Skip to content
Blood Test Tracker

Venous pCO2

The pressure of carbon dioxide dissolved in blood from a vein. It reflects how well your lungs clear CO2 and runs a few mmHg above the arterial value.

AI-verified against cited sources. Awaiting expert reviewAlso called PvCO2, pvCO2, vPCO2LOINC 2021-4

Check your result

Sex (optional)

Type your result to see where it falls.

Nothing you type here leaves your browser.

What it is

Carbon dioxide (CO2) is a waste gas your cells make. Your blood carries it to your lungs, and you breathe it out. pCO2 is the pressure that dissolved CO2 gas exerts in the blood sample. A venous blood gas measures it in blood from a vein. Venous pCO2 is higher than arterial pCO2, by roughly 4 to 6 mmHg, because the blood has just picked up CO2 from the tissues.

Why it matters

pCO2 shows whether your breathing is keeping up with the CO2 your body makes. A high value suggests breathing that is too slow or shallow to clear it. A low value suggests breathing faster than needed. It also shapes your blood pH. Venous pCO2 can screen for a breathing problem, but it varies more from the arterial value than venous pH does, so it is best read with pH, bicarbonate and your symptoms.

If it’s low

A lower result means less CO2 in your blood, usually from breathing faster or deeper than your body needs. Anxiety, pain and fever are common reasons, and pregnancy and high altitude also lower CO2. Some lung and liver conditions can too. Your clinician reads it with your pH and bicarbonate.

Common causes

  • Anxiety, panic or pain that speeds up breathing
  • Fever
  • Pregnancy, where slightly faster breathing is expected
  • High altitude
  • Lung conditions that cause shortness of breath, such as asthma or a blood clot in the lung
  • Liver disease
  • Aspirin overdose

What can help

  • Ask your clinician what is behind the fast breathing
  • Mention symptoms such as dizziness or numbness or tingling in your hands or feet

If it’s high

A higher result means more CO2 is dissolved in your blood. This usually happens when breathing is too slow or shallow to clear CO2, as with lung disease, weak breathing muscles or medicines that slow breathing. Venous values run higher than arterial ones, so a mild rise may not mean the arterial level is high. Your clinician may repeat the test or check an arterial sample.

Common causes

  • Airway or lung disease, such as asthma, COPD or severe pneumonia
  • Weak chest muscles, or nerve and muscle disorders that affect breathing
  • Medicines that slow breathing, such as opioids and sedatives, especially when mixed with alcohol
  • Severe obesity or sleep apnea, which can limit breathing
  • Chest injuries or deformities, such as scoliosis
  • Slow blood flow, which lets more CO2 build up in blood returning from the tissues
  • A sample that sat too long before testing

What can help

  • Talk with your clinician about the cause, because treatment depends on it
  • Tell your clinician about any sedating medicines and your alcohol use
  • If you have sleep apnea or lung disease, ask how often your CO2 should be checked

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: 38–58 mmHg. Source: Clinical Chemistry and Laboratory Medicine 2021;59(5):947-954, Cureus 2024;16:e74223, Medicina (Kaunas) 2025;61:1337, Alberta Precision Laboratories (Alberta Health Services). Labs treat above 68 mmHg as urgent.

What can affect your result

  • Arterial or venous source: venous pCO2 runs about 4 to 6 mmHg higher
  • Sampling site: a central line or mixed venous sample differs from blood from an arm vein
  • Delay before testing: pCO2 rises as a sample sits
  • Blood flow: slow circulation raises venous pCO2
  • Pregnancy and altitude lower CO2, and some medicines raise it by slowing breathing
  • Delay before analysis raises pCO2. In a 2025 study of arterial gases held 60 minutes at room temperature (and then dropped and shaken), pCO2 rose in 85% of samples.
  • Venous pCO2 is higher than arterial pCO2 from the same person, by roughly 4 to 6 mmHg in studies.
  • The sampling site changes the result. Blood from a central line or mixed venous blood does not match blood from an arm vein.
  • Poor blood flow lets more carbon dioxide build up in venous blood, which widens the gap from the arterial value.

It reflects the moment the blood was drawn and can change within minutes with breathing, illness or treatment.

When to talk to a clinician

Call 911 or your local emergency number for severe shortness of breath, unusual drowsiness or confusion, or blue lips. Many US hospital lists call for urgent notification above about 68 to 71 mmHg (9.1 to 9.5 kPa) for venous pCO2, based on a small number of lists. If your lab flagged the result as critical, call the clinician who ordered the test today. For a result outside your lab's range without those signs, talk with your clinician.

