Vodcast 65 - BVM Anatomy
Show Notes for Vodcast 65:
Knowing the anatomy of our medical equipment makes using it in different ways so much easier. Here is an intro to the anatomy of the BVM.
The Back End
So I don't know what these valves are really called, but here is what I named them. At first these valves may seem a little pointless since they are the same size and point different directions from the same area. However, you will soon see that these valves are absolutely necessary for the BVM to function properly.
This valve I call the pressure seal. This is what closes when you squeeze the BVM so that the pressure does not leak out of the back (ensuring your patient gets tidal volume).
This pressure seal opens when you're not squeezing the bag to allow oxygen from the back-end of the bag to flow into the bag itself. This pressure seal is opened up pretty wide when you release the BVM after giving a breath.
If your patient decides to breathe in while you have an ETT in place, or a mask sealed up against their face, the safety valve ensures that they can always draw in more volume. First the pressure seal opens, and you would hope this would mean that the patient would draw in oxygen from the reservoir and the oxygen tubing. However, a very fast inspiratory flow rate (IFR) or if you release your BVM too quickly after giving a breath, the safety valve can open - drawing in room air and diluting the percent of oxygen inside of the BVM.
The relief valve functions when the pressure seal is closed. When you squeeze the BVM, the pressure seal at the back closes - isolating the entire back end of the BVM. If the reservoir is full, the oxygen that is still flowing will leave the relief valve. Why does it do this? Isn't that just wasting oxygen? No. It does this so that the pressure from your flowing oxygen does not create a leak in your pressure seal. If the pressure seal was broken by the oxygen flow from the back of the BVM, it could make it impossible to create forward flow into the patient, meaning you would not deliver adequate tidal volume.
The Business End
Now for the front end of the BVM. This has three ports. One comes from the bag, one goes to the patient, and the other leaves through the PEEP valve.
When your squeeze the bag, the pressure generated causes the duck bill / diaphragm to close off exhalation. The duck bill opens and flow is delivered to the patient.
When the patient exhales, the duck bill closes and the diaphragm lifts off of its circulator post. This diverts the exhaled tidal volume to the only place it can go - through the peep valve.