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    105 CO2 in arterial blood
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    105 CO2 in arterial blood

    Arterial CO₂ (PaCO₂): normal range, hypocapnia and hypercapnia, and the link with ventilation.

    Carbon dioxide in arterial blood is measured as its partial pressure, PaCO₂. CO₂ is produced continuously in the tissues by the metabolism of glucose, fats and amino acids and is removed by the lungs. PaCO₂ therefore reflects the balance between CO₂ production and alveolar ventilation: if ventilation falls, CO₂ rises; if ventilation increases, CO₂ falls. CO₂ is an acid (it forms carbonic acid in water), so PaCO₂ is the respiratory component of the acid–base balance and the main chemical stimulus for breathing.

    Blood gasesVentilationAcid–base balance
    Normal PaCO₂≈ 35–45 mmHg (4.7–6.0 kPa)
    Hypercapnia> 45 mmHg
    Hypocapnia< 35 mmHg
    Measured byarterial blood gas
    Arterial carbon dioxide partial pressure: below 35 mmHg hypocapnia, 35 to 45 normal, above 45 hypercapnia Arterial PaCO₂ (mmHg) Hypocapnia below 35: hyperventilation 35 Normal 35–45 45 Hypercapnia above 45: hypoventilation
    Figure 1. PaCO₂ zones (1 kPa ≈ 7.5 mmHg). Whether a change is acute or chronic is judged from pH and bicarbonate.

    Clinical significance

    Low PaCO₂ (hypocapnia)

    Hyperventilation: anxiety, pain, fever, pneumonia, pulmonary embolism, asthma attack, early sepsis, pregnancy, high altitude, and as compensation for metabolic acidosis. Symptoms: tingling around the mouth and in the fingers, dizziness, muscle cramps.

    High PaCO₂ (hypercapnia)

    Hypoventilation: COPD exacerbation, severe asthma, opioid or sedative overdose, neuromuscular disease, obesity hypoventilation, chest wall deformity; excessive oxygen in people with COPD. Symptoms: headache, drowsiness, confusion, flushed warm skin, tremor; severe hypercapnia leads to coma.

    In acute hypercapnia pH falls quickly (respiratory acidosis); in chronic hypercapnia the kidneys retain bicarbonate and pH may be near normal.

    Seek emergency care

    for increasing drowsiness or confusion with breathing difficulty, particularly in people with COPD or after taking opioids or sedatives.

    References

    1. Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid–base disturbances. N Engl J Med. 2014;371(15):1434–1445.
    2. O’Driscoll BR, Howard LS, Earis J, Mak V; BTS Emergency Oxygen Guideline Group. BTS guideline for oxygen use in healthcare and emergency settings. Thorax. 2017;72(Suppl 1):ii1–ii90.

    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