Arterial pCO2
How much carbon dioxide is dissolved in your arterial blood. It shows how well your breathing clears this waste gas.
AI-verified against cited sources. Awaiting expert reviewAlso called PaCO2LOINC 2019-8
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What it is
pCO2 is the pressure of carbon dioxide (CO2) dissolved in your blood, and in arterial blood it is often written PaCO2. Your cells make CO2 as a waste gas, and your lungs breathe it out. The level in your blood reflects the balance between how much your body makes and how much your breathing removes. It is measured on an arterial blood gas along with pH and oxygen. It is not the same as the 'CO2' result on a standard chemistry panel.
Why it matters
CO2 acts like an acid in the blood, so pCO2 is one of the two main controls of blood pH. Reading pCO2 next to pH and bicarbonate helps your clinician tell whether a change in acidity starts with the lungs or with the body's chemistry. It is also the clearest blood measure of how well you are breathing, which a finger pulse oximeter can't show.
If it’s low
A pCO2 below the usual range means you are breathing off more carbon dioxide than your body makes, which is called hyperventilation. This makes the blood more basic. It is common and often harmless, and it can also be the body's response to another problem.
Common causes
- Anxiety, pain, or fast breathing during the blood draw itself
- Pregnancy, when breathing runs a little faster
- High altitude, where thin air makes you breathe faster
- An air bubble in the syringe, which lets carbon dioxide escape
- Lung conditions that cause breathlessness, such as asthma or pneumonia
- The body breathing faster to offset too much acid in the blood (metabolic acidosis)
What can help
- Ask your clinician whether the cause is anxiety or pain or something else
- If you tend to breathe fast when stressed, ask about breathing techniques
- Ask whether a repeat sample while you are calm would help
If it’s high
A pCO2 above the usual range means carbon dioxide is building up in the blood, usually because breathing isn't clearing it fast enough. This makes the blood more acidic. In long-lasting lung conditions the kidneys adapt over days, so the pH can end up near normal even though pCO2 stays high.
Common causes
- A delay before the sample was tested, which lets blood cells add CO2
- Slow or shallow breathing, for example from sedating medicines, opioids or alcohol
- Long-term lung conditions such as COPD
- Pneumonia or other conditions that interfere with breathing
- Muscle or nerve conditions that weaken the breathing muscles
- The body slowing its breathing to offset too much base in the blood (metabolic alkalosis)
What can help
- Ask your clinician what is behind the change and whether your breathing needs support
- Ask your clinician to review any sedating medicines you take, and don't change them on your own
- If you have a lung condition, ask whether your care plan, including oxygen or breathing support, is still the right fit
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
For adults: 35–45 mmHg. Source: MedlinePlus, National Library of Medicine, South African Family Practice, Medicine International, American Journal of Clinical Pathology, American Journal of Respiratory and Critical Care Medicine, F1000Research, Penn State Health, Northwell Health Laboratories, Clinical Chemistry and Laboratory Medicine. Labs treat below 20 mmHg or above 70 mmHg as urgent.
- Adults (this one applies)35–45 mmHg
Sources: MedlinePlus, National Library of Medicine, South African Family Practice, Medicine International, American Journal of Clinical Pathology, American Journal of Respiratory and Critical Care Medicine, F1000Research, Penn State Health, Northwell Health Laboratories, Clinical Chemistry and Laboratory Medicine
What can affect your result
- How you were breathing at the moment of the draw, including pain, anxiety or breath-holding
- Sample handling: air bubbles and delays before testing
- Altitude: healthy people living high up have lower pCO2
- Pregnancy, when breathing runs a little faster
- Sex: women tend to have a slightly lower pCO2 than men in studies of healthy adults
- Recent changes in oxygen, breathing support or ventilator settings
- Air bubbles in the syringe let carbon dioxide escape and lower the measured pCO2. A delay before analysis lets blood cells keep producing CO2 and can raise it.
- A venous or mixed sample mislabeled as arterial reads higher.
- Pain, anxiety or breath-holding during the needle stick can change your breathing and move pCO2, mostly downward when you breathe fast.
- A recent change in oxygen, breathing support or ventilator settings changes pCO2, so the timing of the draw matters.
Minutes. pCO2 follows your breathing quickly. After a change in oxygen or ventilator settings, care teams wait for the level to settle before drawing a sample.
