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    104 CO2 elimination (VCO2)
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    104 CO2 elimination (VCO2)

    CO₂ elimination (VCO₂): resting value, exercise testing, anaerobic threshold and ventilatory efficiency.

    CO₂ elimination (VCO₂) is the volume of carbon dioxide exhaled per minute. Carbon dioxide is produced in the tissues by the oxidation of carbohydrates, fats and proteins, carried in the blood mostly as bicarbonate, and removed by the lungs. At rest, an adult exhales about 200 mL of CO₂ per minute. During exercise, CO₂ production by the muscles rises sharply; it is transported efficiently because cardiac output may increase more than fivefold and venous CO₂ content rises, from about 42–45 mmHg at rest to 60 mmHg or more at peak exercise. VCO₂ is measured by analysing exhaled gas, for example during cardiopulmonary exercise testing and indirect calorimetry.

    Gas exchangeExercise testingVentilation
    Adult at rest≈ 200 mL/min
    Measured byexhaled gas analysis
    Related to O₂ uptakeRER = VCO₂ / VO₂
    Ventilatory efficiencyVE/VCO₂ slope
    Cardiopulmonary exercise test schematic: oxygen uptake and carbon dioxide output rise with work rate; above the anaerobic threshold CO2 output rises faster than oxygen uptake O₂ uptake and CO₂ output during incremental exercise (schematic) anaerobic threshold VO₂ VCO₂ work rate → Above the threshold, lactate is buffered by bicarbonate, producing extra CO₂ and driving ventilation.
    Figure 1. Schematic responses of oxygen uptake (VO₂) and CO₂ output (VCO₂) during an incremental exercise test.

    What VCO₂ shows

    • Metabolic rate: together with oxygen uptake it gives energy expenditure (Weir equation)
    • Fuel use: the ratio VCO₂/VO₂ is about 0.7 when burning fat and 1.0 when burning carbohydrate
    • Anaerobic threshold: during exercise, the point where VCO₂ starts to rise faster than VO₂ marks the onset of lactate accumulation
    • Ventilatory efficiency: the ventilation needed to eliminate CO₂ (VE/VCO₂ slope) is increased in heart failure and pulmonary hypertension and is a strong marker of prognosis

    Clinical significance

    Lower VCO₂

    Low metabolic rate (hypothermia, hypothyroidism, sedation), low cardiac output (less CO₂ delivered to the lungs), CO₂ retention during hypoventilation.

    Higher VCO₂

    Exercise, fever, sepsis, hyperthyroidism, burns, excessive carbohydrate feeding, buffering of metabolic acids (lactic acidosis), hyperventilation (temporary washout of CO₂ stores).

    Why it matters

    Cardiopulmonary exercise testing combines VO₂, VCO₂, ventilation and ECG to find the cause of breathlessness or exercise intolerance and to assess fitness before surgery.

    References

    1. 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
    2. West JB, Luks AM. West’s Respiratory Physiology: The Essentials. 10th ed. Philadelphia: Wolters Kluwer; 2016.

    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