What Is Partial Pressure of Carbon Dioxide (PaCO2)?

The partial pressure of carbon dioxide (PaCO2) is one of several measures calculated by an arterial blood gases (ABG) test. This is often performed on people with lung diseases, neuromuscular diseases, and other illnesses. PaCO2 specifically evaluates carbon dioxide (CO2) levels in the blood. The ABG test also evaluates the partial pressure of oxygen (PaO2), bicarbonate (HCO3), and the pH level of blood.

These tests can help provide information about lung and kidney function and your body's general metabolic state.

This article explains the purpose and risks of a PaCO2 test. It describes what to expect before, during, and after the test, as well as how the results are interpreted.

Purpose of the Test

An ABG test assessing PaCO2 is useful for getting a glimpse of the body's metabolic and respiratory state. It helps evaluate lung function and the effectiveness of oxygen therapy, and can determine the body's pH or acid-base balance.

Every time you inhale, oxygen is brought into your lungs and delivered to the alveoli. This is where the transfer of oxygen into and the removal of carbon dioxide from the blood occurs.

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

Having too much carbon dioxide is called hypercapnia, a condition common in people with late-stage chronic obstructive pulmonary disease (COPD).

In contrast, too little CO2 can lead to alkalosis, a condition where you have too many bases in your blood (CO2 is an acid).

Risks and Contraindications

An ABG test is a standard blood draw usually taken from an artery. 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.

Additional risks are rare but may include:

• Feeling lightheaded or fainting after the blood draw
• Blood buildup under the skin (hematoma)
• Excessive bleeding
• Infection

Considerations

If you have recently been on supplemental oxygen, your oxygen levels must remain consistent for 20 minutes before taking the test.

Be sure to tell your healthcare provider if you've been taking blood thinners (anticoagulants) such as warfarin or aspirin.

Before the Test

There are no special preparations leading up to the test besides making sure oxygen levels are consistent if you're on supplemental oxygen and making your provider aware of any blood thinners you're taking.

An artery will be selected for the draw. The most common ones are:

• Radial artery in the wrist
• Femoral artery in the groin
• Brachial artery in the arm

If your preference is to draw from the wrist, your healthcare provider will first test the circulation in your wrist to make sure it's an option for the sample.

During the Test

For the test, a small needle is inserted through the skin into the artery to draw the blood. As the needle is inserted, you may feel moderate pain. The insertion tends to be more uncomfortable than when drawing blood from a vein. However, some people only report a prick or some stinging.

In some instances, your healthcare provider may need to switch to a different artery if they find it is difficult to get the blood sample. Arteries can vary in size from one person to another and from one side of the body to the other.

After the Test

After the blood is successfully drawn, the sample is then sent to a laboratory for analysis.

Afterward there may be some throbbing or slight bruising, but this should resolve quickly. If you feel lightheaded, your healthcare provider may have you stay for observation until it passes.

If you have continuous bleeding, pain, or swelling at the injection site or develop a fever after the test, seek urgent medical care. You may have an infection or other complication.

Interpreting Results

The normal range of partial pressure of carbon dioxide is between 35 and 45 millimeters of mercury (mmHg). If the value is higher than 45 mmHg, it's indicative of too much carbon dioxide in your blood. Under 35 mmHg, and you have too little.

Elevated CO2 levels are commonly seen in cases of:

• Diseases causing stiffening of the chest cage
• Diseases causing neuromuscular weakness
• Sedative overdose (opioids, benzodiazepines, some anesthetics)
• Obesity hypoventilation
• Starvation
• Hypothermia
• Severe obstruction of the airway

By contrast, decreased CO2 is frequently seen with:

• Pain
• Anxiety/panic disorders
• Fever
• Brain inflammation/infection
• Aspirin overdose
• Pulmonary embolism
• High altitude
• Pregnancy
• Chronic liver disease
• Kidney dysfunction or failure
• Severe diarrhea
• Anorexia/starvation
• Overuse of chlorothiazide diuretics (used to reduce stroke and heart attack risk)
• Diabetic ketoacidosis

Factors Affecting 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 in a commercial flight) can exert pressure on the body. This can alter how well or poorly blood moves from the lungs to the capillaries and back.

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 injuries and drug use)
• Neuromuscular diseases such as amyotrophic lateral sclerosis (ALS)
• Low concentration of hemoglobin used to transport oxygen and carbon dioxide through the blood

Summary

The ABG test is a relatively low-risk method of evaluating your PaCO2, which can be helpful in determining how efficiently your lungs are working. The PaCO2 measurement is just one tool that should be taken into account with other evaluations respective to your condition. Ask your healthcare provider to help explain the various measures involved in the ABG test and what they mean for you.