“Satting a hundred and slipping away.”

Using the alveolar gas equation to understand one of the potential risks of unnecessary supplemental oxygen.

#Tweetorial #MedThread 1/6

When you admit a patient with COPD flare, or pain crisis requiring lots of IV opioids, which do you order most frequently?

So let’s imagine such a patient. Admission O2 sat is 95% on room air. They’re on the floor. You order continuous O2 sat monitoring so that you’ll know if they worsen. 3/

Now the alveolar gas equation:

PAO2 = FiO2*(760-47) - 1.25*PCO2

On room air, this is

PAO2 = ~150 - 1.25*PCO2

So if they become hypercarbic to 80,

PAO2 = 150 - 1.25*80 = 50.

They desat. Alarm rings. Help comes. 4/

But what if you had placed that patient is on 2L nasal cannula (FiO2 28%)? Now, if they retain to a PCO2 of 80..

PAO2 = 0.28*(713) - 1.25*PCO2

PAO2 = 200 - 100 = 100 mmHg.

Their O2 sat is 99%... alarm doesn’t ring. They’re slipping away with the cannula on. 5/

Take-homes:
- Hypoxemia from hypoventilation/hypercarbia is easily reversed with minimal supplemental O2
- But the hypercarbia remains, and can kill you
- In addition to potential toxicities, unnecessary O2 takes away your ability to detect change in clinical status. 6/6