A Case of CO2 Narcosis

aka Pulmonary Puzzler 007

A 67 year old gentleman with a BMI of 45 was waiting for his respiratory outpatient clinic appointment. While waiting, he fell asleep. A medical emergency was called because he could not be woken up. On arrival of the MET team the patient is found to have a blood pressure of 140/70, a heart rate of 100bpm and oxygen sats of 68% on room air. He is breathing spontaneously but requires airway support. His GCS is 3/15.

Supplemental oxygen is applied and the following arterial blood gas is obtained.

ABG Pulmonary Puzzle 007


Q1. Describe the arterial blood gas?

Answer and interpretation
  • Moderately severe acidaemia due to severe respiratory acidosis.
  • Marked metabolic alkalosis which indicates that there is likely to be severe underlying hypercapnic respiratory failure.
  • The oxygen saturation is 97%.

Q2. If the baseline bicarbonate is 46, what is the baseline CO2?

Answer and interpretation

In a chronic respiratory acidosis the bicarbonate rises by 4 for every 10mmHg rise in the CO2.

So, a bicarbonate of 20 above normal corresponds to a CO2 of 50 above normal. In other words, the baseline CO2 would be about 90mmHg!

Q3. Is supplemental oxygen a good idea in this case?

Answer and interpretation

Yes. Without supplemental oxygen, a CO2 of 132mmHg is not compatible with life. Calculating the PAO2 from the alveolar gas equation for a patient on room air demonstrates this:

While supplemental oxygen may precipitate worsening type 2 respiratory failure in patients with chronic hypercapnia, removing supplemental oxygen in the setting of such extreme hypercapnia will precipitate death.


Pulmonary Puzzler-LITFL 700


Pulmonary Puzzler

Intensivist in Wellington, New Zealand. Started out in ED, but now feels physically ill whenever he steps foot on the front line. Clinical researcher, kite-surfer  | @DogICUma |

One comment

  1. I m a std of respiratory therapist i really appreciate tga method of calculating for oxygen supplement but i ask to you if u have a emergency situation what you would do u would do calculation in this case u have a also a emergrncy i think i give a oxygen bolous / 100% o2

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