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Pulmonary Puzzle 007

January 12, 2010 By Leave a Comment

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.

Pulmonary Puzzle 007 image 1

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:

Pulmonary Puzzle 007 image 2
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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Filed Under: Blood Results, Intensive Care, Investigation [tests], Respiratory Tagged With: , ,
About Paul Young

A proud graduate of The Breakfast Club, Paul is an Intensivist in Wellington, New Zealand. According to his father, Paul studied medicine after performing a cost-effectiveness analysis of his own biomedical fragility – a champion runner as a youth, he now struggles with a zimmer frame. Although he started out in the ED, Paul feels physically ill whenever he steps foot there these days.

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