Questions you could ask

  • Was this sample from an arm vein, an artery or a central line?
  • What could be making my CO2 higher than expected?
  • Do we need an arterial sample to confirm it?
  • Could my medicines or sleep affect my breathing?
  • What could be making me breathe faster than my body needs?
  • Do my pH and bicarbonate give more clues?

Questions people ask

Why is my venous pCO2 higher than the 35–45 mmHg I see for arterial blood?

Blood returning from your tissues has just picked up carbon dioxide, so venous pCO2 is higher than arterial pCO2, by roughly 4 to 6 mmHg in studies. That is why venous gases have their own range.

How do I convert kPa to mmHg?

Multiply kPa by 7.5, so 5.3 kPa is about 40 mmHg. To go the other way, multiply mmHg by 0.133.

Can a venous pCO2 tell me my arterial pCO2?

Only roughly. In one review a venous pCO2 of 55 mmHg could match an arterial value anywhere from about 44 to 57 mmHg. A low venous value (a review cites under 45 mmHg) can help show that arterial CO2 isn't high, but a higher venous value often needs an arterial sample to confirm.

Show the science and sources

Units

  • 1 mmHg = 1 mmHg
  • 1 mmHg = 0.1333 kPa

Open the Venous pCO2 unit converter

For clinicians and researchers

Venous pCO2 equals arterial pCO2 plus the CO2 added by tissue metabolism, so it is higher by about 2 to 5 mmHg under physiological conditions. Studies find a larger and more variable gap: a meta-analysis by Ress et al. found arterial pCO2 4.2 mmHg lower than venous, a random-effects meta-analysis summarized in a 2025 review (Byrne et al.) gave an estimated bias of 4.15 mmHg with arterial lower, and an emergency department study of 154 patients with breathing complaints found a mean venous minus arterial difference of 0.85 kPa (about 6.4 mmHg) with 95% limits of agreement of -0.92 to 2.61 kPa (about -7 to 20 mmHg). Because of this spread, a venous pCO2 of 55 mmHg could match an arterial value anywhere between about 44 and 57 mmHg. Venous pCO2 can help exclude arterial hypercapnia (a 2025 review cites a venous value under 45 mmHg as having a negative predictive value of about 99%), but it does not replace an arterial gas in severe disease. Reported healthy-adult venous intervals are 38–58 mmHg (134 adults) and 35–59 mmHg (182 adults). A review table lists peripheral venous pCO2 as 43–48 mmHg, central venous as 44–45 mmHg (range 35–48) and mixed venous as 41–51 mmHg, so site and analyzer matter. In shock, a venous-to-arterial pCO2 gap above 6 mmHg, usually measured on central venous blood, is linked to worse outcomes. In a 2025 study of 240 ICU arterial samples held 60 minutes at room temperature and then dropped and shaken, pCO2 rose in 85% of samples.

LOINC 2021-4 (primary), 97538-3, 19212-0. Specimen: whole-blood.

Sources

  1. Reference intervals for venous blood gas measurement in adults, Clinical Chemistry and Laboratory Medicine 2021;59(5):947-954 (2021)
  2. Reference Intervals for Venous Blood Gas in Healthy Adults: A One-Year Prospective Study, Cureus 2024;16:e74223 (2024)
  3. The Role of Venous Blood Gas Analysis in Critical Care: A Narrative Review, Medicina (Kaunas) 2025;61:1337 (2025)
  4. Comparison of peripheral venous and arterial blood gas in management of patients with respiratory complaints in the emergency department, PLoS One 2025;20:e0330190 (2025)
  5. Stability of arterial blood gas samples after delayed analysis and mechanical stress, PLoS One 2025;20:e0334710 (2025)
  6. Visualization of Critical Limits and Critical Values Facilitates Interpretation, Diagnostics (Basel) 2025;15(5):604 (survey of critical-notification lists from 50 US medical centers, 2024) (2025)
  7. Critical Decision Thresholds for Urgent Physician Notification of Point-of-Care Testing Results, Diagnostics (Basel) 2026;16(8):1139 (critical-limit lists from 417 US hospitals, 2023-2025) (2026)
  8. Respiratory acidosis, MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine)
  9. Respiratory alkalosis, MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine)
  10. Acidosis, MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine)
  11. LOINC codes verified with the NLM Clinical Tables LOINC search, U.S. National Library of Medicine
  12. Laboratory Bulletin: Changes to Blood Gas Reference Intervals and Critical Values, Alberta Precision Laboratories (Alberta Health Services) (2021)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

Related markers