a small amount of blood from the radial artery in the wrist, the brachial artery in the arm, or the femoral artery in the groin.

After the blood is removed, direct pressure will be applied to the site for several minutes to stop the bleeding. The blood will then be sent to the lab for a quick test.

Normal values

Normal ABG values ​​are within the following ranges:

  • Partial pressure of oxygen (PaO2): 75 – 100 mm Hg.
  • Carbon dioxide partial pressure (PaCO2): 35 – 45 mm Hg pH: 7.35 – 7.45.
  • Oxygen saturation (SaO2): 94 – 100%.
  • Bicarbonato – (HCO3): 22 – 26 meq / liter.

Note: mEq / liter = milliequivalents per liter; mm Hg = millimeters of mercury.

Are there risks involved?

If done correctly, there is a very low risk with an ABG. The most common occurrence includes excess bleeding or bruising at the puncture site, and there is a slight risk of impaired circulation.


If you have COPD, your doctor will want to know what your partial pressure level of carbon dioxide (PaCO2) is.

Why it is important to measure PaCO2

Every time we inhale, oxygen enters the lungs and is sent to the alveoli. The alveoli are where the transfer of oxygen to the blood and carbon dioxide from the blood occurs.

If the partial pressure of oxygen and carbon dioxide is normal, the molecules will move from the alveoli into the blood and return 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 common condition in people with late-stage COPD. Too little CO2 can cause alkalosis.

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, diving, or even taking a commercial flight) can put pressure on the body, which can alter how well or poorly blood passes from the lungs to the capillaries. and vice versa .

Diseases can work in 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 injury 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 done on the radial artery in the wrist or the femoral artery in the groin. In general, it is an uncomplicated procedure, but it can be painful since the arteries are located deeper in the body than the veins. Swelling and bruising can sometimes occur.

The normal range of partial pressure of carbon dioxide is between 40 and 45 mm Hg. If it is more than 45 mm Hg, then you have too much carbon dioxide in your blood. Less than 40mm Hg, and you have very little.