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    106 CO2 venous blood
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    106 CO2 venous blood

    Venous CO₂: typical values, agreement with arterial values and the venous–arterial CO₂ gap.

    Carbon dioxide in venous blood (PvCO₂) is higher than in arterial blood, because blood picks up CO₂ as it passes through the tissues. The difference depends on the metabolic rate of the tissues and on blood flow: when flow is slow, each unit of blood collects more CO₂. A venous blood gas is easier to obtain than an arterial sample and is widely used in emergency departments. Venous pH agrees well with arterial pH, but venous PCO₂ differs from arterial PCO₂ by a variable amount, so it cannot reliably replace PaCO₂ when an accurate value is needed.

    Blood gasesVenous bloodTissue perfusion
    Peripheral venous PCO₂≈ 41–51 mmHg (approx.)
    Mixed venous at rest≈ 42–45 mmHg
    Peak exercise (mixed venous)60 mmHg or more
    Venous–arterial gapreflects tissue blood flow
    Venous carbon dioxide partial pressure: below about 41 mmHg low, about 41 to 51 typical for peripheral venous blood, above about 51 high Venous PCO₂ (mmHg) Low below ≈ 41 41 Typical ≈ 41–51 51 High above ≈ 51
    Figure 1. Approximate venous PCO₂ zones. A normal venous PCO₂ makes arterial hypercapnia unlikely, but a raised value must be confirmed with an arterial sample.

    Interpretation

    In a meta-analysis of 18 studies, venous and arterial pH were comparable, but the difference between venous and arterial PCO₂ varied too widely between patients to use one in place of the other. In intensive care, the venous–arterial CO₂ difference (from a central venous catheter) is used as a marker of whether cardiac output is adequate: a large gap (above about 6 mmHg) suggests insufficient blood flow.

    Clinical significance

    Low venous PCO₂

    Hyperventilation, metabolic acidosis with respiratory compensation, high blood flow states.

    High venous PCO₂

    Hypoventilation and hypercapnia (COPD, sedatives, neuromuscular disease), low cardiac output or shock (slow tissue flow), tourniquet use or prolonged sampling, high metabolic rate.

    Seek emergency care

    for drowsiness or confusion with breathing difficulty, or signs of poor circulation: cold clammy skin, fast pulse and low blood pressure.

    References

    1. Byrne AL, Bennett M, Chatterji R, Symons R, Pace NL, Thomas PS. Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology. 2014;19(2):168–175. doi:10.1111/resp.12225
    2. American Thoracic Society; American College of Chest Physicians. ATS/ACCP statement on cardiopulmonary exercise testing. Am J Respir Crit Care Med. 2003;167(2):211–277. doi:10.1164/rccm.167.2.211

    This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

    Published on 6 May 2024