When to talk to a clinician
An arterial blood gas is usually done in a clinic or hospital, where your care team sees the result right away. Many US hospitals treat a pCO2 below about 20 mmHg (2.7 kPa) or above about 70 mmHg (9.3 kPa) as needing prompt attention and call the ordering clinician. If you are not in a care setting and feel very short of breath, unusually drowsy or confused, or notice bluish lips, call 911 or your local emergency number.
Questions you could ask
- Was this an arterial sample, and was I on extra oxygen at the time?
- How do my pCO2, pH and bicarbonate fit together?
- Is my pCO2 high because of my breathing, a medicine or something else?
- Does my care plan need to change to support my breathing?
- Could anxiety, pain or fast breathing during the draw explain this result?
- Should we repeat the test, and are other tests worth checking?
Questions people ask
Is this the same as the CO2 on my chemistry panel?
No. The 'CO2' on a basic or comprehensive metabolic panel is mostly bicarbonate, measured on a vein sample and reported in mmol/L. The pCO2 on a blood gas is the pressure of dissolved carbon dioxide gas, and its usual range is 35 to 45 mmHg.
My report shows kPa. How do I convert to mmHg?
Multiply kPa by 7.5. For example, 5.3 kPa is about 40 mmHg. To go the other way, multiply mmHg by 0.133.
Show the science and sources
Units
- 1 mmHg = 1 mmHg
- 1 mmHg = 0.1333 kPa
Open the Arterial pCO2 unit converter
Is a change real?
Arterial pCO2 naturally varies about 4.8% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 15% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Desirable Biological Variation Database specifications, 2014 update (Minchinela J, Ricos C, et al.; the 'Ricos database', a compilation of median published CVI/CVG).
For clinicians and researchers
PaCO2 varies inversely with alveolar ventilation at a given CO2 production, so it is the direct blood marker of ventilation. It is measured with an electrode at 37 °C. In the Henderson-Hasselbalch equation, dissolved CO2 is 0.03 x pCO2 (mmol/L, with pCO2 in mmHg), which is why pCO2 and bicarbonate together set the pH. Typical compensation rules from one review: in respiratory acidosis, HCO3- rises about 1 mmol/L per 10 mmHg above 40 when acute and about 4 mmol/L when chronic; in respiratory alkalosis, HCO3- falls about 2 mmol/L per 10 mmHg below 40 when acute and about 5 mmol/L when chronic. In metabolic acidosis, Winter's formula gives an expected PaCO2 of 1.5 x HCO3- + 8, plus or minus 2. The 35-45 mmHg interval is a convention: in 194 healthy nonsmokers aged 40-90, mean PaCO2 was about 35.8 mmHg (SD 3.9), and women had a lower mean pCO2 than men in two studies of healthy volunteers (20 young adults and 182 adults). Air bubbles lower PaCO2, and one review advises analyzing room-temperature samples within 15 minutes and iced samples within 1 hour. For someone breathing on their own, the 1992 AARC sampling guideline advises waiting at least 20 to 30 minutes after an oxygen change before drawing the sample. The 20 and 70 mmHg limits are medians of hospital notification lists, not a clinical standard.
LOINC 2019-8 (primary), 11557-6, 2020-6. Specimen: whole-blood.
Sources
- Arterial Blood Gas (ABG) Test, MedlinePlus, National Library of Medicine
- Mastering blood gas interpretation: A practical guide for primary care providers, South African Family Practice (2025)
- Blood gas analysis: Clinical applications, interpretation and future directions (Review), Medicine International (2026)
- Arterial blood gas standards for healthy young nonsmoking subjects, American Journal of Clinical Pathology (1981)
- Reference values of arterial oxygen tension in the middle-aged and elderly, American Journal of Respiratory and Critical Care Medicine (1995)
- Arterial blood analysis of healthy residents in Huamachuco, Peru (3,164 m): a cross-sectional study, F1000Research (2023)
- Visualization of Critical Limits and Critical Values Facilitates Interpretation, Diagnostics (MDPI) (2025)
- Critical Decision Thresholds for Urgent Physician Notification of Point-of-Care Testing Results, Diagnostics (MDPI) (2026)
- Blood Gas, Arterial, Penn State Health
- Blood Gas Arterial, Northwell Health Laboratories
- Reference intervals and age and gender dependency for arterial blood gases and electrolytes in adults, Clinical Chemistry and Laboratory Medicine (2011)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.