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    101 O2 consumption (VO2)
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    101 O2 consumption (VO2)

    Oxygen consumption (VO₂): resting value, VO₂max, the Fick principle and clinical significance.

    Oxygen consumption (VO₂) is the volume of oxygen used by the body per minute. It reflects the total metabolic activity of the tissues: heart, brain, liver, kidneys, gut and muscles. At rest an adult consumes about 3.5 mL O₂ per kg per minute, roughly 250 mL/min. During exercise VO₂ rises in proportion to work until it reaches a plateau, the maximal oxygen uptake (VO₂max), which is the best single measure of cardiorespiratory fitness. Thyroid hormones (especially triiodothyronine) set the resting level of oxygen consumption.

    MetabolismExercise capacityOxygen transport
    Adult at rest≈ 250 mL/min (≈ 3.5 mL/kg/min)
    Fick principleVO₂ = CO × (CaO₂ − CvO₂)
    Peak VO₂depends on age, sex, fitness
    Measured byexhaled gas analysis
    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.

    Measurement

    VO₂ is measured by analysing inhaled and exhaled gas (indirect calorimetry, cardiopulmonary exercise testing) or calculated by the Fick principle from cardiac output and the difference in oxygen content between arterial and mixed venous blood. Peak VO₂ in heart failure is used to assess prognosis and the need for advanced therapies.

    Clinical significance

    Low VO₂

    At rest: hypothyroidism, hypothermia, sedation, or inability of tissues to use oxygen (sepsis, cyanide). At peak exercise: heart failure, lung disease, anaemia, muscle disease, deconditioning.

    High VO₂

    Exercise, fever, sepsis, burns, trauma and surgery, hyperthyroidism, shivering, increased work of breathing. In critically ill patients, rising consumption must be matched by rising delivery.

    When to see a doctor

    Reduced exercise tolerance, breathlessness or fatigue out of proportion to effort may be investigated with a cardiopulmonary exercise test.

    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. Dunn JO, Mythen MG, Grocott MP. Physiology of oxygen transport. BJA Educ. 2016;16(10):341–348.

    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