I posted about my collar experience in another post, but SleepRider asked me to write about my experience with the Velpeau collar in this thread.
It seems that a lot of my apneic events happen due to my chin tucking. When this happens, there's nothing the CPAP can do to overpower the flow limitation. It tries to, though, and increases the pressure until the mask leaks and I wake up, or otherwise, have OA events.
I've found using a collar to have an immense impact on my sleep apnea treatment. When I wear the Velpeau collar, even loosely, it prevents these flow limitations completely.
I'm sure many collars work just the same, but I got the Velpeau. The dip in the front doesn't prevent it from working. Mine comes with a collar slip cover that can be taken off and washed, which is nice. I have a 15" neck circumference and the large fits fine. A medium would probably work too since a perfect fit is unnecessary.
So long as the collar is fastened tightly enough that my chin can't drop too far, it works fine. This means the collar is quite loose and doesn't touch the front of my neck while in bed. I don't notice it when it's on.
Below are two side-by-side screen shots where the left shows a night's sleep with the collar and the right shows a night without it. The shots are representative of almost all of my nights with one of the aforementioned configurations.
If you're still having trouble with some apneic events that pressure can't seem to fix, I'd try a collar. It might be all you need.
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