aka Pulmonary Puzzle 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.
Questions
Q1. Describe the arterial blood gas?
There is a moderately severe acidaemia due to severe respiratory acidosis. There is a 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?
In a chronic respiratory acidosis the bicarb rises by 4 for every 10mmHg rise in the CO2. So, a bicarb 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?
Yes. Without supplemental oxygen, a CO2 of 132mmHg is not compatable 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.
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