What Is Partial Pressure of Carbon Dioxide (PaCO2)?

Evaluates Impact of CO2 on Obstructive Lung Disease

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The partial pressure of carbon dioxide (PaCO2) is one of several measures calculated by an arterial blood gases (ABG) test 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.

Causes of changes in PaCO2
Verywell/Cindy Chung

Purpose of 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).

Importance of Testing 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 is called respiratory acidosis. When this happens in late-stage COPD (when a person has severely weakened respiratory muscles), the condition may lead to respiratory failure.

Risks and Contraindications

An ABG test is a standard blood draw usually performed on the radial artery in the wrist, the femoral artery in the groin, or the brachial artery in the arm. 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


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.

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 that you have 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 acidosis

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, which 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

A Word From Verywell

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. Be sure to ask your healthcare provider to help explain the various measures involved in the ABG test and what they mean for you.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Castro D, Keenaghan M. Arterial Blood Gas. InStatPearls [Internet]. StatPearls Publishing.

  2. Abdo WF, Heunks LM. Oxygen-induced hypercapnia in COPD: myths and factsCrit Care. 2012;16(5):323. doi:10.1186/cc11475

  3. Cukic V. The changes of arterial blood gases in COPD during four-year periodMed Arch. 2014;68(1):14–18. doi:10.5455/medarh.2014.68.14-18

  4. Williams AJ. ABC of oxygen: assessing and interpreting arterial blood gases and acid-base balance. BMJ 1998; 317:1213.

By Deborah Leader, RN
 Deborah Leader RN, PHN, is a registered nurse and medical writer who focuses on COPD.