Why do COPD patients retain CO2 when given too much oxygen?

Most people think that this is due to loss of hypoxic drive, and subsequent hypoventilation.  Although this may be true for a small minority of patients, the majority will retain carbon dioxide due to V/Q mismatching.

Localised low PaO2 in the lung leads to localised vasoconstriction, to areas of poor ventilation have appropriately poor perfusion, redirecting blood to areas of better ventilation.  This protects the COPD patient from V/Q mismatch.

If a high concentration of oxygen is given to a patient, loco-regional low oxygen concentration will be abolished, and with local hypoxic vasoconstriction.  Thus, perfusion returns to normal, but ventilation is poor.  This is termed a shunt, and results in worsening hypoxia, and most importantly, hypercarbia.

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12 Comments on “Why do COPD patients retain CO2 when given too much oxygen?”

  1. Jasmine Says:

    Thank you for this video, it has made things a lot clearer 🙂

    • dundeechest Says:

      You are welcome. Most people will trot out the hypoxic drive answer, but it really doesn’t fully explain what is happening to our patients in real life.

  2. Alfred Curnow Says:

    I think the constricted and dilated capillaries for the diseased alveoli on the right side are switched in the above diagrams. The appropriately constricted capillary in the 2nd diagram belongs in the first diagram in room air conditions, whereas the inappropriately dilated capillary in the top diagram belongs in the second picture under 100% O2 conditions.

    • dundeechest Says:

      You need to watch the videos – the diagrams do look wrong whilst static, but the moving images make sense.

  3. CO2 patients do NOT retain carbon dioxide when given too much oxygen unless they have advanced disease process. Treating chronic lung patients as well as heart failure patients require that you meet the blood oxygen need in order to address cause and buy time without causing harm. Low blood oxygen saturation is harmful.

    • dundeechest Says:

      The BTS oxygen guideline writing committee, and I, disagree. There is significant mortality, and morbidity associated with giving too much oxygen to patients with chronic type 2 respiratory failure, of any aetiology. There is no evidence that I am aware of to show that outcomes are improved in any disease by keeping oxygen saturations any higher than 90%.

  4. Dan Says:


    Nice tutorial – thanks.
    Do you have any references to primary literature supporting this?


  5. […] — Why do COPD patients retain CO2 when given too much oxygen? (features brief animations showing loss of hypoxic vasoconstriction leading to V/Q mismatch in […]

  6. Jon Says:

    I just spent 3 days trying to find a good explanation. Yours have explained it perfectly….thank you Dundee.

  7. The Chiefs Says:

    Reblogged this on The (unofficial) University of Texas Southwestern Internal Medicine Blog and commented:
    A quick explanation for the real reason behind oxygen-induced hypercarbia in COPD.

  8. Xian Hui Tan Says:

    Wow, I searched for so many sites but from your short explanation, I can understand it straight away!

  9. […] — Why do COPD patients retain CO2 when given too much oxygen? (features brief animations showing loss of hypoxic vasoconstriction leading to V/Q mismatch in COPD […]

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