You’re working in the Cardiac Room as a new PGY-2 and the triage nurse calls you to evaluate a patient. He’s about 40 years old, slightly overweight, known to the ED for chronic alcohol abuse, and appears to be intoxicated yet again.The nurse tells you that when she checked his vitals, his O2 sat was 91%, so she wanted you to take a look at him. You look him over–no signs of obvious trauma. He wakes up a little when you nudge him, but he definitely needs to sober up. You put a non-rebreather (NRB) on him to help with the pulse ox and clear him for the side teams.

You go back into the Cardiac Room and run the patient by your attending, thinking he’ll agree with your plan. But then he jumps up from his chair, goes into triage and promptly takes the NRB off. He tells the nurse to leave the oxygen off, and to make sure the patient goes to a monitored bed.

Wait. What? Why no oxygen?!

Your attending is worried about masking hypercapnia and possibly CO2 narcosis secondary to hypoventilation. Remember that ventilation and oxygenation are two separate processes, and in the patient above, we are more concerned about his ability to ventilate than to oxygenate.

While breathing room air (FiO2 21%), pulse oximetry has been shown to be a reliable indicator of hypoventilation, and you’ll be able to detect ventilatory abnormalities in real time.

Now we add supplemental oxygen. The NRB supplies an FiO2 of about 60-80% with a rate of 15L/min. Without getting into too much of the physiology, an FiO2 in this range will increase the PAO2 so much that even a dangerously elevated PACO2 will not cause the SpO2 to drop low enough for us to detect the desaturation on our monitor. By the time the PACO2 has risen enough to actually see an SpO2 <90%, the patient may be very close to a respiratory arrest and will likely be severely altered from hypercapnia.

So, what to do? The patient mentioned above needs a monitor with a pulse ox and frequent re-assessments. If you want to get fancy, you can put an end-tidal CO2 monitor on the patient, which will help us assess his ventilation status. He may also benefit from a nasal trumpet or two.

Tl;dr: Be extremely cautious when using supplemental oxygen in patients when you are concerned about their ability to VENTILATE. Treat the patient, not the numbers. Remember to take the clinical context into consideration prior to performing any manipulations, even if it’s as simple as giving them a little oxygen.

SourceFu, Eugene S. et al. Supplemental Oxygen Impairs Detection of Hypoventilation by Pulse Oximetry. CHEST, Vol 126, Issue 5, 1552-1558.