THE BOTTOM LINE
A non-rebreather (NRB) mask ONLY delivers high FIO2 when the flow meter valve is opened fully and even then only gets to about 90%.
THE DETAILS
I find great joy in reading old literature. I’m pretty sure I would be camped out in a medical library if it weren’t for the interwebs, thank goodness I can do all this from home…For this installment of the basics we look at how much oxygen that non-rebreather (NRB) mask actually delivering.
First, we need to talk about what a non-rebreather does. It’s mostly in the name but it isn’t always intuitive as I see it often used incorrectly. A non-rebreather should not allow you to re-breathe; yes I know how silly that sounds. However, when the NRB is working correctly the reservoir should not deflate with inhalation otherwise you are rebreathing. The NRB has two one-way valves. One is between the reservoir and the mask to allow oxygen to flow into the mask when you breathe in but not when you breath out. The other is at the side of the mask and allows the exhaled air to escape to the world. Ideally, the 100% FIO2 (Fraction of Inspired oxygen) in the bag is inhaled and thus the patient gets 100% oxygen delivery. Unfortunately in the real world this does not happen. One important reason is most NRB’s only have the one one-way valve so the patient doesn’t suffocate if the wall oxygen were to fail. Also the mask may not fit as tightly so oxygen will escape from the mask further decreasing the FIO2. Finally, we use oxygen on sick patients who may have increased respiratory rates and abnormal tidal volumes. Thus the normal minute volume will be abnormal in a sick patient. The goal of preoxygenation is to get the nitrogen part of normal air washed out and replace it with oxygen so our blood will be “super-saturated” and we will have longer times of apnea without the oxygen saturation dropping while a patient is being intubated. Thus we would like as high as possible FIO2 going into the patient. Also we really cant tell what the PaO2 is from the pulse ox monitor because no correlation can be made once SaO2 is >98%.
Thus we NEED to know the FIO2 of the non-rebreather going in. To figure this out we go back in time to 1991 while this might sound scary to go to a time period without iphones; we only have to be there long enough to see the article by Farias, Delivery of High Inspired Oxygen by Face Mask in the Journal of Critical Care. The recruited 5 healthy male volunteers and changed respiratory rates, tidal volumes and oxygen flow rates to see the effect on the FIO2 of a non-rebreather. Now here is the impressive part of the study “FIO, was measured from a catheter positioned through the nose so that its tip was in the pharynx”. The “catheter” was a 6Fr Foley that was inflated once in the oropharynx! These people volunteered for this? You sure they didn’t get paid something???? Yikes! This is why the old literature is entertaining! Anyway…. All wall oxygen comes through a flow valve with a little metal ball that measures the flow out. Normally, with a NRB the flow is set to 15L on the meter and sadly this is the highest the meter reads. So to test various flow rates they had to use their own flow meter and tested rates of 15, 20,30,40 and 60 L/min. They note, “by opening the valve fully on the flow meter, it was possible to attain an 02 flow of 60 L/min. “. At each flow rate the volunteers produced respiratory rates (RR) of 20, 30, and 40 breaths/min (brpm) and also varying tidal volumes (VT) of 500, 750, and 1000 mL. Normal tidal volume is around 500 ml or 7 ml/kg. For a normal RR of 20 and a “normal” VT of 500ml, when the flow was set to 15L the best FIO2 obtained was just shy of 60%. When the RR was 40 brpm and the VT was 1000 ml, this fell to 40% . The study notes, “When the O2 flow rate was increased to 60 L/min, the FIO2, remained at approximately [90%] despite increasing respiratory frequency and tidal volume.”
Hence, this is the reason I say crank that little knobby thing on the oxygen until it stops. Because only then are you getting 60 L/min of oxygen and only then, regardless of RR and VT, does one approximate FIO2 of 100%…well ok 90% but I’ll take it! The study also looks at ways to improve the masks by doing stuff to them but since I’m just using the NRB before I intubate I’ll be sufficiently happy with a 90% FIO2 to preoxygenate my patients besides I don’t need to look any crazier to the staff by making an art project out of the masks before I intubate someone…
REFERENCE
Farias, E. et al. Delivery of high inspired oxygen by face mask Journal of Critical Care. Volume 6, Issue 3, September 1991, Pages 119-124.
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