The fio2 Fallacy
In school I had always been told that a BVM delivers nearly 100% oxygen (fio2 of 1.0). So, when I got my hands on a gas analyzer from the lair of the nemesis of emesis ( @jducanto ) I decided to give the BVM a test. I made a video of the test and posted it to twitter:
The video got a lot of attention and some great discussion was started! It was awesome!
Haters will say Jesse's hat was photoshopped....
In the video I perform two ways of using the BVM. The first way was squeezing the the bag in the center and allowing it to inflate rapidly. Utilizing the BVM was problematic for a couple of reasons. First, the tidal volume I was using was too high for the average adult. Squeezing the bag in the middle delivers approximately 700 ml (when using one hand). Not only can this cause lung injury, this also means that when you release the bag, the oxygen delivered from tank must meet this larger demand. Second, the demand of the bag inflation is too much for the oxygen delivery system to keep up with. For example, let's say that I released the BVM over the course of 0.5 seconds and it must entrain 700 mL of gas. This would require an inspiratory flow rate of 84 L / min to fill the bag with only oxygen. Too bad the wall oxygen is only set at 15 LPM during an average resuscitation. DOH! The fio2 reading upon squeeze of the bag was usually round 40-50 using this technique. I know what you're thinking... What about the reservoir bag? In this initial experiment, it barely deflated. Why? I have no idea. There were no holes in it, there was nothing wrong with the equipment, it simply barely deflated at all. This will be investigated further in the future. What about the second way I used the BVM?
The second way I utilized the BVM was with a smaller vT and slower inflation. By using a smaller, more appropriate vt, it is safer for the lungs and needs less flow to compensate. This vT was around 400-500mL (we will say 500 just to be safe). The inflation time I used was approximately 6 seconds. This means that I extended the inflation out over the entire time I was not giving a breath. This means that I only needed an inspiratory flow rate of approximately 5L / min in order for the bag to favor the oxygen source rather than room air. This resulted in my fio2 staying around the 90% range throughout the entire time I was 'bagging.' Slower inflation rates FTW! You can imagine that if a higher minute volume was used, you would just need to turn up the oxygen flow on the wall to compensate.
Anatomy of the BVM
A BVM actually has a pretty complicated anatomy. There a ton of little intricacies on this thing! The duckbill does not allow free flowing gas (so you can't use blow-by oxygen with it). There are exhalation ports for the patient. On the back end there is a port that off-gasses excess pressure. There is the reservoir opening. Then, there is this pesky little valve that opens to room air.... This little valve opens so easily, that when the bag is released quickly, the bag seems to favor letting room air in over sucking from the reservoir. If you let the bag go quickly, you can actually hear it opening up.
This BVM observation was very limited, and has lots of room for improvement. Next, I will be enlisting some help from Jim DuCanto and Tyler Christifulli! We will perform a more detailed study of the oxygen concentration from different portions of the BVM, and experimenting with different flow rates. We will be testing the oxygen concentration prior to the duckbill (by drilling a hole in the BVM), in the middle of the bag itself, as well as near the room air inlet. 15LPM, 30LPM, and flush-rate oxygen will be used to test all of these positions.
Please share and contribute!
Do you have any recommendations for how to perform this study? Do you have any budding questions? Comment and let me know! I am very excited to put this vital piece of equipment to further testing!
Much more to come on this! Stay tuned!