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“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
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/
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/
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
- 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
